As I say, even when U1 is completely disconnected, the collector of Q1 stays low, regardless of the trigger input - unless it's disconnected from the T1 gate. I don't see how T1 is pulling it low.

 

hi rj,
With that circuit the SCR will be triggered On, when the transistor Base is low.
E

 

Yes that's what I want. I want the thyristor to be on when the trigger is low. It's the same as what I was trying to do with the P mosfet.

So - the sequence is meant to be
trigger goes high - U1 starts to conduct, and charges C1. C1 won't discharge because thyristor is off.
trigger goes low - U1 stops conducting. Thyristor turns on and discharges C1

 

I do not see how T1 gets turned off once it is turned on.

 

I do not see how T1 gets turned off once it is turned on.

Because thryistors turn off when the current stops flowing through them.
That's the theory I think and it's what happens in practice

 

Hi rj,
If you post your complete circuit, showing component values and how you expect the circuit to operate, I will run the circuit in simulation.
What is the specification for the thermistor?
E

 

My feeling is that the thyristor is probably damaged because I didn't use a resistor to limit the base current. This might explain why the circuit worked for a while but no longer does. I will try with a new one.