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Seeking spec on HV resistors

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LAN_403

MRMILSTAR

Sun Dec 01 2019, 10:48PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

Does anyone know the voltage rating of those red HV 100 mm resistors offered on E-Bay. They are rated for 10 watts but do not state the voltage rating. Here is a typical listing on E-Bay.

Patrick

Tue Dec 03 2019, 02:17AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I must admit the lack of voltage rating makes them useless.

edit: i would find another supplier, i wouldnt trust a seller who doesnt know how to list their products.
I looked at this "uxcell" it looks like clothes retailer and alot of random chinese stuff lumped together with some electronics stuff.

i got all mine either direct from EGB, Caddock or Dale or through real suppliers on ebay that i knew.

Adrenaline

Tue Dec 03 2019, 01:57PM

Registered Member #235 Joined: Wed Feb 22 2006, 04:59PM
Location:
Posts: 80

For continuous power dissipation, using ohms law, looks like 2200 volts. That voltage might break over if not in oil though.
Is that 10W dissipation in free air, or oil?

Sulaiman

Tue Dec 03 2019, 04:45PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Those resistors are large enough to dissipate 10W,
and they are certainly long enough to withstand several kV,
I use VR37 resistors for h.v. - 10mm long and rated for 3.5 kV ... in air.

So I would consider those resistors as 10W, for 500 kOhm that's 2.2 kV, dc or rms.
As the information given indicates that they are suitable for pulse usage,
they could probably handle 5 kV or maybe 10 kV pulses - provided that the average power is kept below 10W.

I personally de-rate resistors to about 1/2 nominal power rating for reduced temperature rise.

klugesmith

Wed Dec 04 2019, 01:43AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

As we know, each resistor part series has a power rating and a voltage rating. I think the voltage rating comes from internal material physics and construction, not on external flash-over, so oil immersion doesn't help if you want the part to serve for thousands of hours.

RMS voltage should not exceed sqrt(P_max * R), to avoid overheating,
and should not exceed V_max to avoid overvolting.

So for each series there's a critical R value, above which the voltage rating gets you first. As Patrick said, you can't infer the voltage rating from power rating or external creep distance, except by saying the unknown part looks just like a documented part.

OTOH, this forum has many readers who regularly abuse electronic components and come to talk about it. :)
Like Sulaiman said, the pictured R's are probably fine for pulse duty far above sqrt(5) kilovolts.

Often you can find good deals on ebay for parts like Ohmite Slim-Mox series.
I see 10 for $20, 2 megohm 108 series (2.5 W, 20 kV). Hey, that series is power limited for R values up to 160 megohms! Does your application need to dissipate lots of power as well as withstand lots of voltage?

Patrick

Wed Dec 04 2019, 02:02AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i have 100mm / 4in soviet resistors and 4 inch caddock resistors. Insulation from skin to another object is typically good for 1-3kv. you could buy them then destructively test one or two to see the breakdown value then divide by a safety factor of two or something. I always use oil above 20 kV for any components not just resistors.

I suspect you could reach 20 kv in air as with mine of similar length, but its a roll of the dice based on the maker. use round or ball bearing terminations at each end too.

Plasma

Wed Dec 04 2019, 02:19AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

I don't fully agree with Patrick, regards oil, but don't know the frequency?. 1W is meant to be 1kv, but its the current when it arcs that will flash over the resistors, basic rule set your final resistors to Voltage/ current of diodes, plus buffer. It will limit your final voltage, but you won't fry the diodes or resistors.

MRMILSTAR

Wed Dec 04 2019, 04:00AM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

My intended application is for a large Cockroft-Walton voltage multiplier that I am building It will have 14 stages driven by a flyback. I anticipate an output of 200 - 250 KV. I will use the resistors in a chain on the output to limit the current. Right now I am thinking about 30 Vishay VR68 (470K ohms) each rated for 10 KV and 1 watt. Everything will be operated in mineral oil.

Patrick

Wed Dec 04 2019, 04:28AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Plasma wrote ...

1W is meant to be 1kv...

i dont understand

MRMILSTAR

Wed Dec 04 2019, 03:42PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

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