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Old 19th Jul 2021, 7:17 pm   #2
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Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 12,173
Default Re: VTVM - AC range capacitor voltage rating?

Originally Posted by 6SN7WGTB View Post
Am restoring a VTVM and the AC range input cap has a rating marked 1000 WVDC. It appears original - plus age and make appear to match all others in the unit.

The VTVM has a maximum input range of 1500 V AC or DC. The 1500 AC is specified as RMS, or 4200 p-p, as you'd expect.

As I understand it, DC rating of a capacitor exceeds the AC rating by definition of the RMS factor of root 2.

Sorry if I'm being a bit dim here, but what makes a 1kV DC cap suited to this 1500V AC application?

The VTVM has a tube rectifier on the AC range.

Thank you for your help.
The voltage across the capacitor will depend on its reactance and the current flowing via it, that will depend on the circuit
impedance, the circuit for the most part will be a series circuit comprising the capacitor and the AC range resistors, once the circuit impedance is known the current flow through the circuit can be calculated by dividing the voltage applied to the circuit by the circuit impedance, once that's known the voltage across the capacitor can be calculated by multiplying its reactance by the current.

As an example, I'm looking at the schematic for the Heathkit V7-A VTVM, the input capacitor on AC is 0.01uF and the total series resistance of the AC range resistors that the capacitor connects to is 1.37 meg, ignoring the effect of the rectification diode for the moment then bunging those figures into a RC series impedance calculator at a frequency of 50Hz gives a circuit impedance of 1.406 meg (approx.) the highest AC range is 1500 Volts therefore the circuit current at that voltage is V/Z which is 0.00106 amps (approx.) therefore the voltage across the capacitor is Xc (318.3k approx) multiplied by 0.00106 amps which equals 337 volts (approx.) the capacitor rating is given as 1,600 volts, that so far as I know will the DC rating.

Here's a calculator for the impedance of C and R in series that you can try:


Last edited by ms660; 19th Jul 2021 at 7:32 pm. Reason: extra info
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