The owner of a broken washing machine recently asked me to examine a damaged circuit board he had found inside. Using the schematic I was able to determine that the charred area formerly contained a device labeled MOV. I found several of these devices on the board and now gather they were Metal Oxide Varistors, which can be used for over voltage protection.
Considering this board appeared to be a low power supply of some sort (transformer, rectifier, transistor etc.) and also contained a blown 0.5A fuse, what was the most likely function of the blown MOV?
In general, what are MOVs used for in PCB designs? Real world circuit examples would be great.
Best Answer
A varistor after the fuse ensures that when the voltage crosses a certain value, the fuse blows and current flow stops. Generally fuses are rated for a current limitation, not a voltage limitation (as in your example). It is possible that the voltage difference across the fuse is such that it doesn't create more than the rated current through the fuse yet is still such that it could cause harm to harm the circuit (or is just unwanted). In that case the varistor is used to increase current through the fuse, causing it to blow and stop the current. When the voltage crosses the upper limit, the varistor resistance is reduced, increasing current through the fuse as in this circuit:
Many PCB circuits contain inductors and capacitors which will create transient states and surges (switching spikes). Too much of this kind of occurrence will harm the device, so varistors are used for protection.