3 - Opening of a resistive circuit supplied with ACcurrent with a loadless rectifier The diagram is given in Figure 26. When the circuit suppliedwith AC current is opened, an overvoltage appears across
the rectifier terminals:- Ldi/dtThe energy stored by the inductance coil (1/2 L I
Applications
Three typical examples of applications are shown toillustrate the “protection” function of zinc oxide
varistors.1 - Protection of relay contacts
It is a well-known fact that a sudden break in an inductivecircuit causes an overvoltage which can seriously damage
the contacts of relay due to arcing. Overvoltages of several
thousand volts can occur across the terminals of unprotected
relay contacts. This disadvantage can be overcome by limit-
ing the overvoltage due to opening an inductive circuit to a
level such that it cannot generate an arc. Such limitation is
achieved by wiring a zinc oxide varistor in parallel across the
terminals of the relay characterized by the value of its induc-
tance coil L and its resistor R (Figure 24).
LR Figure 24
Figure 25
Figure 26 2 - Protection of a diode rectifier bridge
Semiconductor components (silicon diodes, thyristors, etc.)
are especially sensitive to transients and must be protected
so that the overvoltage value is limited to levels which are not
dangerous.An example of protection for a diode rectifier is schematical-ly represented in Figure 25. The varistor is connected to the
transformer secondary at the input of rectifier bridge.If the transformer’s magnetizing current is interrupted whenit reaches its maximum value, a voltage ten times greater
than the normal value can then appear at the terminals of the
secondary winding in the absence of a load.
This overvoltage, which is excessive for the semiconductors,is limited by the presence of the varistor which absorbs the
energy corresponding to the change of state of the primary
circuit.The same varistor can also protect the rectifier bridgeagainst overvoltages coming from the mains and reaching
the secondary circuit via the stray capacitance of the trans-
former.Another practical case to be considered involves
closing ofthe primary circuit. If the circuit is closed when the primary
voltage reaches its maximum value, the secondary voltage
can be two times greater than its steady-state value.
Although this case is less dangerous than the preceding
one, it still may cause damage to the rectifying diodes.
Connection of a varistor in parallel limits this overvoltage to a
value such that it does not cause any damage to the semi-
conductors.
2 rms) istransferred to the protective varistor wired in parallel to the
inductance coil.
L 9