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.
Post Reply
User avatar
Paul W Fontana
Posts: 38
Joined: Fri Mar 13, 2015 7:58 pm
Real name: Paul W. Fontana

Dummy load for HV supply

Post by Paul W Fontana »

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
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Dummy load for HV supply

Post by Dennis P Brown »

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.
User avatar
Werner Engel
Posts: 145
Joined: Sun Aug 18, 2013 11:51 am
Real name: Werner Engel
Location: Vienna, Austria
Contact:

Re: Dummy load for HV supply

Post by Werner Engel »

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
Attachments
IMG_5248.jpg
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Dummy load for HV supply

Post by Richard Hull »

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.

Richard Hull
Attachments
MVC-002F.JPG
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
prestonbarrows
Posts: 211
Joined: Sun Jun 24, 2012 1:27 am
Real name:

Re: Dummy load for HV supply

Post by prestonbarrows »

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.
User avatar
Bob Reite
Posts: 576
Joined: Sun Aug 25, 2013 9:03 pm
Real name: Bob Reite
Location: Wilkes Barre/Scranton area

Re: Dummy load for HV supply

Post by Bob Reite »

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.
The testing isn't over until the prototype is destroyed.
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Dummy load for HV supply

Post by Richard Hull »

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
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Paul W Fontana
Posts: 38
Joined: Fri Mar 13, 2015 7:58 pm
Real name: Paul W. Fontana

Re: Dummy load for HV supply

Post by Paul W Fontana »

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.
User avatar
Paul W Fontana
Posts: 38
Joined: Fri Mar 13, 2015 7:58 pm
Real name: Paul W. Fontana

Re: Dummy load for HV supply

Post by Paul W Fontana »

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
Hank Ball
Posts: 10
Joined: Mon Feb 09, 2015 1:04 pm
Real name: Hank Ball

Re: Dummy load for HV supply

Post by Hank Ball »

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.
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Dummy load for HV supply

Post by Rich Feldman »

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 2:30 am, edited 1 time in total.
All models are wrong; some models are useful. -- George Box
User avatar
Paul W Fontana
Posts: 38
Joined: Fri Mar 13, 2015 7:58 pm
Real name: Paul W. Fontana

Re: Dummy load for HV supply

Post by Paul W Fontana »

@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?
User avatar
Paul W Fontana
Posts: 38
Joined: Fri Mar 13, 2015 7:58 pm
Real name: Paul W. Fontana

Re: Dummy load for HV supply

Post by Paul W Fontana »

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
User avatar
Werner Engel
Posts: 145
Joined: Sun Aug 18, 2013 11:51 am
Real name: Werner Engel
Location: Vienna, Austria
Contact:

Re: Dummy load for HV supply

Post by Werner Engel »

Hank, you are right!
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Dummy load for HV supply

Post by Rich Feldman »

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
fur2.JPG (17.33 KiB) Viewed 9517 times
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
Posts: 1
Joined: Mon Dec 21, 2015 12:40 pm
Real name: Candra Levy

Re: Dummy load for HV supply

Post by CandraLevy »

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
Posts: 90
Joined: Sat Dec 01, 2012 1:25 am
Real name: Andrew Haynes

Re: Dummy load for HV supply

Post by Andrew Haynes »

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
User avatar
Paul W Fontana
Posts: 38
Joined: Fri Mar 13, 2015 7:58 pm
Real name: Paul W. Fontana

Re: Dummy load for HV supply

Post by Paul W Fontana »

@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
John Futter
Posts: 1848
Joined: Wed Apr 21, 2004 10:29 pm
Real name: John Futter
Contact:

Re: Dummy load for HV supply

Post by John Futter »

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
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: Dummy load for HV supply

Post by Rich Feldman »

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
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 2:35 pm, edited 1 time in total.
All models are wrong; some models are useful. -- George Box
User avatar
Paul W Fontana
Posts: 38
Joined: Fri Mar 13, 2015 7:58 pm
Real name: Paul W. Fontana

Re: Dummy load for HV supply

Post by Paul W Fontana »

@John Thanks for the photos! Where do you get those nice corona rings? I was thinking I would make one out of copper tubing, but if I wanted to get a commercial one I have no idea where I would look.

Speaking of, with all the HV experience here someone will be able to tell me - am I likely to need more than one corona ring if I'm going up to 40 to 50 kV? My plan is to connect my resistors in a helix like in the Glassman example Preston posted early in this thread. Probably six resistors per turn, approx. 15 cm diam., with 2.5 cm turn-to-turn separation. That makes about 5 turns for a stack of 30 or so resistors about 13 cm tall. Then a corona ring on top, and a terminator where I can connect my HV cable. Is there a rule of thumb about how often to include corona rings? Seems like I might need one mid-way, at the 25 kV point? Or would it be sufficient to try to keep my connections between resistors as smooth as possible? I can just try it and see and add one as needed, but thought there might be some engineering principle I should learn.

@Rich Thanks for the tip! I knew resistor voltage was limited by flash-over in addition to dissipated power, but wasn't aware that these power resistors could also break down internally even if the power/current rating was not exceeded. A quick study online seems to indicate that exceeding the voltage rating causes a gradual decline of resistance value over prolonged use rather than catastrophic failure. I couldn't find any ratings for resistors like these, so I'll just give it a go and keep an eye on it. Good to be aware of.

-- pwf
User avatar
Bob Reite
Posts: 576
Joined: Sun Aug 25, 2013 9:03 pm
Real name: Bob Reite
Location: Wilkes Barre/Scranton area

Re: Dummy load for HV supply

Post by Bob Reite »

My eight stage CW multiplier has a corona ring at each stage, but that is probably overkill, unless I decide to push it to 100KV. So maybe have a ring for every 12.5 KV?
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
John Futter
Posts: 1848
Joined: Wed Apr 21, 2004 10:29 pm
Real name: John Futter
Contact:

Re: Dummy load for HV supply

Post by John Futter »

The corona rings are 12mm aluminium rod rolled into a circle and welded

Bob
a corona ring every 12 kV is conservative but you will not regret it
I seem to have more trouble after 20kV with the odd sharp edge
prestonbarrows
Posts: 211
Joined: Sun Jun 24, 2012 1:27 am
Real name:

Re: Dummy load for HV supply

Post by prestonbarrows »

On a normal PCB, around 300 V per mil is a conservative design point with uncoated traces. More than that and you need to start looking into adding slots and so on.

In air, breakdown starts at electric field strengths of around a megavolt per meter, or about 10kV per cm. The best geometry for between a given voltage drop is two spheres. Planes, or worse points, will concentrate the field and lead to breakdown at lower absolute voltages.

Past a few 10's of kV, design rules are hard to pin down. Everything goes non-linear and becomes much more empirical. If you have the capability on your setup, adding one stage at a time while appropriately scaling the load voltage and recording the drain current will show you exactly where any corona losses are happening. This will show up as an anomalous current not accounted for by Ohm's law and the basic resistor circuit to ground.

A classic designer's trick is to imagine stretching a rubber sheet over the HV section towards ground. Where the sheet would break first is where you should expect the most arcs.

Always avoid sharp points and keep the voltage gradient headed in one direction without folding it back on itself.
Post Reply

Return to “High Voltage - Fusor Input Power (& FAQs)”