In this condition the varistor is not visibly damaged and outwardly appears functional ( no catastrophic failure ), but it no longer offers protection.
12.
They will also include high energy overvoltage MOV ( Metal Oxide Varistor ) protection, and circuit over-current protection in the form of a Polyswitch.
13.
For example, the pulse energy rating is directly proportional to the volume of " varistor material " that you're heating with each energy pulse.
14.
The device logically belongs in the varistor family, and has the transconductance or transfer impedance of a device having gain, so that this combination is descriptive.
15.
Memristor resistance depends on the integral of the input applied to the terminals ( rather than on the instantaneous value of the input as in a varistor ).
16.
A catastrophic failure occurs from not successfully limiting a very large surge from an event like a lightning strike, where the energy involved is many orders of magnitude greater than the varistor can handle.
17.
The development of the varistor, form of a new type of rectifier ( copper oxide ), originated in the work by L . O . Grondahl and P . H . Geiger in 1927.
18.
As these pulses occur, the " clamping voltage " it provides during each event decreases, and a varistor is typically deemed to be functionally degraded when its " clamping voltage " has changed by 10 %.
19.
Instead, built in Metal Oxide Varistor ( MOV ) protection devices will clamp the voltage and fast overcurrent protection turns off semiconductor switches, so that the over current pulse can be safely absorbed by internal electrolytic capacitors.
20.
However, in general, such effects were undesirable and telephone designers suppressed them by mechanical means with off-normal switches on the dial, or greatly attenuated them by electrical means with a varistor connected across the receiver.
