HV Divider

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Cristiano_Machado
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HV Divider

Post by Cristiano_Machado » Thu Jan 03, 2019 7:57 pm

Hello,

I read the FAQ "Measuring Fusor Voltage Part III - HV Divider" viewtopic.php?f=29&t=8841 and one of the recommendations is to use an array of 20 x 5 meg 1 watt resistors to make a 100 meg resistor.

I would like to know if it is possible to use an array of 10 x 10 meg ohms 1 watt resistors? That means each resistor will have 1.000 Volts across it. Will it breaking down or arcing?

Thanks,

Cristiano

Cristiano_Machado
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Re: HV Divider

Post by Cristiano_Machado » Thu Jan 03, 2019 7:59 pm

Please fell free to transfer/erase if this question belongs to "New User Chat Area".

Regards,

Cristiano

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Rich Feldman
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Re: HV Divider

Post by Rich Feldman » Fri Jan 04, 2019 4:11 am

It's good that you know about resistor voltage limits, that apply independently of the power limits.

Many "1 watt" resistors have formal voltage ratings of only 500 volts (and many 1/2 watt resistors say 350 volts).
These voltage limits are softer than those for capacitors and semiconductor devices.
They're not about breakdown or arc-over, as much as long term stability of R value.

Here's a snip from one datasheet:
cff.JPG
Richard Feldman

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Dennis P Brown
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Re: HV Divider

Post by Dennis P Brown » Fri Jan 04, 2019 10:22 am

A great post, Mr. Feldman; I didn't know that.

As for voltage dividers, do realize that very high meg resistors designed for high voltages are available rather cheap on ebay. They are much longer than most similar resistors but often, a bit short for real use. For this reason I place them under oil. Besides preventing arc over, this eliminates corona issues as well.

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Re: HV Divider

Post by Rex Allers » Fri Jan 04, 2019 11:26 am

If it was me, I'd stalk ebay looking for some high value, high voltage resistors, made for something like this. I like something in the 100M to 1G range and voltage rated for 10-30kv. That should be easier and more stable than a bunch of lower value R's. I've gotten a number this way over the last several years at <$10 each (often <$5 ea).

I like Ohmite Mox resistors if I can find something suitable. You should be able to download datasheets to get an idea what they are and look like. There are several flavors of Mox resistors...
[slim, mini, maxi]-mox

Hopefully you understand how to calculate the divider chain for some practical division ratio. Need ohms law and series/parallel resistance calcs. Also figuring out the minimum wattage of resistors needed for the voltage and resistance you are using.

Searching for something right on ebay is hard with so many resistors on there. Obviously one search term is 'resistor', then might add hv, meg, kv. I've also searched for particular voltage ratings like 10kv, 20kv, 30kv. It's not easy to find the good stuff.

Knowing that the Ohmite mox families are good, I just searched for 'resistor mox'. One reasonable result was:
"Ohmite 208 300m MEG Slim-Mox 1% Resistor Thick Film / SM208023006F" at $6.69 and "ships to worldwide"
(There are a number with US-only shipping but that wouldn't work for you.)

Looking that P/N up in an Ohmite datasheet it's: Slim-Mox 300Meg 5W 20KV

For a 40-50KV source, I'd use at least 3 of these in series in the divider.

So, just giving you an idea of another way to go.
It's a bit tedious to find the good stuff. Some of the hits I saw were very expensive, like >$20 each, and finding good search terms is hard. There may be a lot of China new parts. I bought some lower value power ceramics a while back. The slightest mechanical stress broke the wires from the resistor body.
I also bought some high voltage R's from Ukraine and they seemed OK.
Rex Allers

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Re: HV Divider

Post by John Futter » Sat Jan 05, 2019 2:10 am

I would buy a box of VR68 5M1 series resistors from Vishay available from digikey etc
$0.28 each a box = 500 so about US$150 ish a box share with another member
each resistor is designed to cope with 10kV but this may be less due to actual dissipation limit of 1 watt --you work it out

Cristiano_Machado
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Re: HV Divider

Post by Cristiano_Machado » Sun Jan 06, 2019 12:48 am

Rich, Dennis, Rex and John,

Thank you for all the advises. It really helped.

I will look for an appropriate resistor. Unhappily, here in Brazil it is not so easy to find parts and the import tax are very high.

Regards,

Cristiano

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Dennis P Brown
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Re: HV Divider

Post by Dennis P Brown » Sun Jan 06, 2019 11:42 am

Using lower voltage resistors is fine if you create a long enough string and place them in oil (to prevent arc over and corona issues. Synthetic motor oil is very good for this purpose. The oil submersion makes existing solder joint issues a lot less of a problem.) Using a glass tube to hold the resistor string and contain the oil inside is simple, and further prevents arcing.

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Rich Feldman
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Re: HV Divider

Post by Rich Feldman » Sat Jan 19, 2019 1:38 am

Just got a minor disappointment about resistor working voltage ratings, while making a proportional counter tube bias supply.
This started with a 0-100 microamp meter, whose scale card is now marked for 0-3000 volts.
kolo2.JPG
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So the external resistance needs to be 30 megohms. I made it out of ten 3.01 M resistors. They were handy, and the voltage loading is 300 V per resistor. Even 1 M would be above the critical resistance value for all sizes of ordinary axial-lead resistors. That means the package voltage limit is more restrictive than the package power limit.

The resistors are what I'm used to calling 1/4 watt size. Marked with their ohms value and RN55D, a popular mil-style identifier for standard electrical and package specs. RN55D's can be side-stacked on 0.1 inch centers -- nominal body diameter 0.09 inches, length 0.24 or 0.25 inches. Here set up for a calibration measurement (initial readings 1% low, so 10th R should be replaced with about 2.7 MΩ)
kolo3.JPG
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Some Internet searching, to back up my voltage limit story, found the MIL-R-10509 standard. https://33audio.com/enter/data/M10509G.pdf
That's surprisingly conservative. RN55 is rated 1/8 W, 200 V at 70 degrees C. To get 1/4 W and 300 V, you need a RN60 (typ. body diameter 0.145 inches).
*Today's datasheet for CMF55/RN55 from Vishay Dale calls it 1/8 W, 200 V. https://www.mouser.com/datasheet/2/427/ ... 223788.pdf
*TT Electronics has a data row for RN55D saying "nominal size 1/4 W", but the voltage rating is still 200 V. https://www.mouser.com/datasheet/2/414/RN-1509383.pdf
*The green-hued table I posted early in this thread has a 1/4 W, 250 V resistor series very slightly bigger than my RN55D's.

I'm going to use this meter at 300 V per resistor and not lose any sleep about it. No mass production, high reliability, or life support requirement. The voltage ratings are so low that external arc-over is a non issue, and I would not consider the margins to be improved by oil immersion.
I bet fusor people, like Tesla coil people, all have friends who have gotten away with abusing electronic components. :-)

p.s. Chose not to use these little guys from the surplus store. I found data sheets for similar R's, with radial leads 0.4 inches apart, rated 1/2 watt and 4000 volts. Anybody recognize the part number in picture?
unkolo.JPG
Richard Feldman

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