How to stop PNP transistor from triggering at power up

Question

In a simple PNP switch circuit, any capacitance on the base will make the transistor turn on when the 24V (V1) is first applied. It'll stay on until the cap charges. I imagine there will always be some stray capacitance, especially if the transistor is being controlled from another pcb with a long cable. How can I stop the brief triggering at power on? Is the only solution to minimize the capacitance until the charge time is faster than the transistor can react?

I don't have an actual cap, just estimating stray capacitance. Also, how could you add a filter to the base without leading to false triggering at power on?

I think all I need is a larger cap from emitter to base – Thomas C. Aug 21 '22 at 15:49 — Thomas C., Aug 21 '22 at 15:49
Yes, put the cap between emitter and base instead... – bobflux Aug 21 '22 at 17:34 — bobflux, Aug 21 '22 at 17:34

score 3 · Accepted Answer · answered Aug 21 '22 at 17:43

If you have parasitic from B to ground (e.g. the 100 pF as you show), then you can add a real capacitor from B to E. This will slow down the turn-on.

Note that in this circuit you have a 24 V supply, so as long as C1 is charged to < 24 V, the PNP will turn on. The added B-E capacitor will need to be << 0.6 V to keep the PNP off, so needs to be about 24/0.6 = 40x the value of the parasitic C1.

There are ways to reduce this -- e.g. adding a R in series with the B and another from B-E; then the new C from the added B resistor to supply.

How to stop PNP transistor from triggering at power up

1 Answers1