Re: Comark Early 1905 Insulation Meter Checks
I've plotted the way the voltage presented by the Comark varies as a function of the load, and I think the results are interesting if anyone is using this meter for testing high voltage caps.
Fig1 just shows how the applied voltage (or test voltage) fall in line with the expression VT = Range * (100 - D) /100 , where D is the % meter deflection from the RHS of the meter. There is no linear scale on the 1905, so those are "by eye" measurements, but the line looks good.
Fig2 shows how the voltage falls (the same behaviour applies to all ranges, but the 1000V range is shown) quickly when there's a significant load on test, or even the effect of connecting a "high impedance" DMM on to the 1905. I forced Excel to show a smooth line, and it has put a wiggle in there.
The bottom line here is that if you use a 1905 on the 250 or 500 volt range to test a HT capacitor, it may well be that you only have a fraction of the nominal voltage being applied, so the results need to be considered carefully.
B
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