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Old 19th Jul 2021, 4:18 pm   #1
6SN7WGTB
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Default 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 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.
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Old 19th Jul 2021, 7:17 pm   #2
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Default Re: VTVM - AC range capacitor voltage rating?

Quote:
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:

https://www.translatorscafe.com/unit...-rc-impedance/

Lawrence.

Last edited by ms660; 19th Jul 2021 at 7:32 pm. Reason: extra info
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Old 19th Jul 2021, 7:57 pm   #3
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Default Re: VTVM - AC range capacitor voltage rating?

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Originally Posted by ms660 View Post
V7-A .
V-7A

Lawrence.
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Old 20th Jul 2021, 8:53 am   #4
6SN7WGTB
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Default 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 V-7A 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.
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Old 20th Jul 2021, 9:35 am   #5
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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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Old 20th Jul 2021, 9:43 am   #6
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Default Re: VTVM - AC range capacitor voltage rating?

Quote:
Originally Posted by 6SN7WGTB View Post
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 V-7A have a dedicated HV ladder up front of the rectifier, so does that undo your calculations? Meaning that they should look more like mine?)
The AC input circuit on the highest AC voltage range (1,500V) in the Heathkit V-7A primarily consists of a series circuit comprising of a 0.01uF capacitor in series with a 900k resistor, a 320k resistor and a 150k resistor, the feed to the diode rectifier is taken from across the 150k resistor and as I said in my 1st post the effect of the diode circuit was ignored, the reason being that it wouldn't have greatly affected the outcome of the calculation, the calculations I did seem to be correct to me, not sure about your reactance being given in milliohms, here's a screen shot showing the calculation from my figures.

Lawrence.
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Old 20th Jul 2021, 10:32 am   #7
6SN7WGTB
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Default 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.
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Old 20th Jul 2021, 11:05 am   #8
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Default Re: VTVM - AC range capacitor voltage rating?

Quote:
Originally Posted by 6SN7WGTB View Post
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.
EDIT: Kalee20 - I'm not likely to measure such a voltage, but I would not want the VTVM to be incorrectly restored.

As an aside, one thing my AVO Test Set No. 1 was 'useful' for was measuring the 2.5kV on one of those fly-swatters 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.
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Old 20th Jul 2021, 11:52 am   #9
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Default Re: VTVM - AC range capacitor voltage rating?

I meant to add, for those interested, the VTVM is a Taylor 172A.
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