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Old 20th Jul 2021, 9:35 am   #5
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 6,250
Default Re: VTVM - AC range capacitor voltage rating?

It sounds as though this capacitor is a DC blocking capacitor for the AC ranges.

That being so, it will have only a small AC across it, but will have the full DC offset.

Your calculations... If I understand the circuit from your description, are OK. If you do indeed apply 1500V AC to the input, you'll get 141V across the capacitor (200V peak).

Incidentally, 1500V AC is a serious voltage - how often are you going to need to measure that?

Whatever! If you have a 1,000V DC rated capacitor, you'll be able to measure AC voltages riding on DC of up to 1,000V (such as ripple voltage across HT electrolytics)... and you can also be sure that the capacitor isn't going to die if you measure voltages of 1,500V AC according to what you have worked out. The combination of a high DC voltage with a high AC voltage riding on top, takes a bit more working out.

The fact that the VTVM is rated to 1,500V DC but the capacitor being rated to 1,000V DC does mean that if you switch the meter to AC and then connect to 1,500V DC, the capacitor will be overstressed., which doesn't sound good. It will probably survive, but we can't rely on it...
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