Dummy load for HV supply

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
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Rich Feldman
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Re: Dummy load for HV supply

Post by Rich Feldman » Tue Oct 06, 2015 5:19 am

Paul, let me suggest a different tack.

A. Your OP says "starting at 2.5 megohms", and later you say things like "per leg". Are you planning to make a 2.5-MΩ 1000-watt string for use alone, to get 20 mA at 50 kV? To be followed by identical strings in series for lower current, or in parallel for full current at lower voltage? The resistors will be wastefully under-loaded, power-wise, in anything but the just-one-by-itself case.

Better to build the max configuration as a series-parallel combination of identical sub-strings, which can be reconfigured when you want overall R values higher or lower than 2.5 M. The simplest example is four strings, each 2.5-MΩ 250-W (max 10 mA, max 25 kV).

B. Do you have student labor and want to teach soldering skills? Think about using lower power resistors in large numbers -- they cost less per watt. At Digikey, 2 watt 500-volt Yageo FMP-200's are in stock for $0.02272 each if you buy a whole reel of 2,500 resistors ($56.80 plus shipping). Staying within the ratings, you can meet the OP requirement with 525 resistors ($11.93 worth).

The 120kΩ value has a good balance between max power and max voltage rating. Let's design for 4 mA, 480 V, 1.92 W per resistor. A series string of 105 resistors would draw 4 mA at 50.4 kV. Exactly divisible by 3, 5, or 7 if you want to build it in sections. Five of those strings would draw 20 mA at 50.4 kV.

You can find creative ways to spread out the resistors for good cooling and high-voltage withstanding. Blowing air over them with a fan can give you lots of power margin. I am not sure oil immersion gives any benefit that outweighs the hassle.
Last edited by Rich Feldman on Tue Oct 06, 2015 6:30 am, edited 1 time in total.
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Paul W Fontana
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Re: Dummy load for HV supply

Post by Paul W Fontana » Tue Oct 06, 2015 5:34 am

@Hank Ball: Good points. I did find a data sheet for it (http://www.newark.com/vishay-dale/rh050 ... dp/41K9147) that says it's rated at 2000 Vrms, and I don't think in normal use they'd ever exceed that (across each), so it would just be a matter of keeping them physically isolated. I hadn't thought about heat sinking them, though. The data sheet says without heat sink the dissipation is devalued to 40%, or 20W. I wonder if forced air (which I was planning to use) would get some of that back.

But maybe I should get these cheaper 20W ceramic cement ones instead: http://www.ebay.com/itm/5pcs-Ceramic-Ce ... 5adf0b3270. I could push 14mA through without violating the power rating, though I'd still like to test the supply out up to 20mA if I can get there. That would require them to dissipate 40W. OK for short periods of time?

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Paul W Fontana
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Re: Dummy load for HV supply

Post by Paul W Fontana » Tue Oct 06, 2015 4:13 pm

Rich Feldman wrote:taying within the ratings, you can meet the OP requirement with 525 resistors
I like your design idea, but the thought of soldering and mounting 525 resistors in this network gives me hives, esp. since the dummy load is really just an accessory and not the main event. I wouldn't want to ask the students to do it for a 10-week class either - maybe if they were working on it as a summer research project, or if I had a couple dozen of them working on it, but they can't afford to invest that much time for the advanced lab when I'd rather they get to work trying out configurations of the power supply.

However, you have convinced me of the wisdom of (4) strings, each 2.5Mohm, 250W. I was considering maybe 3 strings, with at least one at 1000W, so it could be used by itself, but also get other resistances by combinations in series and parallel, but you're right - for the sake of an extra string I can drop the power requirements and the cost enormously. So now I'm thinking (4) strings of (25) 100 kohm, 10W ceramics like these:
http://www.ebay.com/itm/151744883697?_t ... EBIDX%3AIT

100 resistors to solder and mount is worse than 25 but a lot better than 525, it gives a lot of flexibility, and ends up being quite reasonably priced. And I don't have to deal with metal cases or heat sinks. Thanks for the suggestions!

-- pwf

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Werner Engel
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Re: Dummy load for HV supply

Post by Werner Engel » Tue Oct 06, 2015 5:57 pm

Hank, you are right!

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Rich Feldman
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Re: Dummy load for HV supply

Post by Rich Feldman » Fri Oct 16, 2015 11:05 pm

I just happened to connect some dots in a way that makes an old ebay prank more sensible.

Paul pointed to some nameless ceramic cement power resistors on ebay. U.S. component distributors like Mouser and Digikey offer familiar brands like Ohmite and Vishay. Turns out there's a Japanese maker called Fukushima Futaba Electric Co. Ebay always has resistors for sale under Fukushima title, bearing the Fukushima logo as seen on company website http://www.fu-futaba.co.jp/english/prod ... up_01.html and this image example:
fur2.JPG
That helps to explain this ebay offering that I captured in 2012, not long after a famous tsunami event in Japan.
ufr.JPG
Can anyone think of a reason to seek resistors of type "Film, Uranium" from FUkushima or FUtaba for a FUsor project?
All models are wrong; some models are useful. -- George Box

CandraLevy
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Re: Dummy load for HV supply

Post by CandraLevy » Mon Dec 21, 2015 5:47 pm

As per my experience Fusor power supplies are generally lethal. Demo fusor's using NST transformers can be relatively safe when used by someone familiar with high voltage.
As for a test load for a high voltage, high current power supply, a torr range gas is an excellent ballast if one is in the right range.
Many lasers operate at 1 - 20 torr with 30 kV supplies and argon is an excellent gas.

Andrew Haynes
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Re: Dummy load for HV supply

Post by Andrew Haynes » Mon Dec 21, 2015 10:48 pm

I think resistor would be out, at V2/R and Pr*r its about 1kv per resistor, at the first foumla you will be looking at 1-5 watt for 50% tollernce, making the price $300-500, a switch and inductor might be better
Andrew Haynes

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Paul W Fontana
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Re: Dummy load for HV supply

Post by Paul W Fontana » Thu Jan 28, 2016 10:46 pm

@Andrew You're right, 1 kV per resistor max, so I would need 50 per string to go all the way up to 50 kV. (I think the tolerance would probably be in my favor, though - I could probably push the heat dissipation higher for short times.)

I just bought these instead:
http://www.ebay.com/itm/25pcs-Ceramic-C ... V#shpCntId

110 kOhm at 20W, so voltage drop of up to almost 1500V across each. That's the highest P*R I could find, and they're less than $1/ea. With a single string of 30 I can go up to 44kV without pushing the rating. That will at least let me test a load of up to 13 mA; for a bigger load I can add a second string, but that should be plenty to start with. Two of these resistors in parallel will also make a nice ballast resistor for the fusor and the other glow discharge tubes.

Now the challenge will be mechanical - I need to build a mount that will let them dissipate all that heat without starting a fire, and connect them in a way that doesn't produce corona.

-- pwf

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Re: Dummy load for HV supply

Post by John Futter » Fri Jan 29, 2016 2:51 am

OK I'll post a few pics of how I make the ballast and divider resistor arrays @ work
The blue resistors are VR68 and are arranged in a spiral picking up each accel plate to set the plate voltage. 120kV total approx 15kV between each plate these resistors are rated @ 10kV each and are cheap by the boxful off digikey about $180 for 500
The metal clad (gold) are RS 25watt 10ks as Ballast for 20kV working note also the array of 26 100 V transil diodes that creates the anode cathode differential
The two big green are WW 30K each ballast for the accel tube these are used @ 120kV hence the corona rings
The group of four green ones are WW 7.5K ohm each all in series these are run up to 40kV note the remoted Glassman supply that floats @ 40kV
DSCN1561.JPG
DSCN1562.JPG
DSCN1564.JPG
DSCN1565.JPG

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Rich Feldman
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Re: Dummy load for HV supply

Post by Rich Feldman » Fri Jan 29, 2016 4:59 pm

Thanks for the story & pictures, John.
By coincidence, this week I chose & ordered some resistors for a HV load spec'd about like Paul's: 2-3 megohms, 200-300 watts. Am looking forward to comparing notes.

Andrew and Paul should know that resistors have model numbers, data sheets, and voltage ratings independent of the R value. In DC service, tolerable voltage is sqrt(P*R) or the voltage rating, whichever is less. For high resistance values, the voltage rating gets you first. Even sooner in AC or low duty cycle cases.

For example,
a 1 watt 10 kΩ resistor is power limited to 100V DC or RMS AC. The power rating allows 316 volts at 10% duty cycle, which is OK for most 1 watt series.
A 1 watt 1 MΩ resistor is power limited to 1000V DC or RMS AC, but most 1-W series have voltage ratings of 350 or 500 volts.
http://www.ohmite.com/cat/res_od_of_oa.pdf
Capture.PNG
Capture.PNG (9.34 KiB) Viewed 3944 times
John's stuff has some axial lead 1 watt R's with 10 kV rating, from a special series designed for high voltage. Don't know the voltage rating of R's like Paul's, but would guess 1500 V is OK. Try browsing similar-looking components at non hobby-oriented stores.
Last edited by Rich Feldman on Fri Jan 29, 2016 7:35 pm, edited 1 time in total.
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