Design problem for scientific lamp current source.

That's a honkin' big transistor for a few mA -- are you just trying to be sure to dissipate the heat, or what?

Given that the data sheet says nothing about the transistor cutoff frequency or input capacitance, I suspect that it's not anything to write home about. That slow of oscillation is surprising, but the 1N4007 is going to slow down the speed that you can turn things off, because at that point you'll be working the C-B capacitance of the predriver against the 1k-ohm resistor.

I'd look at the base voltage of the drive transistor vs. the opamp voltage, and see if you learn anything.

I'd also see if I could find some higher voltage devices. I know they'll be thin on the ground at that voltage level, but there should be something, and then you wouldn't have to worry about your 700V transistor misbehaving with your 800V supply.

Assuming that you could stabilize it, a MOSFET or IGBT should slide right in there.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

This is a slightly Russian circuit.

A more subtle approach is to use a transformer to apply the 600V striking voltage to the lamp for long enough to get the discharge going - less than a microsecond for a glow discharge, some microseconds for an arc - which lets you get away with a supply voltage a lot closer to the 200V running voltage of the lamp.

What you are saying is the circuit is unstable. This means that you need to look at the frequency compensation. That 10nF capacitor is decoupling the op amp from the output stage at higher frequencies, but it may not be the right value to do what you want, and it could be contributing extra phase shift where you don't actually want it.

Get hold of a text-book that talks about stabilising negative feedback loops and read it carefully. "The Art of Electronics" by Horowitz and Hill might be one such textbook - though it covers a lot more than just negative feedback.

Some of our regular posters have published more specialised texts, and may be prepared to stick their necks out further.

--
Bill Sloman, Sydney

SOA says you can only run 600V up to a hair over 10mA, maybe 12mA. BJTs are a bad idea for dropping high voltages due to second breakdown.

Why not choose a tube like 6L6GC? Current production, doesn't take much heater power, and takes the voltage easily. Okay, a MOSFET would be smaller and cheaper (though probably about even if you include the heatsink, come to think of it).

Note you're trying to carry a maximum of (800V - 200V) * 0.06A = 36W, which is a moderate amount for a small transistor. Just in case, you'd be better off with a TO-247 package, or two TO-220s in parallel.

Switching methods are much more efficient, even if you can't run the thing directly on AC (I assume that's part of the design requirement).

Tim

-- Deep Friar: a very philosophical monk. Website:

Why the Darlington?

If you have voltage to throw away you can always try putting in more R.

And sure why not a high voltage FET.

George h.

Tim Wescott wrote in news:ptSdnZVX6KzpSD_NnZ2dnUVZ snipped-for-privacy@web-ster.com:

I am looking at MOSFETS now.

Bill Sloman wrote in news: snipped-for-privacy@vb8g2000pbb.googlegr oups.com:

Your point about separating the run and strike ccts is well taken. I might go that way if I can't find another solution using the present transformer.

I don';t think it is a stability problem because at the point of breaking down, the volts to the base are only dipping, indicating the loop is trying to prevent the output from conducting.

"Tim Williams" wrote in news:k7un7q$hfd$ snipped-for-privacy@dont-email.me:

I agree I'm pushing the limits of the SOA. Now that this is becoming clear that it is the problem, I can tackle it in the best way I can. I had thought about a tube too. For the moment I might try a MOSFET in a TO247 pack. The SOA is a bit more friendly at the high voltage end.

Switch mode is out of the question at present, I know the customer has a huge dislike, so I am doing it the way he wants.

s h

be

g

an upside down buck similar to this should be doable:

could make one with current sense on the low side

-Lasse

I had adapted the circuit from a similar one. I am not using the darlington now. The problem persists, probably second breakdown.

That is worth doing if dissipation becomes a problem.

Trying that next.

Well, it is by definition a stability problem because you're seeing oscillation where you want to see steady operation. The only question is where the oscillation is coming from, which is why I was suggesting looking at other points in the circuit than just the op-amp output.

The circuit topology is correct assuming that there's not a lot of coupling between the collector and base of the transistors, that the Darlington stage isn't too slow. That 10nF cap is in the right place, but Bill is correct that it may be too small.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

Trying to control the break over point on neon gas is going to be tricky with so many slow components in line.

Using darlington adds to the problem with the storage time in the trannies and the use of that diode in the base with nothing but a 1k to pull it up isn't helping much.

That circuit needs the integrator/miller cap greatly reduced, diode removed and possibly a Cap lead network from the feed back to give it a little derivative in the - feed back. At least that will help with the phase error and reduce os-kill-La-Trons! :)

MeSelf, I think a PWM would be nice but then again, we don't know what the adverse effect of flicker would be..

Jamie

g s c s t

is

I wasn't that specific. I just said that it wasn't doing what it had presumably been put in there to do.

--
Bill Sloman, Sydney

Where does the 800V come from?

One thing I've done in the past is to make a weak voltage doubler that drops down to about half when you start drawing serious current. The effective series impedance would help stabilize the circuit, and it would dissipate a lot less power.

With your circuit, you could try a 10K 100W resistor instead of the

330R. They even come in TO247 if that's what you really want.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
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Embedded software/hardware/analog  Info for designers:  http://www.speff.com

That's exactly how many HeNe gas laser power supplies work. The first stage of the CW multiplier have large capacitors. Then it has enough stages with smaller capacitors to make the start voltage. A proper current limiting ballast resistor establishes the run current.

Take a look at the I-V curve for the lamp. You may be entering a region of oscillation in the plasma.

Steve

Spehro Pefhany wrote in news: snipped-for-privacy@4ax.com:

800V is from volatge double from a 270 volt secondary winding at 50VA. The caps are a pair of 450V 47uF. I think that is a good idea too, effectively reducing the supply regulation.

Owen Roberts wrote in news: snipped-for-privacy@kt16g2000pbb.googleg roups.com:

I might just do that too. There is a chance it acts like a relaxation oscillator. The lamp does not misbehave with a simple R in series with a HV supply at 50mA or so.

Try a simple resistor in series with the lamp and adjust the resistor and/or supply for that current range. Very possible the lamp is in a negative resistance region. BTW, the secondary ionization voltage for neon is 40.9V and the third ionization voltage for neon is 63.2V - meaning you better see about 63V across it when operating properly.

tor

ird

Not if it's operating as a glow discharge. Getting electrons out of the cathode by positive ion bombardment needs quite a large voltage drop across the anode glow region.

--
Bill Sloman, Sydney