Help me identify component

[jimmyblacksmith]

Hi,
This circuit is a 240 volt motor speed controller. There's a missing component (blank space above the triac), which looked like a zener diode, but it melted, so I can't identify it. I've tried diodes, thermistors, and diacs, but the motor only runs at top speed, or not at all. Can anyone tell me what the missing component should be?

[Kim Christensen]

Usually it's a DIAC on the gate of the TRIAC:

[MathWizard]

Was this a through hole or SMD part ? I guess most THH zeners are the glass type, so was it some SMD part that got toasted ?

I've done a couple of things with Triac's but not enough to know what was there. There's also Diac's that come up in these circuits, don't they get used sometimes to turn on Triac's ?

If it had any digital controls, it would probably use a high voltage opto-isolator for triggering or whatever it's called.

[jimmyblacksmith]

That was my conclusion after looking at some other circuits. I couldn't get it to do anything other than top speed though.

[jimmyblacksmith]

Through hole part. Really simple circuit, with no SMD. It was a glass component.

[jimmyblacksmith]

Correction to the diagram.

[MathWizard]

Yeah Diac's are also semi-conductor's and often in glass it seems.

[Kim Christensen]

That was my conclusion after looking at some other circuits. I couldn't get it to do anything other than top speed though.

It's highly possible that the TRIAC is shorted since the DIAC was completely cooked.
With the circuit unpowered, put an ohm meter across the main terminals of the TRIAC. It should read open in both directions. Do the same with the capacitors which should also read open.

[jimmyblacksmith]

I've replaced the Triac, which was actually fine. I've also replace C1 which leaked a little when I put 500v across it. All other components test fine. With a diac in place I now have an open circuit.

[eurgenca]

Oh thanks god. We only have one triac and diac.

[eurgenca]

Maybe this will help https://www.electronicshub.org/triac/