“I’m a producer and arranger from Turkey and I’m very happy being in touch with you. But, I’m a bit confused when I read the Word For the Day “MOV” in the latest inSync weekly report. When you describe MOV you wrote that a varistor will lower its resistance as input voltage increases. I think there is a logic mistake about this. If resistance decreases input voltage can pass easily, which means voltage increases.”
I was hoping someone would bring this up. It’s encouraging to know people are paying attention out there. The WFTD as written is actually correct, but doesn’t really spell it out in enough detail to prevent readers from making erroneous assumptions (remember, we were defining MOV, not varistor there).
In the WFTD, notice it says that when the MOV resistance drops the over voltage gets shunted back to ground instead of getting passed on to your gear. From the looks of your message to us it appears as if you were seeing the MOV as a device that is in line with your gear, but it isn’t. This is a common misperception. It’s actually in parallel with it, meaning it is placed between hot and ground (or neutral depending upon the electrical system you use) on your electrical wiring. If it were in line, you’d be correct; it would have to increase its resistance to protect your equipment. However, being in parallel means it can lower its resistance when voltage goes up. When this happens the extra voltage (and current) gets shunted directly to ground instead of being passed on to your gear (electricity always takes the easiest path to ground). This is actually easier (read cheaper) to do and more effective than trying to prevent the excess voltage from entering a system through increased resistance, and is why they are so useful in TVS devices.