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Old 09-22-2017, 02:06 PM
Findm-Keepm's Avatar
Findm-Keepm Findm-Keepm is offline
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Join Date: Jun 2007
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Save your old Varistor if possible - I'd like to have it, to investigate the failure curve.

When Doug did his experiment(s), I took to my own Zenith Varistor stash to see if I had any that had failed. Of 9 unused/NOS, all 9 were good. Of 7 pulls from various chassis, and some identified only by comparing the body color markings to NOS ones, I was able to see only one failure - a beige/purple striped Varistor that was open - no response all the way to 1KV.

Silicon Carbide Varistors (what Zenith and other TVs used) have a different V/I curve than the semiconductor MOVs Doug used. The SiC varistors all have a linear V/I curve, and like MOVs, they are rated by "some voltage at some current" with the most prevalent current being 1mA. So if a SiC varistor is rated at 400V @1mA, applying 400V DC to the varistor in a closed circuit will yield 1mA of current flowing. Some Zenith varistors are rated to 950V, not a problem for me to measure with a Fluke 332D 0-1000VDC (okay, only 999.999V) voltage standard with a current compliance to 20mA, plenty of room for varistor testing.

About 4 years ago, I got a Zenith 800-600 Zenith Varistor/Thermistor kit with 90% of the varistors being new, unused. All had good V/I measurements. My workman stash got raided as well, with only one (used, solder on leads) varistor being w-a-a-y out, topping at 560V for a 410V varistor.

If you wanna try an original, lemme know what you have, and I'll send you a NOS one, free, if I have one - and verified good.
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