Dummy load for HV supply

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Paul W Fontana
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Dummy load for HV supply

Post by Paul W Fontana » Tue Sep 22, 2015 5:08 pm

I think I'm going to have students build an HV supply in lab next quarter so we can all learn the ins and outs, and if it's successful it will be a big step towards a fusor. Tentatively designing for 0-50 kV at 0-20 mA. I'd like to be able to test it under load and give the students the opportunity to iterate on the design a bit before we run it through a plasma. Any tips or suggestions for building or buying a dummy load? I couldn't see anything on it in the FAQ's. It would need to dissipate up to 1 kW, hold off 50 kV, and provide load (ideally adjustable, though if that were in discrete steps it would be acceptable) up to 20mA, so resistance starting at 2.5 Mohm. Thanks for any input!

-- pwf

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Dennis P Brown
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Re: Dummy load for HV supply

Post by Dennis P Brown » Tue Sep 22, 2015 7:42 pm

The supply you just describe sounds lethal. Use by students? Fusor power supplies are generally lethal. Demo fusor's using NST transformers can be relatively safe (when used by someone familiar with high voltage) but require careful use to be safe and frankly, not something I'd let the average person use.
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 (and cheap). Yet no "load system/ballast" will render a lethal power supply safe in any manner. You want a very low current HV system like in the micro-amp range to be safe. (Obviously there are exceptions for HV/high current systems when they are very high frequency but that is beyond me to know when it reaches a safe freq. range for that voltage/current.)
Remember, a NST (under 15 kV) is current limited but even then, if someone is well grounded it could prove highly dangerous.

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Werner Engel
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Re: Dummy load for HV supply

Post by Werner Engel » Tue Sep 22, 2015 8:08 pm

Hi Paul!

I use 6x10k wire resistors with 300W. They can be overloaded for a few seconds up to 1kW - as long as they are cooled.
Works fine. As they are seperated, you can vary the numbers ...

But be aware of corona discharges - they start at about 15 kV!

BR,
Werner
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Richard Hull
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Re: Dummy load for HV supply

Post by Richard Hull » Tue Sep 22, 2015 8:25 pm

I have long spoken of and used high watt, high ohm resistors. These are currently a great rarity, yet were common in the 1930's through the 1960's. They can only be found inexpensively at Hamfests and other electronic surplus sales venues now.
Werner has the right idea. I have assembled a bank of 100 watt resistors that can be tapped into up to 1 megohm. most are 25k ohm to 63 kohm units of wattages from 100 to 225 watts they are all wire wound and hooked in series. I tend to find them in the price range from 50 cents to 5.00 each at hamfests. Even though I have enough already, I buy them whenever I encounter them now as they will never return.

I attach an image of one of my 1.5kw HV load boards. The large resistors are all 225 watts each and the smaller ones are 70 watt units.

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Re: Dummy load for HV supply

Post by prestonbarrows » Wed Sep 23, 2015 1:48 am

Glassman has an excellent write up about high voltage test loads on their site.
http://www.glassmanhv.com/TechNotes/Resistance.shtml

As others have mentioned, you generally want a large array of series-parallel resistors. The lower the voltage rating of the resistors and the higher the voltage output of your supply, the more resistors in series you will need. The lower the current rating of the resistors and the higher the current output of your supply, the more resistors in parallel you will need. You need to make sure the voltage across, the current through, and the power inside each resistor is within its rating.

Wire wound resistors are usually the best style for something like this. Ohmite is one of the few companies still making these things new, but the large power ratings are harder to find.

Going above 10-20kV or so will require special attention to the frame and wiring. All conductors must be rounded to avoid corona. The joints between resistors must have enough clearance between other joints and the frame to prevent arcing and flashover. The frame must also be able to handle the considerable waste heat being thrown off by the resistors.

50kV is doable, but will be quite an undertaking if you are not familiar with high voltage engineering. You absolutely must have an interlocked enclosure around this test load before you energize it; especially with students running around.

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Bob Reite
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Re: Dummy load for HV supply

Post by Bob Reite » Wed Sep 23, 2015 1:56 am

Believe it or not, you can still buy 100 watt 100K wirewound resistors. Ohmite still makes them. You will pay around $17.00 each. Need more power dissipation? 225 watt 100K resistors are still made too, for $30.00 each.
The more reactive the materials, the more spectacular the failures.
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Richard Hull
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Re: Dummy load for HV supply

Post by Richard Hull » Wed Sep 23, 2015 6:33 pm

I knew Ohmite was still making the high watt wire wounds, but the prices are rather prohibitive compared to hamfest finds. Certainly the prices are not high for an educational institutional purchase.

Two weekends ago, I went to a hamfest and rooted through a large box of wirewounds with most of them in the low ohm, high watt range as per normal. However, near the bottom I did find three 75 watt, 85k ohm resistors and got the lot of $1.00! Yes, they were filthy and had old wires just clipped off the lugs. Once home, I desoldered and re-tinned the lugs and cleaned them carefully avoiding cleaning off the value stenciled on them and logged them into my stash of such items.

A couple of my favorite ohmite finds of a few years ago are two 200 watt 100k wire wounds in their original sealed boxes with the classic WWII yellow label on them with a minute-man standing with his flintlock by his side saying "support the war effort - buy war bonds and savings stamps".

As noted, such mammoth resistors were a common thing in the vacuum tube era.

Richard Hull
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Paul W Fontana
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Re: Dummy load for HV supply

Post by Paul W Fontana » Wed Sep 23, 2015 9:06 pm

Thanks, all, for the thoughtful input.
Dennis P Brown wrote:The supply you just describe sounds lethal. Use by students? Fusor power supplies are generally lethal.
Have you been talking to my department chair? :D I appreciate your concern, and safety will be a major part of the lesson plan. I want the students to learn the proper respect, attitude, and practices, so they can deal with electricity safely and with confidence. (And I do have training and experience with high voltages and currents - not as high voltage as this, but many orders of magnitude higher stored energy and current.) To clarify, I'm designing for 50 kV/20mA, but I'll be happy if we make it to 30 kV carrying a 10 mA load.
Dennis P Brown wrote:a torr range gas is an excellent ballast if one is in the right range.
That's a good idea - we do have argon we can fill our glow discharge tubes with. I was just hoping to separate the challenges of building and characterizing the p.s. from applying it to a plasma, looking for a load that would be somewhat more linear (or at least have most of the nonlinearity due to the p.s. itself). Also wanted to put off dealing with the x-rays until the p.s. was working. If that turns out to be too cost-prohibitive, though, we'll just hook up the plasma tubes.
Werner Engel wrote:I use 6x10k wire resistors with 300W.
Thanks. That looks like it would work up to 1kV/15mA. I'm afraid I need a lot more resistance than that to test a 30-50kV supply. It's good to know the power rating on those wire wound resistors can be pushed a bit for a few seconds at a time.

Bob Reite wrote:elieve it or not, you can still buy 100 watt 100K wirewound resistors. Ohmite still makes them. You will pay around $17.00 each. Need more power dissipation? 225 watt 100K resistors are still made too, for $30.00 each.
It's helpful to know the parameter range that's available. I found those at DigiKey for a little less than you quoted. Still, to keep my load down, I need pretty high resistance, at least a couple Mohm. By stacking many of these I can accept a lower power rating, but for (25) 100kohm/50W it's getting close to $300, and fewer resistors of higher value at 50W seems to come out in the same ballpark no matter what. We do have a bigger budget than many amateurs, but that's getting a bit steep just for a dummy load whose only purpose is to test this p.s. (and maybe to heat the room), which itself is part of the bigger plasma physics instructional setup.
Richard Hull wrote:Certainly the prices are not high for an educational institutional purchase.
That would be true if I didn't need so many. At half the price, I could probably justify it. Since there are other options available (e.g. using the plasma), I think it's a bit out of reach. Between upgrading the vacuum system and vacuum chamber and putting together the HV power supply parts, things are starting to add up, and the budget is not unlimited.
prestonbarrows wrote:Glassman has an excellent write up about high voltage test loads on their site.
http://www.glassmanhv.com/TechNotes/Resistance.shtml
Perfect! That's just the sort of information I was looking for!

Update: Out of curiosity, I contacted one of the vendors on eBay selling 1000W low-ohm dummy loads intended for audio amplifiers, because the listing said "any resistance" was available. I gave him my specs (including the voltage!), and he came back with a quote for $87.80. Now we're talking! At that price I'm tempted to get two at 5Mohm, or even three at 7.5Mohm. Then I can put them in series for smaller loads or in parallel for the full load. I'm just skeptical that it can really survive such high voltage, esp. if they squeeze it into the same relatively compact package.

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Paul W Fontana
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Re: Dummy load for HV supply

Post by Paul W Fontana » Mon Oct 05, 2015 9:57 pm

Still waiting to hear back from the eBay source. Assuming I can't count on their load to stand off tens of kV, any reason not to use 25 of these for each 2.5Mohm leg?
http://www.ebay.com/itm/390356576642?_t ... EBIDX%3AIT

They are 100kohm, 50W. If I can trust them up to 2000V across each, they should work, and the price is a lot more reasonable than the Ohmite ones. I could build the whole load for $70 per leg.

(I think I will also use two of them in parallel as my 50kohm ballast resistor.)

-- pwf

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Re: Dummy load for HV supply

Post by Hank Ball » Tue Oct 06, 2015 1:41 am

I wouldn't use a metal-body resistor like that for high voltage because the close clearance between the resistor lead and the body greatly reduces the effective creepage distance and voltage capability. You would be better off with a ceramic type. Those flange mount resistors require a heatsink to achieve the 50W rating anyway, and there's no easy way to do that using many of them at high voltage.

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