If you connect the resistor and the condenser, then it will be possible one of the most useful and universal chains.
About numerous ways of application of which I decided to tell today. But at first about each element separately:
The resistor is its task to limit the current. This is a static element whose resistance does not change, they are not talking about thermal errors - they are not too great. The current through the resistor is determined by the law of Oma - I \u003d u / rwhere u voltage at the conclusions of the resistor, R is its resistance.
The capacitor is more interesting. He has an interesting property - when it is discharged, then behaves almost like a short circuit - the current through it flows without restrictions, rushing in infinity. And the voltage on it tends to zero. When it is charged, it becomes like a break and current over it ceases to flow, and the voltage on it becomes equal to the charging source. It turns out an interesting dependence - there is current, no voltage, there is a voltage - no current.
To visualize yourself this process, imagine gan ... um .. The balloon which is filled with water. The flow of water is the current. Water pressure on elastic wall - voltage equivalent. Now look when the ball is empty - water flows freely, high current, and there are almost no pressure - there is little voltage. Then, when the ball is filled and starts to resist pressure, due to the elasticity of the walls, the flow rate will slow down, and then stop at all - the forces were equal, the condenser was charged. There is a tension of stretched walls, but no current!
Now, if you remove or reduce the external pressure, remove the power source, then the water under the action of elasticity rushes back. Also, the current from the condenser will flow back if the circuit is closed, and the source voltage is lower than the voltage in the condenser.
Capacitance capacitor. What is it?
Theoretically, in any ideal condenser you can download the charge of an infinite size. Just our ball is stronger than the walls and the walls will create a greater pressure, infinitely a long pressure.
And what then about Farad, what do they write on the side of the capacitor as a capacity of the container? And this is just the dependence of the voltage of charge (Q \u003d Cu). A low-capacity capacitor has a voltage growth from charge will be higher.
Imagine two glasses with infinitely high walls. One narrow, like a test tube, another wide, like a basin. The water level in them is the voltage. The bottom area is a container. And in that and in another one and the same liter of water can be harvesting - equal charge. But in the test tube, the level jums up a few meters, and in the basin it will splash at the bottom. Also in capacitors with a small and large capacity.
You can pour how much you like, but the voltage will be different.
Plus, in real life, capacitors have a punching voltage, after which it ceases to be a condenser, and turns into an anger conductor :)
How fast charged the capacitor?
In ideal conditions, when we have an infinitely powerful voltage source with zero internal resistance, ideal superconducting wires and an absolutely flawless condenser - this process will occur instantly, with a time equal to 0, as well as discharge.
But in reality, there are always resistance, obvious - such as a banal resistor or implicit, such as wire resistance or internal resistance of the voltage source.
In this case, the charge rate of the capacitor will depend on the resistance in the circuit and the capacitance of the considera, and the charge itself will go exponential law.
 |
And this law has a couple of characteristic values:
- T - constant timeThis time at which the value will reach 63% of its maximum. 63% were not accidental here, there is a direct tie to such a formula Value T \u003d MAX-1 / E * MAX.
- 3T - and with a three-time constant value to reach 95% of its maximum.
Constant time for RC chains T \u003d R * C.
The smaller the resistance and less capacity, the faster the capacitor is charging. If the resistance is zero, then the charge time is zero.
Calculate for how much charges by a 95% condenser with a capacity of 1UF through a resistor in 1kom:
T \u003d C * R \u003d 10 -6 * 10 3 \u003d 0.001C
3T \u003d 0.003C Through this time, the voltage on the condenser will reach 95% of the source voltage.
The discharge will follow the same law, only upside down. Those. Through the blemishes in the condenser, only 100% remain - 63% \u003d 37% of the initial voltage, and after 3T, and less - a pitiful 5%.
Well, with the feed and removal of voltage everything is clear. And if the tension filed, and then they raised steady, and also discharged the steps? The situation here will not change here - the voltage has risen, the capacitor predicted to him by the same law, from the same time constant - through the time of 3T its voltage will be 95% of the new maximum.
Slightly decreased - it was damaged and through the time 3T voltage on it will be 5% higher than the new minimum.
Why am I telling you, better to show. Gang here in the multiscimation cunning stepped signal generator and filed a chain to integrate RC:
 |
See how the sausage :) Please note that the charge and discharge, regardless of the height of the step, always one duration !!!
And to what size the capacitor can be charged?
In theory to infinity, a sort of ball with endlessly pulling walls. In real life, the ball sooner or later burst, and the condenser breaks and burst. That is why all capacitors have an important parameter - limit tension. On the electrolytes, it is often written on the side, and on ceramic it is necessary to look at the directories. But there it is usually from 50 volts. In general, choosing Conder should be monitored that its limit voltage is not lower than that in the chain. I will add that when calculating the condenser on an alternating voltage, you should select the limit voltage 1.4 times higher. Because On alternating voltage indicate the active value, and the instantaneous value at its maximum exceeds it by 1.4 times.
What follows from the above? And the fact that if there is a constant voltage on the condenser, then it will simply charge it all. This fun will end.
And if you submit a variable? It is obvious that it will be charged, then discharged, and in the chain it will be there and back to walk the current. Motion! There is a current!
It turns out, despite the physical breakdown of the chain between the plates, alternating current flows through the capacitor, but constant weakly.
What gives us this? And the fact that the capacitor can serve as a sort of separator for the separation of AC and constant to the corresponding components.
Any time-changing signal can be represented as the sum of two components - variable and constant.
 |
For example, in classic sinusoids there is only a variable part, and the constant is zero. At the constant current, on the contrary. And if we have a shifted sinusoid? Or permanent with interference?
The variable and the constant component of the signal are easily divided!
A little higher, I showed you how the capacitor is disappeared and is uninstalled when the voltage changes. So the variable component through Conder will be held with a bang, because Only she causes the capacitor to actively change its charge. The permanent as it remained and stuck on the condenser.
But that the capacitor effectively divided the variable component from the constant frequency of the component variable must be no lower than 1 / t
Two types of inclusion RC chains are possible:
Integrating and differentiating. They are low frequency filter and high frequency filter.
The low frequency filter without changes passes the constant component (because its frequency is zero, there is no place below) and suppresses everything higher than 1 / t. The constant component passes directly, and the variable component through the condenser is extinguished on the ground.
Such a filter is also called an integrating chain because the output signal is integrated. Do you remember what is the integral? Square under the curve! Here it turns out at the exit.
A differentiating chain is called it because at the output we obtain the differential input function, which is nothing more than the rate of change of this function.
 |
- In the section 1, the capacitor charge occurs, which means there is a current and a voltage drop on the resistor.
- In section 2, there is a sharp increase in the charge rate, which means the current will sharply increase, and behind it and the voltage drop on the resistor.
- On the section 3 condenser simply holds the already available potential. There is no current through it, which means that the voltage resistor is also zero.
- Well, on the 4th plot, the condenser began to discharge, because The input signal has become lower than its voltage. The current went in the opposite direction and on the resistor already a negative voltage drop.
And if you submit to the inlet of a rectangular pulse, with very steep fronts and make the capacitance capacitor in bed, then we will see such needles:
rectangle. Well, what about? That's right - the derivative of the linear function is a constant, the slope of this function determines the constant sign.
In short, if you have a Matana course now, you can score on the momentive Mathcad, averted Maple, throw Matthaba Matric Herage and, deliver a handful of analog scatter from the honeycomb,
True, the integrators and differentiators are usually not done on only the resistors of the consides, there are operational amplifiers here. You can google it up for these things, curious thing :)
And here I filed an ordinary regeneration signal into two high and low filters. And the exits from them to the oscilloscope:
Here, a little more than a single section:
When you start, the Conder is discharged, the current is poured through it to the full, and the voltage on it is meager - at the entrance of the RESET signal reset. But soon the capacitor charges and through time it will voltage will already be at the logical unit level and the reset will stop the reset signal - the MK will start.
And for AT89C51 It is necessary to organize RESET with accuracy to the opposite - at first to file a unit, and then zero. Here the situation is reverse - until the Conder is charged, then the current flows large, UC - the voltage drop on it is meager Uc \u003d 0. So, the RESET fits the voltage to a little less power supply UPIT-UC \u003d Upit.
But when Conder charges and the voltage on it will reach the supply voltage (Ufully \u003d UC), then the reset is already up to the reset.
Analog measurements
But FIG will take off with a discharge chain where funny use the possibility of RC chains to measure the analog values \u200b\u200bof microcontrollers in which there is no ADC.
It uses the fact that the voltage on the condenser grows strictly to the same law - the exhibitor. Depending on the consideration, resistor and supply voltage. So it can be used as a reference voltage with pre-known parameters.
It works simply, we supply voltage from the capacitor to the analog comparator, and on the second input of the comparator we will put the measured voltage. And when we want to measure the tension, then you just first pull out the output down to discharge the capacitor. Then returning it to Hi-Z mode, draw and launch the timer. And then the Conder begins to charge through the resistor and as soon as the comparator reports that the voltage with RC caught up with the measured, then stop the timer.
 |
Knowing what kind of law on time is the increase in the support voltage of the RC chain, as well as knowing how many natikal timer, we can quite accurately know what the measured voltage was equal to the time of the comparator's work. Moreover, it is not necessary to consider exhibitors. At the initial stage of charging, the Condere can assume that the dependence is linear. Or, if you want more accuracy, approximate to the exponent with piecewise linear functions, and in Russian - to draw its approximate form by several straight or groan the table of dependence of the value from time, in short, the methods of the car simply.
If you need to raise an analog twist, and there is no ADC, you can not even use a comparator. To jump with a foot on which the capacitor hangs and give it to charge through the permno resistor.
By changing the T, which reminds T \u003d R * C and knowing that we have C \u003d const, you can calculate the value of R. And again, it is not necessary to connect the mathematical apparatus here, in most cases it is enough to measure in any conditional parrots, like Tim timer. And you can go to another way, do not change the resistor, but to change the container, for example, connecting the container of your body ... What happens? Right - Touch buttons!
If something is not clear, then do not worry soon I will write an article about how to fasten an analog figovine to the microcontroller without using the ADC. There are all the breaks in detail.
Consider consistent RC chainconsisting of a sequentially connected resistor and capacitor.
Tension at the chain clamps
According to the second law of Kirchhoff, the same voltage can be defined as the amount of stress drops on the resistor and the condenser.
where
Then the first expression can be rewritten in the following form
Current in the chain is equal
Substituting in the expression above, and after performing integration, we get
Voltage on the resistor is equal 
Voltage on the condenser 
As can be seen from the last expression, the voltage on the condenser lags behind the current to the angle π / 2.
Reactive (capacitive) condenser resistance is equal
With a decrease in frequency capacitive resistance of the capacitor increases. With a constant current, it is equal to infinity, since the frequency is zero.
Phase shift in serial RC - chains can be determined by the formula
RC-chain resistance
Amplitude current
Consider an example of solving a RC chain problem
Full resistance consistent RC- Chains are 24 ohms. The voltage on the resistor is equal to 10 V, and its resistance is 20 ohms. Find C,UC., U., I., shift phaseφ . Build a vector diagram.
We find the current flowing through the resistor. Since the connection is consistent, then this current will be common to the entire chain.
Knowing the current and resistance of the chain, we will find the voltage
Capacitive resistance condenser
Knowing resistance, we will find the voltage and container
Shift phases
We construct the RC circuit vector chart, while we consider that the voltage on the condenser is behind the current (this is visible by the phase shift sign).
First, the current current is postponed in the chain, then the voltage on the resistor and the voltage on the condenser. The general voltage vector is then built as the sum of the voltage vectors on the condenser and on the resistor.
The effect of arc discharges on the stability of the operation of the relay contacts is so large that the engineer knowledge of the calculation and application of protective schemes is simply a prerequisite.
Sparkling chains
To reduce damage to contacts with arc discharges apply:
- special relays with large contact intervals (up to 10 mm or more) and high shutdown speed provided by strong contact springs;
- magnetic blowing contacts implemented by the installation of a constant magnet or an electromagnet in the plane of the contact interval. The magnetic field prevents the appearance and development of the arc and effectively protects contacts from the burning;
- sparkling chains installed in parallel to the relay contacts or parallel to the load.
The first two methods guarantee high reliability through constructive measures when developing a relay. The external elements of the protection of contacts are usually not required, but special relays and magnetic blowing contacts are quite exotic, roads and differ in large sizes and solid power of the coil (near the relay with a large distance between the contacts strong contact springs).
Industrial electrical engineering is focused on inexpensive standard relays, so the use of sparkling chains is the most common way to clean arc discharges on contacts.
Fig. 1. Effective protection significantly prolongs the life of contacts:
Theoretically, many physical principles can be used to harvest arcs, but in practice the following efficient and economical schemes are applied:
- RC chains;
- return diodes;
- varistors;
- combined schemes, for example, varistor + RC chain.
Protective chains can be included:
- parallel inductive load;
- parallel to the contacts relays;
- parallel to the contacts and load at the same time.
In fig. 1 shows the typical switching on the protective circuits when working on a constant current.
Diode scheme (for DC circuits only)
The cheapest and widely used scheme for suppressing self-induction voltage. The silicon diode is turned on by parallel with the inductive load, when contacting contacts and in the steady mode does not have any impact on the operation of the scheme. When the load is turned off, self-induction voltage occurs, the operating voltage inverse, the diode opens and shuts the inductive load.
Do not assume that the diode limits the reverse voltage at the level of direct voltage drop, equal to 0.7-1 V. due to the final internal resistance, the voltage drop on the diode depends on the current through the diode. Powerful inductive loads are able to develop self-induction pulse currents up to dozen amps, which for powerful silicon diodes corresponds to a voltage drop of about 10-20 V. Diodes extremely effectively eliminate arc discharges and protect the contacts of the relay from burning better than any other sparkling schemes.
Rules for selecting a reverse diode:
- the operating current and the reverse diode voltage must be comparable to the nominal voltage and the current current. For loads with an operating voltage up to 250 ѵds and the working current of up to 5 A, a common silicon diode 1N4007 with a reverse voltage of 1000 ѵds and a maximum pulsed current up to 20 A is fully suitable;
- the conclusions of the diode must be as short as possible;
- diode should be soldered (screwed) directly to the inductive load, without long connecting wires, it improves the EMC in the switching processes.
The advantages of the diode scheme:
- low cost and reliability;
- simple calculation;
- maximum achievable efficiency.
Disadvantages of a diode scheme:
- the diodes increase the off time of the inductive loads of 5-10 times, which is very undesirable for the load type or contactors (contacts are blocked slower, which contributes to the burning), while the diode protection works only in DC circuits.
If a restrictive resistance is in series with a diode, the effect of diodes on the shutdown time is reduced, but additional resistors determine higher return voltages than only protective diodes (voltage falls on the resistor according to the Ohm's law).
Stabilians (for variable and direct current circuits)
Instead of a diode parallel to the load, a stabilion is installed, and for the AC circuits, two counter-sequentially included stabilion. In such a scheme, the reverse voltage is limited to the stabilion to stabilization voltage, which somewhat reduces the influence of the spark-protecting chain at the time of turning off the load.
Considering the inner resistance of Stabilon, the reverse voltage on powerful inductive loads will be greater than the stabilization voltage by the voltage drop on the differential resistance of the stabitron.
Selection of stabilion for protection scheme:
- the desired limitation voltage is selected;
- the required power of the stabilion is selected, taking into account the peak current developed by the load when self-induction voltage occurs;
- the true voltage of the restriction is checked - an experiment is desirable for this, and when measuring the voltage, it is convenient to use the oscilloscope.
Advantages of Stabilians:
- less shutdown delay than in a diode scheme;
- stabilians can be used in circuits of any polarity;
- stabilians for low-power loads cheap;
- the scheme works on alternating and constant current.
Disadvantages of Stabilians:
- less efficiency than in a diode scheme;
- for powerful loads, expensive stabilids are required;
- for very powerful loads, the scheme with stabilids is technically unrealized.
Varistor scheme (for variable and direct current circuits)
The metal-oxide varistor has a volt-ampere characteristic similar to bipolar stabilong. Until the application to the outputs of the voltage limit, the varistor is almost disconnected from the diagram and is characterized by microampeakers of leakage and internal capacity at the level of 150-1000 PF. With an increase in voltage, the varistor begins to open smoothly, shunting its internal resistance inductive load.
With very small sizes, the varistors are capable of removing large pulse currents: for a varistor with a diameter of 7 mm, the discharge current may be equal to 500-1000 A (pulse duration less than 100 μs).
Calculation and installation of varistoric protection:
- set by the safe voltage of the inductive limit
load; - calculates or measured the current given by inductive load during self-induction to determine the required current varistor;
- the varistor is selected by the catalog to the desired limit voltage, if necessary, varistors can be installed sequentially to select the desired voltage;
- you must check: the varistor should be closed in the entire range of operating stresses on the load (leakage current less than 10-50 μA);
- the varistor must be mounted on the load according to the rules specified for diode protection.
Advantages of varistoric protection:
- varistors operate in variable and direct current circuits;
- normalized limit voltage;
- minor effect on shutdown delay;
- varistors are cheap;
- varistors perfectly complement the protective RC circuits when working with high voltages on the load.
Lack of varistor protection:
- when applying only varistors, the contact protection of the relay from the electrical arc is essentially worse than in diode chains.
RC chains (for direct and alternating current)
Unlike diode and varistor schemes, the RS circuit can be installed both parallel to the load and parallel to the relay contacts. In some cases, the load is physically not available for mounting on the sparking elements on it, and then the contact of contacts is shunting RC-chains, the only way to protect the contacts.
The principle of action of the RC-chain is the fact that the voltage on the condenser cannot change instantly. The self-induction voltage is pulsed, and the pulse front for typical electrical devices has a duration at the level of 1 μs. When applying such a pulse to the RC circuit, the voltage on the condenser begins to increase not instantly, but from a time constant determined by the values \u200b\u200bof R and C.
If you count the internal resistance of the power supply to zero, the connection of the RC circuit parallel to the load is equivalent to turning on the RC-chain parallel to the relay contacts. In this sense, there is no fundamental difference in the installation of elements of the sparkling chain for different inclusion schemes.
RC chain parallel to relay contacts
Condenser (see Fig. 2) When opening contacts, the relay begins to charge. If the charge time of the capacitor before the ignition voltage of the arc on the contacts is chosen in large than the time of discharge of contacts by the distance, in which the arc may not occur, the contacts are completely protected from the appearance of an arc. This case is ideal and in practice is unlikely. In real cases, the RC-chain helps when operating a chain to maintain low voltage on the relay contacts and thereby weaken the effect of the arc.
Fig. 2. Protective elements can be included both in parallel to the contacts and parallel load:
When only one capacitor is turned on in parallel with the relay contacts, the defense scheme is also in principle, but the discharge of the capacitor through the contacts of the relay when they are closed leads to a current throw through the contacts, which is undesirable. RC-chain in this sense optimizes all transient processes both during closure and when the contacts are blurred.
RC-chain calculation
The easiest way to use the universal nomogram shown in Fig. 3. According to the well-known voltage of the power supply U. and current load I. Find two points to the nomogram, after which a straight line is carried out between the points, showing the desired resistance value R.. Tank value FROM Squeezed on the scale next to the current scale I.. The nomogram gives the developer fairly accurate data, with the practical implementation of the scheme it will be necessary to select the nearest standard values \u200b\u200bfor the resistor and the condenser RC-chain.
Fig. 3. The most convenient and accurate nomogram to determine the parameters of the protective RC chain (and this graph is for more than 50 years!)
Selection of capacitor and resistor RC-chain
The condenser should be used only with a film or paper dielectric, ceramic capacitors for high-voltage spark-protection chains are not suitable. When choosing a resistor, it is necessary to remember that there is a high power during transition. It can be recommended to apply 1-2 W Resistors for RC-chains, and must be checked if the resistor is calculated to the high pulsed self-induction voltage. It is best to use wire resistors, but metal and carbon and carbon with fill with ceramic compounds are well working.
The advantages of the RC chain:
- good arc harvesting;
- lack of influence on the time of turning off inductive load.
Features RC chains: the need to use high-quality capacitor and resistor. In general, the use of RC chains is always economically justified.
When installing a sparkling chain in parallel with alternating current contacts, the leakage current, determined by the RC-chain impedance, will occur with open contacts of the relay. If the load does not allow leakage current or it is undesirable in terms of schematic reasons and for the security of personnel, it is necessary to install the RC chain parallel to the load.
Combination RC-chain and diode scheme
Such a scheme (sometimes called the DRC chain) is limited in its efficiency and allows you to reduce all unwanted effects from the effects of an electric arc to the relay contacts.
The advantages of the DRC chain:
- the electrical resource of the relay is approaching its theoretical limit.
Disadvantages of the DRC chain:
- the diode causes a significant delay in turning off the inductive load.
Combination RC-chain and varistor
If the varistor is installed instead of the diode, the parameters scheme will be identical to the usual RC-sparkling chain, but the limit of the self-induction voltage variant on the load allows you to use less high-voltage and cheaper condenser and resistor.
RC chain parallel to load
It is used where it is undesirable or impossible installation of the RC-chain parallel to the relay contacts. For calculation, the following orientation values \u200b\u200bare proposed:
- C \u003d 0.5-1 μF per 1 and current load current;
- R \u003d 0.5-1 Ohm per 1 in stress on the load;
- R \u003d 50-100% of load resistance.
After calculating the ratings R and C, it is necessary to check the additional load of the relay contacts arising from this during the transition process (the charge of the condenser), as described above.
The values \u200b\u200bof R and C are not optimal. If the most complete contact protection is required and the implementation of the maximum relay resource is required, then it is necessary to carry out an experiment and experimentally to select a resistor and a condenser, observing transient processes using an oscilloscope.
The advantages of the RC circuit parallel to the load:
- good suppression of the arc;
- there are no leakage currents through open relay contacts.
Disadvantages:
- with a load current, more than 10 and large capacity values \u200b\u200blead to the need to install relatively and large capacitors in size;
- an experimental check and selection of elements are desired to optimize the scheme.
The photos show the voltage oscillograms on the inductive load at the time of opening of the power without shunting (Fig. 4) and with the Race chain installed (Fig. 5). Both oscillograms have a vertical scale of 100 volts / division.
Fig. 4. Disabling inductive load causes a very complex transition process
Fig. 5. Properly selected Protective Rsetpoint completely eliminates the transition process
A special comment here is not required, the effect of installing a sparkling chain is visible immediately. The process of generating high-frequency high-voltage interference is striking at the time of opening the contacts.
Photos are taken from the university report on optimizing RC-chains installed in parallel to the relay contacts. The author of the report conducted a complex mathematical analysis of the behavior of the inductive load with the shunt in the form of the RC-chain, but as a result, the recommendations on the calculation of the elements were reduced to two formulas:
C \u003d І 2/10
where FROM - RC circuit capacity, ICF;I. - workload current, A;
R \u003d E about / (10I (1 + 50 / e))
where E O.- voltage at load; IN, I. - workload current, A; R. - Resistance RC-chain, Ohm.
Answer: C \u003d 0.1 μF, R \u003d 20 ohms. These parameters are perfectly consistent with the nomogram shown earlier.
In conclusion, we will get acquainted with the table from the same report, which shows the practically measured voltage and the delay time for various sparkling chains. An electromagnetic relay with a coil voltage 28 ѵDS / 1 W was served as an inductive load, the sparkling chain was installed parallel to the relay coil.
| Shunt parallel to the relay coil | Peak voltage emissions on the reel coil (% of the operating voltage) | Switching off the relay, MS (% of the passport value) |
| Without Shunta | 950 (3400 %) | 1,5 (100 %) |
| Condenser 0.22 μF. | 120 (428 %) | 1,55 (103 %) |
| Stabilirt, working voltage 60 V | 190 (678 %) | 1,7 (113 %) |
| Diode + resistor 470 Ohm | 80 (286 %) | 5,4 (360 %) |
| Varistor, 60 V | 64 (229 %) | 2,7 (280 %) |
Inductive Loads and Electromagnetic Compatibility (EMC)
EMC requirements are a prerequisite for the work of electrical equipment and are understood as:
- equipment's ability to work normally under the conditions of exposure to powerful electromagnetic interference;
- property Do not create electromagnetic interference when working more prescribed level standards.
The relay is smallly sensitive to high-frequency interference, but the presence of powerful electromagnetic fields near the relay coil affects the power on and off voltage and turn off the relay. When installing a relay next to transformers, electromagnets and electric motors, it is necessary to experimental verification of the correctness of the operation and turn off the relay. When installing a large number of relays, close on the same mounting panel or on the printed circuit board, there is also mutual influence of the operation of one relay to the voltage on and off the rest of the relay. The catalogs sometimes provide guidance on the minimum distance between the same type of relays that guarantee their normal operation. In the absence of such instructions, you can use the empirical rule for which the distance between the centers of the relay coils should be at least 1.5 from their diameter. If you need a tight installation relay on a printed circuit board, an experienced testing of relay mutual influence is required.
Electromagnetic relay can create powerful interference, especially when working with inductive loads. Shown in fig. 4 High-frequency signal is a powerful interference that can affect the normal operation of sensitive electronic equipment, operating next to the relay, the interference frequency ranges from 5 to 50 MHz, and the power of this interference is several hundred MW, which is completely unacceptable by modern EMC standards. Sparkling chains allow you to bring the level of interference from relay equipment to prescribed safe level standards.
The use of the relay in the grounded metal housings is positively affected by the EMC, but it is necessary to remember that with the grounding of the metal case, most relays decreases the insulation voltage between the contacts and the coil.
Isolation between relay contacts
Between open contacts relays there is a gap, depending on the design of the relay. Air in the interval (or inert gas for gas-filled relays) performs the role of an insulator. It is assumed that insulating materials of the case and the relay contact group are characterized by higher punching voltages than air. In the absence of contamination between contacts, consideration of the insulating properties of contact groups can be limited to the properties of only the air gap.
In fig. 6 (slightly lower in the article) shows the dependence of the breakdown voltage from the distance between the relay contacts. In the catalogs you can find several options for the values \u200b\u200bof the limit voltage between the contacts, namely:
- the limit value is permanently attached to two voltage contacts;
- pulse insulation voltage value (Surge Voltage);
- the limit value of the voltage between the contacts for a certain time (usually 1 minute, during this time the leakage current should not exceed 1 or 5 mA at the specified voltage value).
If we are talking about the impulse isolation voltage, the pulse is a standard test signal ИЗ-255-5 with an increase in the rise to the peak value of 1.2 μs and the recession time up to 50% amplitude of 50 μs.
If the developer needs a relay with special requirements for contacting the contacts, then it is possible to obtain information on this requirements either by the manufacturer's company or by conducting independent testing. In the latter case, it must be remembered that the relay manufacturer will not be responsible for the measurement results obtained in this way.
Materials for Relay Contacts
From the material of the contacts, such parameters of the contacts themselves and the relay as a whole, as:
- current load capacity, that is, the ability to efficient heat removal from the contact point;
- the possibility of switching inductive loads;
- transient contact resistance;
- limiting ambient temperature during operation;
- stability of the material of contacts to migration, especially when switching inductive loads on constant current;
- stability of the material of contacts to evaporation. The evaporating metal maintains the development of an electric arc and worsens insulation when the metal is precipitated on the insulators of the contacts and the relay housing;
- stability of contacts to mechanical wear;
- elasticity of contacts for the absorption of kinetic energy and preventing excessive stray;
- metal resistance Contacts to the effects of corrosion gases from the environment.
Fig. 7. Each material is designed to work in a certain range of currents, but can be used and caution to switch weak signals
Some useful quality materials do not exclude each other, for example, good current conductors always have high thermal conductivity. At the same time, good conductors with low resistivity are usually too soft and easily resistant to wear.
Melting temperature is higher in special contact alloys (for example, AGNI or AGSNO), but such materials are not at all suitable for switching microtons.
As a result, the developer of the relay stops on a certain compromise between the quality, price and dimensions of the relay. This compromise led to the standardization of applications of various relay contacts, as shown in Fig. 7. The areas of application of various materials for contacts are sufficiently conditional, but the developer should understand that when working on the border of the "allocated", the range of currents and voltages may require experimental verification of the reliability of such an application. The experiment is very simple and consists in measuring the transitional resistance of contacts for a party of the same type of relay, and it is desirable to test the relay not just that have come from the conveyor, and the transportation and the past transportation and settled in a warehouse. The optimal period of "deployment" in the warehouse is 3-6 months, during which time the processes of aging in plastics and compounds of metallplastic are normalized.
Calculation of RC circuit, voltage changes on the condenser depending on time. Constant time. (10+)
RC - chain. Constant time. Charging and discharge condenser
Connect the condenser, resistor and voltage source as shown in the diagram:
If at the initial moment, the voltage on the condenser differs from the voltage of the power source, then the current will flow through the resistor, and the voltage on the condenser will change over time, approach the power supply voltage. It is useful to be able to calculate the time for which the voltage will change from the specified initial to the specified end value. Such calculations are necessary for designing delay chains, relaxation generators, sources of sawtooth voltage.
Unfortunately, errors occur periodically in articles, they are corrected, the articles are complemented, developing new ones. Subscribe to news to keep up to date.
If something is incomprehensible, be sure to ask!
Ask a Question. Discussion of the article.
More articles
RC - high, low frequency filter. High frequency low-frequency. R...
Online calculation of high and low frequency filters RC. Determining the signal phase ...
Practice design of electronic circuits. Electronics tutorial ....
The art of device development. Elemental base of radio electronics. Typical schemes ....
Overview of batter-informator power sources ...
Pulse power supply circuit. Calculation of different stresses and currents ....
Inductance. Henry. Henry. GN. Units. Shares, Milligeni, ...
The concept of inductance. Units. Inductance coils ....
Calculation of the online devastating capacitor of the batran-informator power supply ...
Detector, sensor, detector of hidden wiring, breaks, cliffs. CX ...
The diagram of the device for detecting hidden wiring and its gaps for self-like ...
Svetomuska, light-music prefix with her own hands. Scheme, construction ...
How to collect light music yourself. Original design of the light-musical system ...
Switching windings The relay in the DC circuits of relay protection and automation are usually accompanied by significant overvoltages that can be dangerous for semiconductor devices used in these circuits. To protect transistors operating in switch mode, protective chains began to be used (Fig. 1), which are connected parallel to the winding relay (Fig. 2 - here the switched relay winding is represented by the substitution scheme - the inductance L, the active component of the resistance R and the resulting interspervic capacity with ) and reduce overvoltages arising between the winding clamps 1 and 2.
Fig.1 - Protective chains used to reduce switching overvoltage
Fig.2 - Protection of the VT transistor using a protective chain
However, at present, the determination of the parameters of protective chains and the assessment of their impact on the operation of relay protection devices is not deemed enough attention. In addition, when developing and designing relay protection devices using semiconductor diodes exposed to switching overvoltages, diode protection in many cases is not provided.
This leads to a rather frequent output of diodes and failure or incorrect action of the device. An example of circuits where overvoltage can affect the diode, the diagram shown in Fig.3 serves. Here, the VD separation diode turns out to be influenced by switching overvoltage and can be damaged when the Ki contacts are blurred and the closed position of the contacts K2. For the protection of this diode to the climb 1 and 2 windings of the K3 relay must be attached a protective chain. To protect diodes, the same protective agents that are used to protect transistors can be used (Fig. 1).
Fig.3 - Chains in which the VD separating diode may be exposed to switching overvoltage
2. Determining the parameters of protective chains
The values \u200b\u200bof the parameters of protective chains are determined based on the condition of reducing the effect of overvoltages to the protected semiconductor device to a permissible level. This is achieved by creating an additional circuit for the current passing in the relay winding.
Switching overvoltage of the UP, acting on the semiconductor device during transition, is defined as [L1]:
- E - voltage of the power supply of operational current;
- UC - Switching overvoltage on the relay winding.
The overvoltage of the UP must correspond to the condition [L2]:
UP< 0,7*Uдоп (2)
where: Udo - the maximum allowable value of the semiconductor device.
Based on equality (1), the maximum allowable voltage on the winding of the switched relay in the case of the use of protective chains:
Um \u003d 0.7UDE.-E (3)
Condition (3) is the initial to determine the parameters of protective chains:
2.1 Diode Stabitron
When using a protective chain diode-stabilion, the stabilization voltage of the UM, determined from the equality (3).
2.2 Diode Resistor
The resistance values \u200b\u200bof the resistor when switching a series of common relay protection and automation techniques are determined using curves depicted in Figure 4, and the corresponding point of intersection of the curve UM \u003d F (Rp) with a straight line (0.7 * udop.-E) parallel axis RR. Curves are obtained by measuring overvoltages using a radial oscilloscope using a high-level ohmic voltage divider. The capacity of the resistor does not play a significant role and 1-2 watt can be accepted.
Fig.4 A) - dependence UM \u003d F (Rp) for relay: RP-23/220 (curve 1), RP-252/220 (curve 2), EV100 series relay (without a brawling circuit (curve 3)
Fig.4 b) - Dependence UM \u003d F (Rp) for relay RU21 / 220
Fig.4 c) - dependence UM \u003d F (Rp) for relay: RPU-2/220 (curve 1), RP222-U4 / 220 (curve 2), RP255 / 220 (curve 3), RP251 / 220 (curve 4 )
2.3 Protective Diode
When using a protective diode UC \u003d 0 and the voltage on the protected semiconductor device according to (1) UP \u003d E.
2.4 Choice Protective RC - Chain
The resistance value R (resistivity of the RC chain resistor) is determined from the condition of limiting the current load on switching contacts from the charge current of the SZ capacitance (the capacity of the RC-chain condenser) permissible load, i.e.
Iaz \u003d e / rz< Iдоп. (4)
The resistance of the RC chain resistor, based on the allowed switching ability of the contacts most common in the protection and automation devices of the relay, with a sufficient margin of 2 com, and the power is 1-2 watt.
The value of the SZ container is determined by graphically and corresponds to the point of intersection of the dependence curve Um \u003d F (SZ) with a straight line (0.7 * udop.-E), parallel axis SZ (see cris.5).
Rated voltage Ur. Capacity SZ must comply with the condition< 0,7*Uном.
Fig.5 a) - dependence UM \u003d F (SZ) for relay: RP-252/220 (curve 1), RU21 / 220 (curve 2)
Fig.5 b) - dependence UM \u003d F (SZ) for relay: RP-251/220 (curve 1), RP222-U4 / 220 (curve 2), RPU-2/220 (curve 3)
Fig.5 c) - dependence UM \u003d F (SZ) for relay: RP-23/220 (curve 1), EV100 series relay (without a brawling circuit (curve 2), RP-255/220 (curve 3)
2.5 Choosing Diodes Protective Chains
The choice of diodes of protective chains is made according to the maximum allowable voltage of the diodes, based on the condition:
E.< 0,7*Uдоп. (5)
3. The effect of protective chains on an increase in current load on switched contacts
The protective chains under consideration practically do not increase the current load on switching contacts: if there is an increase in the current load in the protective chain of the semiconductor diode, an increase in the current load is at the value of the diode reverse current, which, having a value of up to several tens of micronomer, is quite small compared to the current in the relay winding. Additional load on switching contacts In the case of applying a protective RC - the chain is determined by the current of the actively leakage of the capacitor, which is also very small and may not be taken into account. It should be noted that protective chains, reducing the magnitude of switching overvoltages, facilitate the conditions for the operation of commuting contacts.
To protect the semiconductor devices used in the DC circuits of relay protection and automation, it is recommended to use RC chains and a diode resistor, since damage to any of the elements included in them does not lead to failure of the device.
5. A method for reducing switching overvoltages when using a transistor as a switching element
Switching overvoltages arising when the current is turned off in the relay winding using the transistor can be reduced to a safe level by increasing the transistor switching time from the open state to locked up to 1 MC (L3). Considering that the transistor's own switching time is in the range from one to several microseconds, it can be enlarged by inclusion in the control circuit of the transistor parallel RC circuit (Fig. 6).
Fig.6 is a way to reduce switching overvoltages by increasing the transistor switching time using R2-C
This method can be used in cases where, by the nature of the device, an increase in the switching time is permissible, and the installation of additional elements (protective chains) in the transistor load chain is undesirable. In relation to those who have found the use in the practice of static relays, the specified method seems to be most appropriate, since for detuning the interference in some cases, their action is slowed down.
6. Examples of choosing the protection of diodes from switching overvoltages
In Fig. P-1A - P-5A, the diagrams of the relay protection circuits applied in practice with separating diodes are depicted. In some of these schemes, separating diodes may be affected by switching overvoltages.
1. Fig. P-1A with a closed position of contacts K1 and opening contacts K2, almost all currents in the winding of the K4 relay are turned off. At the same time, between the clips of the winding of the K4 relay (in the winding of K4, the reverse current of the saturation of the VD diode, which constitutes a micronomer unit) occurs, the switching overvoltage occurs, and the potential of the positive clamping of the winding becomes much lower than the potential of the negative power supply pool. The separating Diode VD is under the influence of the return voltage exceeding the maximum allowable diode voltage of D229B.
Fig.P-1A - K3, K4 - Relay Winding, respectively, RP255 / 220, RP251 / 220; VD, VD1 - diodes D229B; VD1, R - protective chain
2. Fig. P-2a. VD1, VD2 diodes are exposed to switching overvoltage with a closed position of contacts K1 and opening contacts K2, since it turns off almost all the current in the winding of the K6 relay, and the potential of its positive clamp turns out to be much lower than the potential of the negative pole.
Fig.P-2 - K3, K4, K5 - RP252-U4 / 220 relay winding; K6 - RPU-2/220 relay winding; VD1-VD6 - diodes D229B; VD5, R4 is a sparking circuit; VD6, R5 - protective chain
3. Fig. P-3A. When the current is turned off in the winding of the K7 relay, K2 contacts when contacts K1 are in a closed position, the transient process occurs similarly to the above. Switching overvoltage affects VD1, VD2 diodes.
Fig. P-3 - K3 - Winding of the index relay; K4, K5, K6 winding relay RP252-U4 / 220, K7 - RPU-2/220 relay winding; VD1-VD6 - diodes D229B; R1, R2 - resistors, respectively, 3000 and 2000 ohms; VD5, R6 is a brawling contour; VD6, R7 - protective chain; SX - Overlay
4. Fig. P-4. In this scheme, separating diodes are not exposed to switching overvoltages.
Fig. P-4 - K3, K4 - winding of indicative relays; K5 - serial winding of the intermediate relay; K6, K7 RP222-U4 / 220 relay winding; VD1, VD2 - diodes D229B; R is a 1000 ohm resistor;
5. Fig. P-5A. Diode-resistor chains attached parallel to the relay windings (see also Fig. P-2A, P-3A) and intended to reduce spurs on contacts, to some extent limit switching overvoltage on separating diodes. Use in these chains of two, instead of one, successively connected diodes with parallel resistors attached to them (serving for uniform distribution of reverse voltage by diodes) is undertaken with the purpose of preventing the breakdown of diodes of these chains from the effects of overvoltages.
However, the possibility of the effect of switching overvoltage on diodes-resistor chains in the Scheme of Fig-5a (as well as in P-2A, P-3A schemes) is excluded (it is assumed that overvoltages cannot also get into the Scheme of Fig. P-5A from the source Power). Therefore, all these comparatively complex chains are advisable to replace on the chains of the diode resistor (Fig. P-2B, P-3B, P-5B). Moreover, with a slight probability of the breaking chain of separating diodes, it is possible to apply instead of three one common chain a diode resistor by attaching it parallel to the winding of the K8 relay (Fig. P-5V).
The overall protective chain of the diode resistor, along with a decrease in the level of switching overvoltages acting on the VD1-VD4 separating diodes, contribute to reducing sparking on contacts.
Fig. P-5 - K4, K5 - RP223 / 220 relay winding; K6, K7, K8 - Relay winding RP23 / 220; VD1-VD14 - D229B diodes; R1 is a resistor of 1000 ohms;
7. Choosing a protective chain
Recommended in guidelines for the use of a protective chain of a diode resistor and RC chain are equivalent from the point of view of their protective properties (RC-chain is less effective when the condenser is not pre-charged). Choose a chain of a diode resistor as having smaller dimensions.
8. Select the parameters of protective chains
8.1 Choosing diodes
The diodes of protective chains are chosen on the basis of the condition:
E.< 0,7*Uдоп. (5)
Considering that E \u003d 220 V, choose a diode type D229B, which has udop \u003d 400V.
8.2 Selection of resistors
The resistor resistance values \u200b\u200bare determined by the curves in Fig. 4 and correspond to the intersection point of the curve UM \u003d F (Rp) with a straight 0.7 * udop.-e \u003d 0.7 * 400-220 \u003d 60V parallel to the RR axis.
In the schemes shown in Fig. P-1B, P-2B, P-3B of resistivity resistor of the protective chain is determined by the curves for the RP-251 relay, RPU-2 and, respectively, r \u003d 2.4 com, R5 \u003d 4.2 com , R7 \u003d 4.2 com.
Calculated for the circuit in Fig. P-5B is the case of turning off the contacts K3 three parallel to the connected windings of the relay K6, K7, K8 with a closed position of contacts K1. In this case, if there is no protective chain in the scheme in Fig. P-5V, then the VD1 diodes, Vd2 are exposed to switching overvoltage. The resistance of the protective chain resistor is defined as equivalent to three parallel to the resistance, one of which (RR) is determined by the curve Fig.4 for the RP-23 relay:
R2 \u003d RR / 3 \u003d 2.2 / 3 \u003d 0.773
In the scheme shown in Fig. P-5B, the consideration of the issue of the possibility of activating the K8 relay when opening contacts K2 is deserved. The answer to this question in the case under consideration can be obtained by comparing the maximum value of the current passing, and the winding of the K8 relay in the transition mode, with the minimum current of the operation of this relay. Current I passing in the winding of the K8 relay when opening contacts K2, it consists of current I1 representing a portion of the current sums in the windings of the K4 relay, K5 and current I2 - part of the current sums in the windings of the K6 relay, K7. The maximum values \u200b\u200bof currents I1, I2, I are defined as follows:
Here: IK4, IK5, IK6, IK7 - Currents, respectively, in the windings of the relay K4, K5, K6, K7.
- 220 - power supply voltage (B);
- 9300, 9250 - resistance to DC-23, respectively, the winding of the RP-23 relay and consistently connected to the email resistor of the RP-223 relay (OM).
The minimum switching current of the relay K8 (RP-23):
Thus, the value of the current passing in the winding of the K8 relay when opening contacts K2 is insufficient to trigger the relay (if IM\u003e ISR.K8, then the relay K8 will work when performing the condition
tB\u003e TSR, where:
- tSR - time, during which II\u003e ISR.K8;
- tB is the response time of the relay K8.
9 References:
- Fedorov Yu.K., Analysis of the effectiveness of the protection of semiconductor devices from switching overvoltages in the DC circuits of relay protection and automation, "Electrical stations", No. 7, 1977
- Handbook of semiconductor diodes, transistors and integrated circuits. Under the general ed. N.N. Goryunova, 1972
- Fedorov Yu.K., overvoltage in the illuminated disconnection of inductive chains of DC in relay protection and automation systems, "Electrical stations", №2, 1973
- Alekseev V.S., Varganov G.P., Panfilov B.I., Rosenblum R.Z., Protection relay, ed. "Energy", M., 1976
