Again with an oops.

How much -HV voltage and current?

HV is ~180V in and ~100 V out at the moment. Currents are low, 1mA in and 100uA out.

The FET is only spec'd for 200V, I've got some ~500V parts on order. I blew it by f'ing around with the load. (not really sure.)

George H.

Assuming your NFET can turn on at say 4V, why can't the opa2134 drive the 5k resistor to say 5 volts?

You could use my high-voltage optocoupler thing. Much simpler.

Variation on this:

Right, I works fine... but f-ing around down stream I somehow fried the Nfet and then the output drive of the opamp. I'm not sure how. I need some more prophylactics.

George H.

An LND150 in series with the source of the NFET might help.

Cheers

Phil Hobbs

Oops, scratch all that. I blew out the zener? Not sure how. (well too much power obviously.)

George H.

Right I've got a few unipolar ones sprinkled around. I was thinking I should make 'em all bipolar. Cause I can afford the 0.6V drop and that gives me twice the protection... both ways.

George H.

How about this?

Sure, (the more resistance the merrier) How do I choose the collector R and the source R? (I sometimes get the idea, that I should first start a design with ~100 ohms between every node, and then increase or decrease with need.)

George h.

I'd start with some assumed maximum voltage across the source resistor, maybe 5 volts, to set the current limit. Add the mosfet Vgs-on voltage to get the drop across the gate resistor. Etc.

This circuit limits the mosfet s-g voltage (hence current) two ways, but that's safe and well controlled. There should be no way to blow out the gate no matter what the opamp does.

I should have labeled the parts.

Spice it and we can play with it.

I still like the HV optocoupler thing better. An opto-depletion fet cascode is cool for higher powers.

Scratch scratch... I'll have to sleep on it. I'm not sure where any voltage limit comes from... I've mostly been thinking of currents. So the opamp (+15) and 5k output R shoots 3mA max into everything above. (TBH I blew it up after I cranked up the positive supply to 25V... for a different part of the circuit, so I that's more current, I should have adjusted the R's.) I like the source resistor, say 1 k ohm, then my GS zener should be safe.

That sounds great. I'm using this to adjust the output voltage from

Ohh I don't think it was the opamp that blew the fet. After some other stuff, I'm back to playing around with my spad. I've flipped the whole thing over, voltage wise. I think I had a simple quench circuit working... with too large of a reset R.. And then I tried to make it smaller...

oops no workie.

I figure it started to oscillate but who knows? (Well I should have some idea in the future.)

So I think it was down stream of the nfet. But I could be wrong.

Hmm, I'm adding some source R, and bipolar lnd150 current limits and getting back to the fun/important part of the circuit.

I am interesting in making this part more bullet proof, so maybe sometime in the future.... if the final thing doesn't work, then it's silly to waste time here.

HV optocoupler, are you speaking of your circuit the opto-C leds in the power rails of an opamp. With the output driving HV fets.

I've always wanted to try that one.

George H.

My circuit doesn't need a zener. The resistor divider ratio limits gate voltage.

A voltage reg can be simpler. Just the one HV coupler. That eliminates all that level shifting and blowing out fet gates and stuff.

Sweet! milli-second speed is fine for this.

What's it like down near zero? (leds have a limited linear range, IR are better. I sound like a pirate :^)

George H.