triac firing using pnp transistor

[petercl14]

Anybody got any idea how the attached electronic circuit works. It appears you would need a reverse current through the pnp transsistor. If somebody knows and would kindly add resistor values I can test it in a spice simulation. The circuit is from a textbook on power electronics.

 

power electronics triac gate drive circuit.asc

[HarryA]

You are starting with a very primitive circuit. There are many complete TRIAC circuits to be had via internet search.

LTspice does not have a TRIAC model because Analog does not manufacture them.

If you search on TRIAC on Youtube.com you  will find numerous videos on the subject. This one has information on finding and download a TRIAC model for LTspice. https://www.youtube.com/watch?v=lRz1gql-BWw  Expand the 'show more'.

Also missing are the requirements for your circuit; 1 amp or a 100amp?.

[petercl14]

Hi Harry, I would never use this circuit myself. A triac optocoupler is much better. Either phase control(random) or zero crossing. The thing is the circuit is from a book called Power Electronics by a professor of Electrical Engineering at Edinburgh university in Scotland. So the circuit must work. What bugs me and I hope you and others on this forum also is how does it work? It would seem to require a reverse current to the pnp. Lets say 1 amp load, 240 resistor for R2. The text doesn't explain how it works. It gives this much. R5 is to stop the triac turning on falsely. This resistor is to drain a capacitor effect at the gate.

I don't know about you but for me I don't like to leave the puzzle of how this works unsolved. It is in our nature. So if anybody knows on this forum please let us know.

Thanks for the triac reference Harry. Also no triac optos on LTSpice.

[HarryA]

See figure 5 here for similar circuit: https://www.homemade-circuits.com/simple-triac-triggering-circuits-explained/

[petercl14]

Hi Harry, none of those circuits are the one I posted with the pnp transistor connected to the gate. I was hoping somebody could explain how that circuit works? It doesn't look possible. It is the circuit you also posted asking for 1 amp or 100 amps. I suggested 1 amp. Do you or anybody else on this forum know how it works?

[HarryA]

"none of those circuits are the one I posted with the pnp transistor connected to the gate. "

How about this one?  With thanks to homemade-circuits.com  In LTspice simulation all I ever get is the TRIAC off or fully on. As there is  no phase control,  because there is no relationship between the transistor and the AC supply. It simple allows the TRIAC to conduct or not.

In  theory you can connect the transistor to your microcontroller and use the circuit as a  replacement for a mechanical relay. There is a problem with the lack of isolation from the ac mains. That is why optical isolators are used.