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.
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.
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.)
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.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
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
0 to -200V or so.
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.
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