Current source?

Transistor Q6 should have a turn-on resistor (Q6 base to ground), instead of having the load turn it on, when the switch is released.

Brian Ellis

Use a 555 one-shot to contol your output transistor. Replace everything to the left of Q5 & its resistors with the one-shot.

Ed

Yes I guess that's what I'll do.

I was trying to do it on the cheap with penny transistors and thick film resistors. The SCR at half the price of a TLC555 kind of defeats that purpose though.

Thanks all for the suggestions

I thought I'd give it one more try. I used a jfet P-channel to discharge the cap at switch off. It drops the cap voltage to zero in 230uS not spectacular but good enough. Load current drops almost instantaneously. I tried a JFET before and it wasn't fully discharging the cap. I must have had a wire crossed somewhere because it works well now. With a lower RDSON JFET I could improve that but for 23 cents for the FET I can't complain.

New schematic.

The load is really a photo mosfet driver not two 1n4001's, R10 is for current sampling. So the total parts count is 11 and price about 50 cents.

Thanks again

It looks like J1 is a dead short to ground. You probably want an N-MOSFET instead, and you'll need a resistor to pull the gate to ground when the switch closes.

Regards, Bob Monsen

No that's a P-channel JFET. It will only conduct when the switch is open Vgsoff (max) =4V.When the switch is closed the JFET is off. When the switch opens it provides a discharge path for the capacitor RDSON*C. Once the cap discharges the JFET drain is at zero volts, the switch is open so there is nothing for it to conduct. RDSON for the J176 is 250 ohms.

I didn't take direct current measurement but there is no need. I just measured the rise time of the cap which is 750uS through the 820 Ohm R4. The fall time is roughly 1/3 that so RDSON must be roughly 1/3 R4. It just gets warm (not hot) after repeated switching for testing.

Odd, I must have looked at it wrong before. Sorry about that.

Regards, Bob Monsen