Metal Oxide Varistor (MOV)

Recently a viewer asked me whether I had opened the extension board with multiple sockets and spike protector and mentioned that they contain Metal Oxide Varistor (MOV).

Had a look at the reverse side of the extension board and there were just 6 star edged screws which seemed to be easy to remove. Started removing the screws one by one using a star edged screwdriver. It took me a few minutes to finish the job. Of course it would have been much faster with a mechanised device which most gig workers use.

As expected, there was a small printed circuit board on one side and the sockets with switches along the length of the extension board. Only the blue Metal Oxide Varistor (MOV) was visible on the under surface of the PCB. Other components including the light emitting diode indicators were mounted on the opposite side of the PCB. Tried to see if the PCB could be taken out easily. It seemed to be well fixed and I did not want to disturb it just see the other side of a functioning extension board!

Tilted the blue MOV at right angles so that I could see the other side of it. Could not see any inscriptions on either side to indicate the specifications, even on zooming in. Tilted it back to the original position and closed the extension board with screws.

This is a close up photograph of the Metal Oxide Varistor. It is a voltage dependent resistor that protects circuits from voltage spikes and surges by rapidly decreasing its resistance beyond a threshold voltage. MOV shunts away the excess current from the circuit. MOVs are composed of zinc oxide grains and has high resistance under normal voltages, permitting only a small leakage current. During a voltage surge, it shunts the extra current and absorbs the excess energy which is dissipated as heat. If the surge goes beyond the capacity of the MOV, an associated fuse in the circuit blows off, protecting from further damage to the electronic circuit. When the surge is not strong enough to blow off the fuse, the MOV returns to the basal state of high resistance as soon as the spike is over and does not shunt any current. Response time of MOV is very fast, in the range of 40 to 60 nanoseconds so that overvoltage surges are quickly suppressed.