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Diabolical Artificer · 27th Jul 2020, 7:36 am

Bless you Dave...had another crack at this yesterday and have still have a low OP voltage, so pulled out Q2 as suggested, no change. something is dragging the gate down, measured across the current sense R - R1 = 44mV, therefore don't think it's going into current limiting. So pulled Q7 for good measure, still SNAFU'd.

This was after changing the resistor I'm using for CCS I3, which is now 6k, I'm still getting 23/24v across this so now we have 3.8mV, better. I also rebuilt it twice to make sure I'd made no mistakes, the gate is still sitting low, without a load about 80v, so OP about 80v.

Gate still low so at a loss I pulled out R7 - 330k, this is with Q2 & 7 replaced, no OP V. I then changed R7's value to 220k, haha, OP V now higher.

After mucking about changing other component's mainly the resistor divider I've found the circuit does work, twiddling R2 does change OP V, but something is keeping the gate low.

Not being that good with transistors it's taking me a while to get my head round things but I'm slowly creeping up on this. Some thoughts which could be completely wrong. Q2's base is always at 24v, I have a 24v zener as D2, is this right? Looking at a blown up schematic it looks like 30v

Whichever this is if Q2's base is at a low voltage surely the gate of the mosfet will also be low? The gate V of the fet seems to me to be mainly dictated by a potential divider D2/Q2/D3/R7 D2 & D3 are fixed (ish) so Q2 Vce and the voltage drop across R7 are the only variable's. However Q2's collector voltage will be set by Q2's base current, but if Q2's base is always at a constant current due to the CCS, so how can Q2 Vce change?

Just thinking out loud here but looking at this another way my circuit differs from yours Dave mainly because I'm only running this off 320v instead of 400 odd V, but don't think that matters.Also V2 and D2 (maybe) are different, but am pretty sure Q4/5/6 are working as per at least for quick tests. Q5 & 6 bases are @ 2.5v so there's no disparity there, if they're right Q4 should be. Q4 is the next candidate for closer inspection, i'll get there....

Andy.

PS thanks for the offer of bits, much appreciated.

27th Jul 2020, 7:36 am	#69
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,663	Re: High voltage regulator circuit. Bless you Dave...had another crack at this yesterday and have still have a low OP voltage, so pulled out Q2 as suggested, no change. something is dragging the gate down, measured across the current sense R - R1 = 44mV, therefore don't think it's going into current limiting. So pulled Q7 for good measure, still SNAFU'd. This was after changing the resistor I'm using for CCS I3, which is now 6k, I'm still getting 23/24v across this so now we have 3.8mV, better. I also rebuilt it twice to make sure I'd made no mistakes, the gate is still sitting low, without a load about 80v, so OP about 80v. Gate still low so at a loss I pulled out R7 - 330k, this is with Q2 & 7 replaced, no OP V. I then changed R7's value to 220k, haha, OP V now higher. After mucking about changing other component's mainly the resistor divider I've found the circuit does work, twiddling R2 does change OP V, but something is keeping the gate low. Not being that good with transistors it's taking me a while to get my head round things but I'm slowly creeping up on this. Some thoughts which could be completely wrong. Q2's base is always at 24v, I have a 24v zener as D2, is this right? Looking at a blown up schematic it looks like 30v Whichever this is if Q2's base is at a low voltage surely the gate of the mosfet will also be low? The gate V of the fet seems to me to be mainly dictated by a potential divider D2/Q2/D3/R7 D2 & D3 are fixed (ish) so Q2 Vce and the voltage drop across R7 are the only variable's. However Q2's collector voltage will be set by Q2's base current, but if Q2's base is always at a constant current due to the CCS, so how can Q2 Vce change? Just thinking out loud here but looking at this another way my circuit differs from yours Dave mainly because I'm only running this off 320v instead of 400 odd V, but don't think that matters.Also V2 and D2 (maybe) are different, but am pretty sure Q4/5/6 are working as per at least for quick tests. Q5 & 6 bases are @ 2.5v so there's no disparity there, if they're right Q4 should be. Q4 is the next candidate for closer inspection, i'll get there.... Andy. PS thanks for the offer of bits, much appreciated. __________________ Curiosity hasn't killed this cat...so far.