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First Thread : A challenge and an opportunity for all of you here!

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LAN_403

HV Enthusiast

Thu Jul 22 2010, 05:41PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Actually, the solution to your problem is VERY simple and economical too.

All you need is a low cost, off-the-shelf audio amplifier feeding an audio modulation transformer. We've done this with some vacuum tube plate supplies modulating +/-10kV at 20-20kHz. You could probably get a solution for this less than $500.

If you're looking for something completely solid state, we have a MOSFET based topology that will do this as well which has already been developed, although it would require tweaking for audio frequency (right now its designed to modulated from 100kHz+.

But in my professional opinion, the off-the-shelf audio amplifier (i.e. car stereo) and modulation transformer is basically your simplest and least expensive option.

Pinky's Brain

Thu Jul 22 2010, 06:52PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

Do those things really have the turns ratio to step up 12 volt to 5 kV?

HV Enthusiast

Thu Jul 22 2010, 10:51PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Pinky's Brain wrote ...

Do those things really have the turns ratio to step up 12 volt to 5 kV?

Its not 12V. Thats the input voltage.
The output voltage is higher and depends on the power of the amplifier and driving capabilities of the amplifier. I have some el cheapo chinese 1000W mono car stereo amplifiers which have around 50V rails, so you only need a 100:1 impedance matching transformer.

We use this arrangement to drive one of our high power VTTC audio modulated coils. The 100:1 transformer we use is an old 1kW RCA modulation transformer.

cdanjo

Fri Jul 23 2010, 09:34PM

Registered Member #2995 Joined: Mon Jul 12 2010, 10:12PM
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Hi all! I'm back after a busy week to respond to some of the latest and greatest!

Thanks for the ideas.
Yes, I'm looking for more or less arbitrary control with audio-frequency bandwidth or more, so EastVoltResearch's comment about an OTS DC+AC supply with 100kHz BW would be a home run, but probably also cost a few arms and legs. Ultimately, it would probably come to that, in a nice compact form.

B/c of the arb waveform need, any kind of sine or square output wouldn't be flexible enough, unless it's about 200kHz or so switching for a class-D approach.

Yes, a FET stack would probably work, but it might be a pain to develop from scratch. But if you can make it work, it could be a useful start.

Also, keep in mind this is a class-A modulation scheme: AC output (up to ~+/-5kV) with a DC offset (~15-20kV) at about 10mA, so a step-up transformer would have to sink a lot of DC current through its windings/core, which means monster transformer!!

Has anyone got any advice on using vacuum tubes? It seems like it might be a match to high voltage and low current applications like this. Obviously, I'd prefer solid-state, but there just isn't anything easy to use yet.

HV Enthusiast

Sat Jul 24 2010, 04:47PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

wrote ...

B/c of the arb waveform need, any kind of sine or square output wouldn't be flexible enough, unless it's about 200kHz or so switching for a class-D approach.

Well, this contradicts your requirements above. You stated you only need up to 20kHz bandwidth max.

wrote ...

Also, keep in mind this is a class-A modulation scheme: AC output (up to ~+/-5kV) with a DC offset (~15-20kV) at about 10mA, so a step-up transformer would have to sink a lot of DC current through its windings/core, which means monster transformer!!

Uh . . . no. You state you need +/-5kV at 50mA, which is about a 100k impedance. Thats nothing. And then on top of that you say the DC current is about 10ma. Not sure how you say "LOTS of DC current" when you say its only 10mA.

I think it would be most beneficial to state in a more concise manner of what you are looking for because everything you have stated so far contradicts each other.

It would be a waste of our time to come up with a solution to your problem when you yourself don't have a clear idea of what you are looking for.

wrote ...

Has anyone got any advice on using vacuum tubes? It seems like it might be a match to high voltage and low current applications like this. Obviously, I'd prefer solid-state, but there just isn't anything easy to use yet.

Again. You're sayings lot of current in one sentence, and now we're back to "low current applications." I already provided you with one inexpensive and proven solution to your problem. We're already doing this for VTTC audio modulated applications (high side modulation) and sourcing much more current that the 50mA and 10mA you are claiming to require.

cdanjo

Sat Jul 24 2010, 09:15PM

Registered Member #2995 Joined: Mon Jul 12 2010, 10:12PM
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Sorry for throwing some contradictions into the mix... The truth is I don't know how much current I need. Initially, we stated up to 50mA (see OP), but someone pointed out that that's 1kW total power, which, in retrospect, is honestly more I than I want to deal with!

I'm afraid I wasn't about the purpose of the BW: It is an amplifier with a DC offset -- Imagine putting in +/-1v AC (audio BW) and getting out +/-5kV AC plus a DC offset of up to 15kV.

The 200kHz switching frequency is not a contradiction: I'm talking about class-D, so switch a square wave a at 200kHz between GND and HV with varying duty cycle and filter it so only the 20kHz BW gets through. The bottom line is I prefer 20kHz BW or more. It sounded like some of the previous posts were talking about outputting either a sine wave or square wave at 20kHz (not 200kHz), which is not what I want.

"Low" vs. "High" current: I was comparing the vacuum tube (typically few hundred volts to a few kV at 10's of mA) to the FET (typically 10's of volts up to about 1kV at up to 100's of amps), hence in comparison to the FET, the tube current capability is indeed "low," and voltage capability "high". In my experience, the secondary side of a step up transformer (say 100:1) would have to be quite chunky to handle 10mA DC without high resistive loss, not to mention a large core to prevent saturation.

Here are the clarified, concise requirements with an adjustment to the current requirement for a more reasonable value:

> Home-run: A HV amplifier capable of up to +/-5kV at 10mA output with floating outputs (up to 20kV) so we could connect it in series with a DC supply. The AC part must be the audio frequency band (20Hz to 20kHz). THD better than 3%.

> Grand-slam: An amplifier that combines both DC capability up to 20kV DC with +/-5kV AC amplifier capability, all at 10mA or more. The AC part must be the audio frequency band (20Hz to 20kHz or more). The output terminals would be GND and DC+AC. THD ~1% desired.

Pinky's Brain

Sun Jul 25 2010, 03:30AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

Well the AM modulation transformers do seem pretty much custom made for it ... the only downside is size and weight.

sashua

Tue Aug 24 2010, 11:12PM

Registered Member #2984 Joined: Fri Jul 09 2010, 02:55PM
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Posts: 4

Bringing this thread back to life with a request.

While we are trying to figure out how to build the ideal PS, we need something to run tests with in the meantime.

I need to rent (or buy on the cheap) one DC HV Power Supply 25-30kV at 10mA (or thereabouts).

Does anyone here have one to lend, rent or possibly point me to one without paying full price to Glassman/Trek/etc.

Thanks,
Russ

dmg

Tue Aug 24 2010, 11:36PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

Unforchantly, I cant help you in regards to pointing out a supply, but why do you intend to buy? a supply with the requirements you decribed can be built at home.

quicksilver

Wed Aug 25 2010, 12:19AM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

If you're looking for a DC HV Power Supply 25-30kV at 10mA (or thereabouts) simply check out "power supply's from 9k-25 or 25k-50. I think it's found via an alphabetical listing under "P" (for power supply) & just about everything you may want is somewhere there. They are decent folks.

They do have some "boat anchors" that are quite repairable for pretty damn cheap. Some are just "parts collections" but they have some very tough to get goodies inside.
They also have some things from Los Alamos that would fill the bill. It all depends on what you want to pay. IF you want to keep the cost low buy something at the power level you need and repair it. That may be one of the fastest ways of getting something cheap and up and running without designing something from the ground up.
I've bought from them in the past and been very happy with everything they have sold me. They even have sent the schematics of two power supplies that were really worth the money I spent.

Surplus Sales of Nebraska

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