Triac Gate Turn-on Question

[bostonman]

My Weller Solder Station broke today.

I found the schematic online, but I don't have experience with Triac's except on paper.

Why was a 2u2 (2.2uF ?) capacitor used between the gate of the Triac and the collector of BC547? Seems like driving it directly with the current through (or voltage at) the collector would have been better.

Unfortunately the only other part of this schematic that's a mystery is the program in the PIC, but otherwise, it's a simple circuit. Now I need a soldering iron to fix my soldering iron. Amazing, the piece I need to fix my soldering iron is my soldering iron.

[Ian.M]

Probably safety.  The PIC *MUST* produce a pulse train to fire the TRIAC.  Any steady state signal cant fire it, vastly reducing the risk of a heater 'runaway'.

[bostonman]

That makes a bit of sense, but how does the DC on the transistor side change the gate voltage because the diode is pulling things down to approximately 0.7v?

[Andy Watson]

Triacs can be triggered with gate-signals of either polarity. The capacitor charges and then the transistor is turned-on to drive a negative pulse into the triac.

[TurboTom]

It's even recommended to use negative drive polarity for TRIAC operation since the trigger current in this quadrant is lowest. See the Datasheet and this Application Note for some additional information on that topic.

[Zero999]

The TRIAC you're using should trigger at 15mA.
https://www.st.com/resource/en/datasheet/t1250.pdf

I like using a pulse transformer to trigger a TRIAC. It's good because it's isolated, although unnecessary in this application.

[bostonman]

My confusion is that it's a DC circuit with a DC block (i.e. capacitor) driving the gate of the triac.

I think if the gate side had some voltage, then the capacitor would charge/discharge, but to me, all it seems is the gate is being pulled to ground via the diode.

[bdunham7]

My confusion is that it's a DC circuit with a DC block (i.e. capacitor) driving the gate of the triac.

I think if the gate side had some voltage, then the capacitor would charge/discharge, but to me, all it seems is the gate is being pulled to ground via the diode.

It's not a DC circuit at all!   

[Berni]

Triacs love taking it rough up there gate.

They switch the most efficiently when you dump a large spike of current into the gate since this makes sure all of its internal structure turns on quickly and simultaneously. For small devices like these it doesn't matter that much if you drive the gate with a gentle current, once one point in the internal structure starts conducting it spreads over all of it, on such a small die at small powers it happens instantly. This is more important for the giant hockey puck sized triacs and SCRs that switch 1000A, those have such a large die that going too gentle on the gate might not turn it on fast enough and blow them up.

So in this circuit the capacitor gently charges up and then once the transistor turns on it dumps all of that capacitor into the gate as one big spike of current.

Tho as others have brought up the fact that the MCU has to continuously provide pulses to keep it on is also a nice feature to protect from a heater runaway in case the MCU crashes. You can see that the MCU also taps the transformer to get a 50/60Hz line synchronization signal. This allows it to precisely time where on the sine wave it wants to turn on the heater, giving it dimming capability.