Surge Protection with a Varistor (MOV Surge Protection)
This is the metal oxide varistor rapidly dropping its resistance when the voltage across it exceeds a certain threshold, thereby absorbing the surge energy. It is a common layer of protection in panels supplied with 220V AC against short-duration voltage surges arising from lightning or from switching inductive loads.
At normal operating voltage the varistor is high-resistance and behaves as if it were not in the circuit. When a surge arrives its conductivity increases and, by passing the current through itself, it limits the voltage seen by the protected device. The energy turns into heat in the varistor; for this reason every surge leaves a certain amount of ageing behind.
- Selection: the maximum continuous operating voltage (MCOV) must be above the mains voltage, otherwise the varistor heats up during normal fluctuations. The energy and surge current ratings are chosen according to the application.
- End of life: an ageing varistor can heat up with leakage current and go to short circuit; for this reason modules with thermal disconnectors are used together with a protective device in series.
- Scope: a varistor limits the surge; it does not solve a sustained overvoltage.
The receiver of the SF100 and ST100 sets can be supplied with 12/24V DC or 220V AC; in mains-supplied panels this layer of protection is worth considering in terms of the life of the control electronics.
The context in which this term is used: Wireless Crane Remote Control Selection Guide