Seeking spec on HV resistors
MRMILSTAR, Sun Dec 01 2019, 10:48PMDoes 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.
Re: Seeking spec on HV resistors
Patrick, Tue Dec 03 2019, 02:17AM
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.
Patrick, Tue Dec 03 2019, 02:17AM
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.
Re: Seeking spec on HV resistors
Adrenaline, Tue Dec 03 2019, 01:57PM
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?
Adrenaline, Tue Dec 03 2019, 01:57PM
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?
Re: Seeking spec on HV resistors
Sulaiman, Tue Dec 03 2019, 04:45PM
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.
Sulaiman, Tue Dec 03 2019, 04:45PM
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.
Re: Seeking spec on HV resistors
klugesmith, Wed Dec 04 2019, 01:43AM
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?
klugesmith, Wed Dec 04 2019, 01:43AM
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?
Re: Seeking spec on HV resistors
Patrick, Wed Dec 04 2019, 02:02AM
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.
Patrick, Wed Dec 04 2019, 02:02AM
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.
Re: Seeking spec on HV resistors
Plasma, Wed Dec 04 2019, 02:19AM
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.
Plasma, Wed Dec 04 2019, 02:19AM
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.
Re: Seeking spec on HV resistors
MRMILSTAR, Wed Dec 04 2019, 04:00AM
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.
MRMILSTAR, Wed Dec 04 2019, 04:00AM
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.
Re: Seeking spec on HV resistors
Patrick, Wed Dec 04 2019, 04:28AM
Patrick, Wed Dec 04 2019, 04:28AM
Plasma wrote ...
1W is meant to be 1kv...
i dont understand1W is meant to be 1kv...
Re: Seeking spec on HV resistors
MRMILSTAR, Wed Dec 04 2019, 03:42PM
MRMILSTAR, Wed Dec 04 2019, 03:42PM
Re: Seeking spec on HV resistors
Plasma, Wed Dec 04 2019, 05:55PM
Just read that ever Watt for a resistor is good for 1kV.
V = sqrt(P *R).
I think it might be a ball park rather than dead fixed.
Plasma, Wed Dec 04 2019, 05:55PM
Just read that ever Watt for a resistor is good for 1kV.
V = sqrt(P *R).
I think it might be a ball park rather than dead fixed.
Re: Seeking spec on HV resistors
Patrick, Wed Dec 04 2019, 10:06PM
power and a functional voltage limit are different... you can hit the voltage limit long before or after the watt limit would indicate by calculation. right ?
Patrick, Wed Dec 04 2019, 10:06PM
power and a functional voltage limit are different... you can hit the voltage limit long before or after the watt limit would indicate by calculation. right ?
Re: Seeking spec on HV resistors
2Spoons, Thu Dec 05 2019, 12:47AM
Voltage rating has almost nothing to do with power, and everything to do with construction. A long skinny resistor will handle more volts than a short fat one rated at the same power. Even choice of construction materials will affect voltage handling.
Short answer: for voltage handling physical size is a better indicator than power rating .
2Spoons, Thu Dec 05 2019, 12:47AM
Voltage rating has almost nothing to do with power, and everything to do with construction. A long skinny resistor will handle more volts than a short fat one rated at the same power. Even choice of construction materials will affect voltage handling.
Short answer: for voltage handling physical size is a better indicator than power rating .
Re: Seeking spec on HV resistors
Plasma, Thu Dec 05 2019, 01:09AM
Just saying what I read
Plasma, Thu Dec 05 2019, 01:09AM
Just saying what I read
Re: Seeking spec on HV resistors
2Spoons, Thu Dec 05 2019, 01:20AM
Thats just ohm's law, and not really relevant to the voltage rating of a resistor except at DC. Pulse handling is a different beast. Its entirely possible to have a 1Meg resistor that will handle a 15kV pulse, but only have a power rating of a watt (which would imply a limit of 1000V ).
2Spoons, Thu Dec 05 2019, 01:20AM
Thats just ohm's law, and not really relevant to the voltage rating of a resistor except at DC. Pulse handling is a different beast. Its entirely possible to have a 1Meg resistor that will handle a 15kV pulse, but only have a power rating of a watt (which would imply a limit of 1000V ).
Re: Seeking spec on HV resistors
klugesmith, Thu Dec 05 2019, 09:44PM
Plasma, the book page you cited might be confusing to noobs.
Right after the introduction of voltage rating, it mentions V = sqrt (P * R). Could be more clear immediately that putting P-max into that formula does not give you V-max, which is an independent limit in the data sheet.
Fortunately, it then mentions critical resistance value.
At foot of the page, it's beginning to say that for resistors whose R value is above critical, the V-max rule gets you before the P-max rule.
( Both rules apply to steady state operation, and have exceptions for pulses and small duty cycles. )
To beat more on the dead horse, and certainly not to say anything MrMilStar doesn't know:
Consider a series of 1/4 watt 200 volt resistors.
The first part does not mean all R values can dissipate 1/4 W, it means stay below 1/4 W to avoid overheating.
The second part does not mean all R values can withstand 200 V, it means stay below 200 V to avoid overvolting.
I agree with all that 2Spoons has said about material and construction details, which underlie the P and V ratings.
Some materials are more tolerant than others of high temperature.
Some resistive materials and dielectrics are more tolerant than others of high electric field strength, even at high temperature.
klugesmith, Thu Dec 05 2019, 09:44PM
Plasma, the book page you cited might be confusing to noobs.
Right after the introduction of voltage rating, it mentions V = sqrt (P * R). Could be more clear immediately that putting P-max into that formula does not give you V-max, which is an independent limit in the data sheet.
Fortunately, it then mentions critical resistance value.
At foot of the page, it's beginning to say that for resistors whose R value is above critical, the V-max rule gets you before the P-max rule.
( Both rules apply to steady state operation, and have exceptions for pulses and small duty cycles. )
To beat more on the dead horse, and certainly not to say anything MrMilStar doesn't know:
Consider a series of 1/4 watt 200 volt resistors.
The first part does not mean all R values can dissipate 1/4 W, it means stay below 1/4 W to avoid overheating.
The second part does not mean all R values can withstand 200 V, it means stay below 200 V to avoid overvolting.
I agree with all that 2Spoons has said about material and construction details, which underlie the P and V ratings.
Some materials are more tolerant than others of high temperature.
Some resistive materials and dielectrics are more tolerant than others of high electric field strength, even at high temperature.
Re: Seeking spec on HV resistors
Patrick, Fri Dec 06 2019, 07:24PM
Patrick, Fri Dec 06 2019, 07:24PM
2Spoons wrote ...
... Even choice of construction materials will affect voltage handling...
which is why we almost always see ruthenium film instead of carbon for HV.... Even choice of construction materials will affect voltage handling...
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