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19th Jul 2021, 4:18 pm  #1 
Pentode
Join Date: Apr 2021
Location: Redhill, West Sussex, UK.
Posts: 106

VTVM  AC range capacitor voltage rating?
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 pp, 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. 
19th Jul 2021, 7:17 pm  #2  
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 11,628

Re: VTVM  AC range capacitor voltage rating?
Quote:
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 V7A 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: https://www.translatorscafe.com/unit...rcimpedance/ Lawrence. Last edited by ms660; 19th Jul 2021 at 7:32 pm. Reason: extra info 

19th Jul 2021, 7:57 pm  #3 
Dekatron
Join Date: Apr 2011
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Re: VTVM  AC range capacitor voltage rating?

20th Jul 2021, 8:53 am  #4 
Pentode
Join Date: Apr 2021
Location: Redhill, West Sussex, UK.
Posts: 106

Re: VTVM  AC range capacitor voltage rating?
Thanks Lawrence for taking the time to reply.
I’ve now got the full schematic which makes it clear. In the two HV AC ranges (500 and 1500) the circuit is as follows: 1. 0u1 input cap 2. A ladder of its own amounting to 388k total going to common 3. A takeoff from between 0u1 and 388k going to the rectifier tube going via a 47n 4. A full ladder ‘below’ the tube of about 30M So, Xc at say 50Hz is 32k (btw that calculator gave me milliohms instead of kilohms). The rectifier and 30M ladder has little parallel effect on the 388k, so the Z of the 0u1 and 388k is root sum squares = 389k. Current at 1500V thus ca. 4mA. Voltage across cap thus ca. 32/388 x 1500 = 141V or ca. 200 WVDC. However, I assume the theoretical edge case is if you apply DC to the AC input, I.e. frequency = zero Hz. Surely at this point the voltage drop approaches 1500V DC? So I’m still unconvinced about my cap being rated 1000 WV DC… (Also BTW doesn’t the V7A have a dedicated HV ladder up front of the rectifier, so does that undo your calculations? Meaning that they should look more like mine?) Last edited by 6SN7WGTB; 20th Jul 2021 at 9:02 am. 
20th Jul 2021, 9:35 am  #5 
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,923

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... 
20th Jul 2021, 9:43 am  #6  
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 11,628

Re: VTVM  AC range capacitor voltage rating?
Quote:
Lawrence. 

20th Jul 2021, 10:32 am  #7 
Pentode
Join Date: Apr 2021
Location: Redhill, West Sussex, UK.
Posts: 106

Re: VTVM  AC range capacitor voltage rating?
Thanks both.
Lawrence  my mistake  the freq was set to MHz... It correctly gives k when set to Hz. Kalee20  agree. As it happens I can get a modern Nichicon film cap of actually slightly smaller dimensions which is rated at 2k5 VDC. 
20th Jul 2021, 11:05 am  #8  
Pentode
Join Date: Apr 2021
Location: Redhill, West Sussex, UK.
Posts: 106

Re: VTVM  AC range capacitor voltage rating?
Quote:
As an aside, one thing my AVO Test Set No. 1 was 'useful' for was measuring the 2.5kV on one of those flyswatters as I worked on minor mods to 'uprate' it...! However, not too sure I'd be keen on working on a low impedance HV source. 

20th Jul 2021, 11:52 am  #9 
Pentode
Join Date: Apr 2021
Location: Redhill, West Sussex, UK.
Posts: 106

Re: VTVM  AC range capacitor voltage rating?
I meant to add, for those interested, the VTVM is a Taylor 172A.
