Switching relay windings in circuits direct current relay protection and automation is usually accompanied by significant overvoltages, which can be dangerous for semiconductor devices used in these circuits. To protect transistors operating in the switching mode, protective circuits (Fig. 1) began to be used, which are connected in parallel with the winding of the switched relay (Fig. 2 - here the winding of the switched relay is represented by the equivalent circuit - inductance L, the active component of the resistance R and the resulting turn-to-turn capacitance C ) and reduce overvoltages that occur between winding terminals 1 and 2.

However, at present, sufficient attention is not being paid to determining the parameters of protective chains and assessing their impact on the operation of relay protection devices. In addition, in the development and design of relay protection devices using semiconductor diodes subject to switching overvoltages, protection of diodes in many cases is not provided.

This leads to a rather frequent failure of the diodes and the failure or incorrect operation of the device. An example of circuits where overvoltages can affect the diode is the circuit shown in Fig. 3. Here, the separating diode VD is under the influence of a switching overvoltage and can be damaged when the contacts KI open and the contacts K2 are closed. To protect this diode, a protective circuit must be connected to terminals 1 and 2 of the winding of relay K3. Diodes can be protected by the same protective equipment that is used to protect transistors (Fig. 1).

8.1 Selection of diodes

Protective circuit diodes are selected based on the condition:

E< 0,7*Uдоп. (5)

Considering that E=220 V, we choose a diode of the D229B type, which has Udop=400V.

8.2 Choice of resistors

The resistance values ​​of the resistor are determined using the curves in Fig. 4 and correspond to the point of intersection of the Um=f(Rp) curve with the straight line 0.7*Uadm.-E=0.7*400-220=60V, parallel to the Rp axis.

In the circuits shown in Fig. P-1b, P-2b, P-3b, the resistance of the protective circuit resistor is determined from the curves for the relay RP-251, RPU-2 and, accordingly, are equal to R=2.4 kOhm, R5=4.2 kOhm , R7=4.2 kOhm.

The calculation for the circuit in Fig. P-5c is the case of switching off by contacts K3 of three parallel-connected relay windings K6, K7, K8 with the closed position of contacts K1. In this case, if there is no protective circuit in the circuit in Fig. P-5c, then the diodes VD1, VD2 are exposed to a switching overvoltage. The resistance of the protective chain resistor is defined as equivalent to three equal resistances connected in parallel, one of which (Rp) is determined from the curve in Fig. 4 for the RP-23 relay:

R2 \u003d Rp / 3 \u003d 2.2 / 3 \u003d 0.773 kOhm

In the circuit shown in Fig. P-5c, consideration of the possibility of the K8 relay operation when the K2 contacts are opened deserves attention. The answer to this question in the case under consideration can be obtained by comparing the maximum value of the current passing through the winding of relay K8 in transient mode with the minimum operating current of this relay. The current I passing in the winding of the relay K8 when the contacts K2 open is the sum of the current I1, which is part of the sum of the currents in the windings of the relays K4, K5 and the current I2 - part of the sum of the currents in the windings of the relay K6, K7. the maximum values ​​of the currents I1, I2, I are determined as follows:

Here: Ik4, Ik5, Ik6, Ik7 - currents passing, respectively, in the windings of the relay K4, K5, K6, K7.

• 220 - power supply voltage (V);
• 9300, 9250 - DC resistance, respectively, of the RP-23 relay winding and the RP-223 relay winding connected in series with an additional resistor (Ohm).

Minimum actuation current of relay K8 (RP-23):

Thus, the amount of current passing in the winding of the relay K8 when the contacts K2 is opened is not enough to operate the relay (If Im > Iav.k8, then the relay K8 will operate when the condition
tb > tav, where:

• tav – time during which Im > Iav.k8;
• tb - relay K8 operation time.

9 References:

• 1. Fedorov Yu.K., Analysis of the effectiveness of the means of protecting semiconductor devices from switching overvoltages in DC circuits of relay protection and automation, "Electric Stations", No. 7, 1977
• 2. Handbook of semiconductor diodes, transistors and integrated circuits. Under the general editorship. N.N. Goryunova, 1972
• 3. Fedorov Yu.K., Overvoltage during arc-free shutdown of inductive DC circuits in relay protection and automation systems, "Electric Stations", No. 2, 1973
• 4. Alekseev V.S., Varganov G.P., Panfilov B.I., Rosenblum R.Z., Protection relay, ed. "Energy", M., 1976

It is used where it is undesirable or impossible to install an RC circuit in parallel with the relay contacts. The following approximate values ​​of the elements are offered for calculation:

C \u003d 0.5 ... 1 microfarad per 1 A of load current;

R = 50...100% of load resistance.

After calculating the ratings R and C, it is necessary to check the additional load of the relay contacts that occurs during the transient process (capacitor charging), as described above.

The R and C values ​​given are not optimal. If the most complete protection of contacts and the realization of the maximum resource of the relay are required, then it is necessary to conduct an experiment and experimentally select a resistor and a capacitor, observing transients using an oscilloscope.

Advantages of an RC circuit in parallel with the load:

good arc suppression, no leakage currents to the load through open relay contacts.

Flaws:

at a load current of more than 10 A, large capacitance values ​​lead to the need to install relatively expensive and large capacitors; experimental verification and selection of elements is desirable to optimize the circuit.

The photographs show the voltage waveforms on the inductive load at the moment of power disconnection without shunting (Fig. 33) and with the RC circuit installed (Fig. 34). Both waveforms have a vertical scale of 100 volts/div.

No special comment is required here, the effect of installing a spark-extinguishing circuit is immediately visible. The process of generating high-frequency high-voltage interference at the moment of opening the contacts is striking, we will return to this phenomenon when analyzing the EMC of the relay.

The photos are taken from a university report on optimizing RC circuits installed in parallel with relay contacts. The author of the report conducted a complex mathematical analysis of the behavior of an inductive load with an RC shunt, but in the end, the recommendations for the calculation of the elements were reduced to two formulas:

Figure 33
Turning off an inductive load causes a very complex transient

Figure 34
Correctly selected protective RC chain completely eliminates the transient process

where C is the capacitance of the RC circuit, microfarads, I is the operating current of the load. BUT;

R \u003d Eo / (10 * I * (1 + 50 / Eo))

where Eo is the voltage on the load. V, I - load operating current. A, R - resistance of the RC circuit, Ohm.

Answer: C \u003d 0.1 microfarads, R \u003d 20 ohms. These parameters are in excellent agreement with the nomogram given earlier.

In conclusion, let's get acquainted with the table from the same report, which shows the practically measured voltage and delay time for various spark-extinguishing circuits. The inductive load was electromagnetic relay with a coil voltage of 28 VDC/1 W, the spark-extinguishing circuit was installed in parallel with the relay coil.

) and today we will look at another fundamental element - namely capacitor. Also in this article, we will look at differentiating and integrating RC circuit.

Simplified, we can say that a capacitor is a resistor, but not an ordinary one, but a frequency-dependent one. And if in a resistor the current is proportional to the voltage, then in the capacitor the current is proportional not just to the voltage, but to the rate of its change. Capacitors are characterized by such a physical quantity as capacitance, which is measured in Farads. True, 1 farad is a hell of a lot of capacitance, usually capacitances are measured in nanofarads (nF), microfarads (uF), picofarads (pF), etc.

As in the article about resistors, let's first look at parallel and series connection of capacitors. And if we again compare the connections of capacitors with the connections of resistors, then everything is exactly the opposite)

Total capacity in case parallel connection capacitors will be equal to .

Total capacity in case serial connection capacitors will be like this:

With the connections of capacitors to each other, in principle, everything is clear, there is nothing special to explain, so let's move on 😉

If we write down the differential equation relating the current and voltage in this circuit, and then solve it, we will obtain an expression in accordance with which the capacitor is charged and discharged. I will not load unnecessary mathematics here, just look at the final result:

That is, the discharge and charge of the capacitor occurs according to an exponential law, look at the graphs:

As you can see, the value of time τ is separately noted here. Be sure to remember this value - this is the time constant of the RC circuit and it is equal to: τ \u003d R * C. The graphs, in principle, indicate how much the capacitor charges / discharges during this time, so we will not dwell on this again. By the way, there is a useful rule of thumb - in a time equal to five time constants of the RC circuit, the capacitor is charged or discharged by 99%, well, that is, we can assume that completely)

What does all this mean and what is the chip of capacitors?

And everything is simple, the fact is that if a constant voltage is applied to the capacitor, then it will simply charge and that's it, but if the applied voltage is variable, then everything will begin. The capacitor will then be discharged, then charged, respectively, current will run in the circuit. And as a result, we get an important conclusion - it easily flows through the capacitor alternating current, but the constant cannot. Therefore, one of the most important purposes of a capacitor is to separate the DC and AC components of the current in the circuit.

We figured it out, and now I'll tell you about differentiating and integrating RC circuits.

differentiatingRC chain.

The differentiating chain is also called a high-pass filter - a high-pass filter, its circuit is presented below:

As the name implies, yes, in fact, this can be seen from the scheme - RC circuit does not pass the constant component, and the variable calmly passes through the capacitor to the output. Again, the name hints that at the output we will receive the differential of the input function. Let's try to apply a rectangular signal to the input of the differentiating circuit and see what happens at the output:

When the input voltage does not change, the output is zero, since the differential is nothing more than the rate of change of the function. During voltage surges at the input, the derivative is large and we observe surges at the output. Everything is logical

And what should we submit to the input of this RC chains, if we want to get rectangular pulses at the output? That's right - sawtooth voltage. Since the saw consists of linear sections, each of which at the output will give us a constant level corresponding to the rate of change of voltage, then in aggregate the output differentiating RC chain we get rectangular pulses.

IntegratingRC chain.

Now it's time for the integrating chain. Also called a filter low frequencies. By analogy, it is easy to guess that the integrating circuit passes the constant component, and the variable goes through the capacitor and does not pass to the output. The schema looks like this:

If you remember a little mathematics and write down expressions for voltages and currents, it turns out that the output voltage is the integral of the input voltage. This is how the chain got its name.

So, we examined very important, albeit at first glance, simple schemes. It is important to immediately understand how it all works and why all this is needed at all, so that later, when solving specific problems, you can immediately see a suitable circuit solution. In general, see you soon in the following articles, if you have any questions, be sure to ask 😉