WTB - Isolation tranformer...

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WTB - Isolation tranformer...

Postby Joe Jarski » Wed Apr 20, 2011 9:13 pm

I've been trying to figure out best the way to provide an adjustable-bias voltage (~500V) that will ride along with my cathode voltage (~30kV for now). Anyway, I forgot about these isolation transformers similar to this or maybe a X-ray filament transformer would work. I only need probably 10-15VA output I think and I can use a CCFL inverter to step it up if it's only a few volts output or a CW multiplier if it's in the higher voltage range. An isolation voltage of 60kV or so would provide some decent headroom for when I upgrade to a better PS.

So, if anyone happens to have or has seen one of these for a reasonable price - please let me know.
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Re: WTB - Isolation tranformer...

Postby Doug Coulter » Wed Apr 20, 2011 9:40 pm

We bought one X ray iso/filament transformer, and it was darned expensive (and big and heavy). If you want it, I think we paid $100 for it. Kinda overkill.

For that job, I'd just run a CCFL/rectifier off a gel cell and let the whole mess float -- the one I use is 12 watts, right in the range. I did that for my electron gun experiments, and adjusted the pots with a special screwdriver I made out of an electric fence post/rod with a metal blade stuck in the end. I made a grounded ring halfway down the shank for sparks to jump to instead of me - works great. You might find some "isoplys" or "iso-formers" around, I have a few, and stick one into oil for that. They are very good isolation, but not into the high kv. Neither the primary or secondary touches the torroidal core, with about 1/2" air gap as they are made.

If you really want to do the work, you can take a decent ferrite core (like from a TV flyback or switching supply) and wind it with gaps between the wires and the core, stick it in oil, drive it with something at some khz to cut the turns you need down. I can put those kinda parts in your hand next time I see you.

Oops, didn't mean to cover this up before the rest see it -- hopefully members know how to use the "recent posts" and "unread posts" stuff. Here's link to the ccfl stuff I use.
viewtopic.php?f=9&t=290&p=1582
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: WTB - Isolation tranformer...

Postby Joe Jarski » Wed Apr 20, 2011 11:26 pm

Thanks Doug. Running the CCFL inverter off of a battery was my initial plan, but I didn't have any good (simple) ideas on how to adjust it while it was running - using an eleven foot pole never crossed my mind (I'm not sure I'd touch it with a ten foot pole :lol: ). My 40kV probe is a foot long or so, so that would probably do for 30kV.

The only transformer of that style that I've found so far is on epay for $200, which is quite a bit more than I wanted to spend, but the ratings are right about what I was looking for. I still have a lot of work to do before I really need one, but I'm trying to plan ahead as I work out some of these details. I'll have to take a look at what you have next time I'm down there.
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Re: WTB - Isolation tranformer...

Postby chrismb » Thu Apr 21, 2011 12:35 pm

Can you give a bit more on what you're trying to do, Joe? If it is for AC on top of the DC, you may be able to use a charge-pump arrangement, isolated with a cap.
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Re: WTB - Isolation tranformer...

Postby Doug Coulter » Thu Apr 21, 2011 12:50 pm

Well, here is what I did. I do admit that I also want to build something like JohnF just did (motor generator) but perhaps on a smaller scale, as in a small 24 volt DC servo driving a 24v stepper motor used as the generator....For one thing, that looks like just plain fun to fabricate and be amazing with. You can really get the volts out of a stepper even not turning it too fast if the power is in-range for you, and they are real cheap these days. Rectify both sets of windings (phases) and get a nice low-ripple output.

First, the magic screwdriver:
MagicScrewdriver.jpg
Screwdriver for HV, made of fiberglass. Note ground between me and the business end...tested to 40kv, probably good for more.

I keep this around, it's useful.
This is the setup I had going then. I used some more of those fiberglass rods to support a little table floating off ground, and used full sized pots to control several small HV sources run off gel cells for the electron gun stuff. I used full sized pots with big slots so it was easy to adjust them.
Isoply.jpg
Overall setup

As a side note, you can parallel those CCFL's to get more current, after rectification. They might just parallel fine without that, and get into sync, I've not tried with these but it's worked with other things. You can also cut the ground track on the transformer secondary and have a bit of float that way too - but not any 30kv, just enough for noise purposes when killing ground loops and so on.

The thing sitting down on the main table with red/blue wires is an iso-ply, in this case a 300v regulated one (gas tubes). They're way cool, only a few pf coupling in<>out and would take oil immersion to do high KV, but would take it. I happen to know the guy who designed them from way back, but he's long out of that business. They show up at hamfests sometimes -- and no one knows what they are, so they go cheap. Nice thing to have laying around.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: WTB - Isolation tranformer...

Postby chrismb » Thu Apr 21, 2011 1:28 pm

Like this Joe. [Is this right?]

You can run a charge pump on top of an isolation capacitor to get your voltage bias, recitfying the AC up at the high voltage [or add -ve signs, to taste!!]. If you want AC, just delete the upper diode and parallel cap.

(Turn the volts up slow, so you don't roast the diode)
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chargepump.jpg
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Re: WTB - Isolation tranformer...

Postby Doug Coulter » Thu Apr 21, 2011 2:11 pm

That should work if the stored charge on the initial series cap doesn't make troubles. If the 30kv is arced to ground bad things can happen at the AC supply end...
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Re: WTB - Isolation tranformer...

Postby chrismb » Thu Apr 21, 2011 2:24 pm

Quite so. Yes, best to add some sort of transient coupling from the top of that AC to ground as a sacrificial short circuit in case the isolation cap has a breakdown, or gets pushed to -ve polarity under a breakdown on the other side. A spark gap or transient diode maybe?
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Re: WTB - Isolation tranformer...

Postby Joe Jarski » Thu Apr 21, 2011 3:35 pm

There are a lot of good ideas here, maybe I should move this to the HV forum...

Chris, my intentions right now are to just provide a negative DC bias voltage (relative to the cathode potential) to a "suppressor grid". In the future I may be looking at using AC in a similar setup for bunching, but not until I have something operational.

Doug, how would you manage voltage regulation and adjustability on the motor-generator setup? Wouldn't it be sensitive to load and speed changes or would you have a regulator on the output?

What the heck is an iso-ply? That's a new one to me and I couldn't find anything on a quick search.
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Re: WTB - Isolation tranformer...

Postby Doug Coulter » Thu Apr 21, 2011 4:27 pm

Depends on how picky you are. The motor speed would tend to make a certain output voltage, and if the HV supply runs off that, it would change with motor speed. Closing the loop would be a trick -- a long distance optocoupler or something like that would be required. I'd just use a regulator and pot on the HV side, myself. I'd guess you could make a decent long range optocoupler with a glass rod (cane) or something like that, or just buy some fiber - if you wanted fancy and accurate, pwm the signal on it so it could ignore absorption variations and stray light. I've got a bunch of thin Plexiglas rod that might do fine for purer homebrew.

An iso-ply (made with an iso-former) is a thing John Walton, sometimes business partner with my dad, invented. It's a toroidal tape wound core, with primary and secondary windings with large air gaps between each and the core, the idea being to give you a very isolated floating power source. Johnny also made audio versions, which were nice for seeing tiny signals floating on a lot of other trash - a near-perfect differential input with near-infinite common mode range. They have almost infinite isolation resistance (an inch of air) and very low cap coupling, depending on whether you tie the tape-core to something or not. Very nice for some things. He wound up making tons of them, most of which wound up in test labs.

Johnny was always into something odd. He started maybe 20 companies, and had a lot of fun, from building large fed-back speaker systems (eg servoed -- but in 1950) to motor phase dropout protectors, to long lived exit signs (didn't do the T ones, though).

I suggest if one of us builds one of these schemes, yeah, we should put it in the HV thread -- complete with the "it almost worked" stuff, as there are a lot of lurkers learning here if the emails I get are any indication. Personally, I want to do the motor thing as it just looks like fun - and I've got a 180kv supply in the wings that might need something like that. I'd use different motors than JohnF did -- but for the same reasons -- my junkbox has in it what it has, and some of it is good stuff for this sort of project. So I'll have to root around for some shaft couplers....probably have some somewhere. I'd guess a belt drive would introduce other interesting issues as the belt would carry some charge around -- would be interesting but probably not useful.
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