P-MOSFET Latch Turns Itself On

[ParkingLotLust]

I'm working on a small latch circuit, which I would like to default to OFF when the power is first applied.

When I simulate the following circuit, it works as expected. However, when I build it on perfboard, on power being applied the LED turns on by itself. I can turn the latch off and on correctly from here, but powering itself on has me scratching my head. I assume there's something obvious I'm overlooking

The P fet is an IRF9630, the N fet is a 2N7002

Also, I think the falstad circuit sim has the arrows reversed in its P & N fets

 

[ronsimpson]

on power being applied the LED turns on by itself.

Try putting a capacitor across the switch that turns off. 1uF or 0.1uF This should be like pushing the switch for a very short time at power up.

 

[ParkingLotLust]

Ron, you nailed it! Adding a 100n cap solved it.

Out of curiosity, any idea why this is necessary in real life (but not in the sim)?

 

[ronsimpson]

The voltage at which the FET turns on, varies from one part to another.
At turn on there probably is a race to see who turns on first.

 

[crutschow]

................
Out of curiosity, any idea why this is necessary in real life (but not in the sim)?

That's a caution when using simulations.
They generally give reasonably accurate results for the circuit models used but those models have "typical" idealized parameters with no tolerances, which are highly unlikely to be the exactly the same as the particular parts used in an actual circuit.
So certain undefined conditions, like which state a bistable circuit takes when power is applied, can be different in the sim as compared to the actual circuit. You must be aware of such situations and add power-on reset circuitry to insure a particular state, if that is important.

 

[audioguru]

Your schematic was an odd negative so I changed it to a normal positive image.
Your Mosfets looked odd, I made them look normal.

EDIT: I also had them wrong, I fixed it again.