Solid State “rheostat” that will withstand high electrostatic fields

[abstruse1]

I’ve been using large and expensive rheostats to vary the voltage to my flyback driver.  I have about run out of capacity and I’m starting to cook these things. I expect there is a solid state way to do this.

My previous experience with solid state devices around hundreds of kilovolts hasn’t been good. I assume the electrostatic field messes up the solid-state components.

The maximum voltage input will be 72 V DC and the maximum current draw will be around 60 Amps.

What kind of the devices do I need to accomplish this?

Thanx, guys!

[Mike]

A buck converter is the simplest solid state solution I can think of, though if you're needing 72V at 60A it's certainly not a small undertaking to design and build one, anything off the shelf will be very expensive at theses power levels. If interested is start by looking at what the QCW guys have built, some of these might manage for a while at least and will provide an estimate for what you're in for of you go that route.

Another option might be a phase controlled power supply (glorified lamp dimmer) on the output of a suitably sized linear transformer but the capacitor requirements are a little eye watering.

[abstruse1]

It just occurred to me that maybe one could strobe the DC output from my batteries (input to the driver) like one dims an LED by strobing.

Or I could just buy two or more rheostats and run them in parallel.

[MRMILSTAR]

Instead of rheostats you may want to consider using a large variac followed by a full-wave bridge rectifier if you use wall power instead of batteries. The large 120 volt variacs are rated for continuous duty of about 50 amps. The 240 volt variacs are rated for 25 to 30 amps continuous duty. You can run them at 2 or 3 times that for short durations. A variac will be much more efficient than a rheostat.

[abstruse1]

Is a 22 amp 120v variac capable of 44 amps at 60v output?

[abstruse1]

And do I need to do anything to prevent HV spikes from going to the mains ?

[MRMILSTAR]

Using the Superior Electric 1156D Powerstat as an example, its rated for 50 amps from 0 to 120 volts. It can be operated at twice that current for short periods. Superior Electric has derating curves for over-current applications. Its basically limited by heating.

As far as voltage spikes are concerned it depends on your application. If its a CW voltage multiplier then I don't think you have anything to worry about. I've been using a SMPS to power my CW voltage multiplier with no problems. However, if its a Marx generator, then I would definitely use batteries. A Marx generator can make short work of an electronic power supply. For some peace of mind you could attach a suitably-rated varistor to the DC power supply output.

[abstruse1]

I’m on it, Steve, thanx.

[davekni]

However, if its a Marx generator, then I would definitely use batteries. A Marx generator can make short work of an electronic power supply

My Marx generator is powered by a home-built SMPS, and uses two small commercial SMPS for 18V (control electronics) and 12V (fans).  No issues with SMPS.  However, it tripped building fire alarms both years I demonstrated it at OMSI (Oregon Museum of Science and Industry).  Not from actual fire, rather due to fields coupling to alarm wiring somewhere above 14' ceiling.  (Did have one tiny fire when insulation failed in my +-22kV input supply, but that was after alarms were disabled to allow demonstrations to continue.)

[MRMILSTAR]

The bigger problem may be getting sufficient power from a 120 volt outlet. You mentioned 44 amps at 60 volts. That is about 2600 watts. I doubt if you can get that much out of a 120 volt circuit without tripping the circuit breaker. If you have 2 independent 120 volt circuits on separate circuit breakers you can parallel them for twice the current. Typical home 120 volt circuits in the USA are rated for 20 amps so even connecting 2 in parallel would be marginal.

You could also consider 240 volt power. That would require a 240 volt variac. They are typically rated for around 28 amps at 0 to 240 volts. You could operate one at 60 amps for short periods. The derating curve will tell you the safe run-time for over-current. You also have the even more expensive option of 2 variacs in parallel.

[MRMILSTAR]

However, if its a Marx generator, then I would definitely use batteries. A Marx generator can make short work of an electronic power supply

My Marx generator is powered by a home-built SMPS, and uses two small commercial SMPS for 18V (control electronics) and 12V (fans).  No issues with SMPS.  However, it tripped building fire alarms both years I demonstrated it at OMSI (Oregon Museum of Science and Industry).  Not from actual fire, rather due to fields coupling to alarm wiring somewhere above 14' ceiling.  (Did have one tiny fire when insulation failed in my +-22kV input supply, but that was after alarms were disabled to allow demonstrations to continue.)

Good to hear Dave. I'm glad you got your Marx working with a SMPS. I've seen your video of it in operation and its quite impressive. I blew one SMPS out and have read of a few others that have done likewise. After that I got paranoid about it and just went to batteries. Batteries also ensure that there is no physical connection from the Marx generator to the house wiring. As you mentioned the EM pulse from my Marx generator sometimes causes nearby electronic devices to do strange things.

[abstruse1]

My shop has 240 VAC power (single and three phase) for welders and machines, so I could use a 240 variac.

But here's another maybe goofy idea. Let's say I know I want to run somewhere between 48 and 60 VDC. Could I wire 4x12v batteries in series and then take a 5th battery in series with a rheostat, and put that output in series with the 48 v. battery pack?  If so, would that mean that I could get by with a lower amperage rheostat since it'd be regulating only the output from a single battery?

[MRMILSTAR]

Any use of a variable resistor (rheostat) is going to waste power. I especially wouldn't want to do it with batteries because it just dumps the excess voltage as heat. Why not consider some switches that can be used to select 12, 24, 36, 48, or 60 volts? I have done that sort of thing. It doesn't waste any power. Do you really need any finer adjustment than that?

[abstruse1]

Steve, that’s essentially what I have now. I can choose the output of one or all of my batteries in series and then the rheostat simply fine tunes the total.

I’ve been asking myself why I think I need a precise output. My rheostat is 10 ohms and I select the starting point by plugging into the battery point I want to start with, e.g. 36, 48, 60 or 72 vdc, then turning the rheostat down (lower resistance) until either I get a desired arc or something croaks elsewhere. I always turn The voltage off by cranking back on the rheostat so that the semiconductors don’t experience such a drastic turn off. A long time ago, This seemed to have helped prevent mosfet blowouts.

I’ve also thought about using lower voltage batteries in series, e.g. 6V or even lower, to give me finer adjustment.

I need to think about what’s really going on here. Maybe I’m doing something that is no longer necessary.

Stay tuned.

[AkashaStar]

Hi there,

I had a couple of thoughts on reading through your post. I hear what you say about how you traditionally used the rheostat in a way which suggested it was protecting the MOSFETs in your circuit.  That's a kind of voltage regulator, if you like.

I do have my doubts about the efficacy of that, although this is obviously what your senses have told you.  The reason is that the 'slew rate' when you operate a rheostat like that is going to be very slow indeed.   It will take you a good fraction of a second to turn down a small device and longer to turn down a larger one.  With the numbers you're showing here, I'm guessing the one that you're frying is a hella big beast!

The time during which your MOSFETS are switching off is let's approximate to a couple of  hundreds of nanoseconds.  That's an infinitesimally tiny amount of time compared to your manual intervention. In circuits where these sorts of big silicon 'switches' mustn't switch off under full load, precautions are taken in the design, but not in the way you describe.  It simply wouldn't work fast enough and there are much better ways.

It doesn't matter if you use a physical resistor or a semiconductor to drop volts from a power supply.  They will both heat up to do the job. Think of old linear power supplies of the past, which had banks and banks of transistors on giant heatsinks acting as a nice radiator in the workshop.

Here's a thought experiment. Say you got a household kettle, rated for 3,000W output.   If you used that in series with your circuit and a voltage source of 70V , it would pass a current of close to 43A.  That's with its resistance being very similar to that of your rheostat at a lower setting (1.633R)  No wonder your rheostat is getting a bit hot under the collar! There's nothing to soak up all that heat fast enough & no pour of coffee at the end of it, either.

If the power supply is very stiff, like a car battery, and capable of supplying many tens of A continuously and thousands of A in a short circuit, then components can get hot very quickly, or, as you've found, go up in smoke.

I do wonder whether your flyback driver actually has an output of 4.2kW, from the numbers in your description? That's a giant amount of power.  Is it possible that you are basing your ratings on peak current (pulsed) rather than average?

For example, it's common to run devices at duty cycles down to 5% if there are limitations that require it.  I'd be curious to know more about how you're using the flyback driver and what the visible or measurable effects are that you're after?

In terms of withstanding 'electrostatic fields', it's more that a circuit would have to be badly designed for that to be an issue.

Hope to hear how you're getting on