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Old 2nd Apr 2019, 11:00 am   #1
G6Tanuki
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Default Voltage rating of resistors.

I'm in the process of building a high-voltage power supply, which as-is-traditional involves several chains of series-connected rectifier-diodes and big electrolytic capacitiors, which need to be equipped with the equally-traditional parallel voltage-equalising resistors.

Tradition says 470K 2-Watt, but the modern 2-watt carbon-film resistors are tiny compared with their old carbon-stick equivalents. Given that they may have 500V or so across them, are they really up to taking this? I'm sceptical, but what to do otherwise? Series-connect pairs of 220K ones?
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Old 2nd Apr 2019, 11:21 am   #2
GrimJosef
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Default Re: Voltage rating of resistors.

The resistor value should be low enough that the current down the resistor stack dominates over the leakage current through any of the capacitors. If the capacitors are very large they will tend to have more leakage (although decent modern components are pretty good in this regard). So stacks of great big caps tend to be shunted by lower value resistors (this also speeds up the bleed-down discharge after switch off, which is a handy secondary feature of these resistors).

Manufacturer's data sheets will give you the safe operating voltage for the resistors. It isn't hard to find 500V rated ones from reputable manufacturers. Above that they tend to become specialist HV components. Vishay PR03s https://www.vishay.com/docs/28729/pr010203.pdf are under 20mm long (body length) and rated for 750V.

Cheers,

GJ
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Old 2nd Apr 2019, 12:07 pm   #3
Craig Sawyers
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Default Re: Voltage rating of resistors.

Yup. Farnell 2614563 is the 470k ones of those (but a wide range of resistances are in large stock quantity). 47p each before VAT.

Power rating is in free air of course. They actually say >1mm from a circuit board.

Craig
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Old 2nd Apr 2019, 12:19 pm   #4
Neutrino
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Default Re: Voltage rating of resistors.

I used Vishay VR37 High Ohmic / High Voltage Resistors with a limiting voltage of 3500 V d.c. to replace defective voltage-equalising resistors in my Oscilloscope. They are 9 mm long with a diameter of 4 mm. Examples are in these links:

https://uk.rs-online.com/web/p/throu...stors/6835290/

https://uk.farnell.com/vishay/vr3700...ilm/dp/2614569

David

P.S. I used 2.2 M ohm which keeps well within the power rating at 500V.

Last edited by Neutrino; 2nd Apr 2019 at 12:26 pm. Reason: Power rating
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Old 2nd Apr 2019, 12:27 pm   #5
GrimJosef
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Default Re: Voltage rating of resistors.

Quote:
Originally Posted by Craig Sawyers View Post
Yup. Farnell 2614563 ... 47p each before VAT.
29p each if you can think of a reason for buying 10 of them.

Cheers,

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Old 2nd Apr 2019, 12:33 pm   #6
G6Tanuki
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Default Re: Voltage rating of resistors.

Cheers! I think you've found me precisely the answer I needed!
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Old 2nd Apr 2019, 12:43 pm   #7
GrimJosef
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Default Re: Voltage rating of resistors.

500V across 470k puts a milliamp or so down the resistor stack. As a rule of thumb I like that to be about ten times the maximum leakage through any of the caps. So as long as none of them is leaking more than about 100uA 470k should be fine for keeping the voltages across individual resistors within 10% of one another.

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Old 2nd Apr 2019, 12:47 pm   #8
Lucien Nunes
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Default Re: Voltage rating of resistors.

I also prefer 3W MF resistors for this application. Voltage rating of typical 2W CF and MF parts ranges from around 350V to 500V, but 3W gets you the option of 750V and a lower surface temp when dissipating significant power.

For repair work it is a pity that 3W carbon film resistors are now generally unavailable. The conventional beige finish and large dimensions allowed them to blend in unobtrusively amongst most kinds of post WW2 components, while being electrically suitable for more or less any application they would physically fit.
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Old 2nd Apr 2019, 1:25 pm   #9
MrBungle
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Default Re: Voltage rating of resistors.

Vishay VR37 here. VR68 if you need more watts.
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Old 2nd Apr 2019, 9:12 pm   #10
Philips210
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Default Re: Voltage rating of resistors.

Hi.

Agreed, the VR series metal glaze resistors are ideal for high voltage circuits. They were previously a Philips/Mullard part. They're easily recognised by their light blue colour body. VR25 (0.25W), VR37 (0.5W) and VR68 (1W). An odd feature is the 5% tolerance band, normally gold, is actually a yellow coloured band with this series of resistors.

They're excellent for use in TV line output stages and focus control circuits to replace carbon composition types. They're a very safe resistor and are often employed in switch mode power supplies as well.

Regards,
Symon.
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Old 3rd Apr 2019, 10:19 am   #11
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Default Re: Voltage rating of resistors.

Quote:
Originally Posted by MrBungle View Post
Vishay VR37 here. VR68 if you need more watts.
And here. And they are available in the tens of megohms too!
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Old 5th Apr 2019, 9:47 am   #12
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Default Re: Voltage rating of resistors.

You have 500V electrolytics?

What safety margin are you applying for each cap, and what voltage are you applying to each string of caps, and then to the total stack?
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Old 5th Apr 2019, 6:50 pm   #13
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Default Re: Voltage rating of resistors.

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Originally Posted by trobbins View Post
You have 500V electrolytics?

What safety margin are you applying for each cap, and what voltage are you applying to each string of caps, and then to the total stack?
500V electrolytics? Yes - standard COTS mainstream components, Vishay MAL215759101E3 is what I'll be using, available from Mouser, RS, Digikey, Farnell etc on next-day delivery....

[The application is a voltage-doubler fed from a 350-0-350V transformer with the centre-tap ignored. I'll be using a pair of said capacitors paralleled, then two such pairs series-connected, in each 'half of the 'staircase'].

It needs to deliver 1500VDC unloaded; with my proposed design there will be four 500V-rated electrolytics in series across the '1500VDC' output. So the capacitors are not being run to their rated-voltage.

That's a good safety-margin to me. If they fail, I will send them back and complain bitterly!
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