Detailed description of varistor connection

It is well known that varistor is a parallel and effective protection circuit, and its function is equivalent to a Zener diode. Under normal conditions, the resistance is very large. Above a certain voltage value, the resistance becomes small, so that the high voltage cannot damage the protected circuit.

The varistor degrades its own performance after passing a large current, and the varistor is connected in parallel with the discharge tube to overcome this disadvantage. Before the discharge tube has not been turned on. The varistor starts to operate, clamps the transient overvoltage, and discharges the amplified current when the discharge tube is discharged. It will be shunted in parallel with the varistor, which reduces the through-flow pressure to the varistor, thereby shortening the time for the varistor to pass current and helping to slow down the performance degradation of the varistor. In this parallel combination.

If the reference voltage Uima of the varistor is selected too low, it is possible that the discharge tube will not discharge during the transient overvoltage action. The overvoltage is most fully vented by the varistor, which is detrimental to the varistor, so the value of Uima must be chosen to be larger than the DC discharge voltage of the discharge tube. Must be pointed out. This well combination circuit does not solve the problem of freewheeling that may occur in the discharge tube. It should not be used for the protection of AC power systems.

Another combination of varistor and discharge tube is in series, as shown in Figure 2. The varistor has a large parasitic capacitance, and when it is applied to the protection of an AC power system, it tends to generate a considerable leakage current under normal operating conditions. For example, a 2 nF varistor with a parasitic capacitance is installed in a 220V, 50hz AC power system with a leakage current of zero. 14 mA (rms), such a large leakage current will often affect the normal operation of the system. After connecting the varistor in series with the discharge tube.

Since the parasitic capacitance of the discharge tube is small, the total capacitance of the entire series branch can be reduced to a microfarad. In this series combination branch. The discharge tube acts as a switch. When there is no transient overvoltage, it can isolate the varistor from the system, leaving almost no leakage current in the varistor, which reduces the varistor reference voltage Uima. It is not necessary to take into account that this will cause an increase in leakage current, so that the degradation of the varistor performance can be effectively slowed down.

During the transient overvoltage, since the varistor reference voltage Uima can be chosen to be low, the series branch can give a lower clamping voltage than a single varistor as long as the discharge tube can be quickly discharged. In practical applications. It is often not an easy task to determine the parameters of the discharge tube and varistor. usually. For the protection of the AC power system, the DC discharge voltage of the discharge tube should be greater than the maximum operating voltage amplitude of the system to cut off the freewheeling of the glow region of the discharge tube when the system operating voltage crosses zero.

The varistor should be selected to ensure that the arc zone of the discharge tube is freewheeled. When the discharge vessel is turned on in the arc zone, the voltage across it is very low. Only about 20 V, the residual voltage of the entire series branch can be regarded as falling on the varistor, which can lead to a conservative approach. That is, the highest operating voltage of the system is considered to be falling on the varistor. At this time, the current in the varistor should be smaller than the freewheeling of the arc region of the discharge tube, so that the freewheeling of the arc region can be effectively cut off after the transient overvoltage has passed. In most instances. The critical value of this current can be conservatively taken to be about 50 mA.