Problems with TRIAC circuit.

Thread Starter

Domophone

Joined Sep 10, 2013
65
Hey,
I'm having issues understanding the following TRIAC circuit. Can somebody explain to me the purpose of the resistor I've highlighted, as shown in the MOC3032/4042 datasheet. The 5V input signal is constant when the load is to be powered. Is it to drain the energy in the gate capacitance when switched off?

Thanks
 

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DickCappels

Joined Aug 21, 2008
10,152
That resistor is meant to keep off leakage current from the small triac inside the MOC3032 and MT2 to gate current inside the larger triac from triggering the larger triac before the MOC3032 actively triggers the larger triac..
 

Thread Starter

Domophone

Joined Sep 10, 2013
65
I'm currently using a 2N6344. I noticed it has a peak gate voltage of 10V. If i where to bring the 5V input high while the Hot line is above 10V, would the gate not see a the line voltage (minus the resistor drop) until the large triac triggers on? This seems like it would violate the absolute max ratings. Does the zero crossing circuit in the MOC3032/42 only turn on & off on the next zero crossing?

I'm having intermittent problems with the circuit, I've got one that failed, and others that work as expected. The failure results in it being constantly almost turned on (MT2 is at 117V, and MT1 is at 99V)
 

Thread Starter

Domophone

Joined Sep 10, 2013
65
That resistor is meant to keep off leakage current from the small triac inside the MOC3032 and MT2 to gate current inside the larger triac from triggering the larger triac before the MOC3032 actively triggers the larger triac..
thanks for the reply!
 

Thread Starter

Domophone

Joined Sep 10, 2013
65
And that's why that resistor is there.
Sorry, could you expand on that? I don't think I understand. The resistor drop i'm talking about would be from the Hot line through 180Ohm to the gate (before triggered). Gate has a Max triggering current of 50mA, meaning a voltage drop of 9V.

(170V - 9V) >> 10V.

Once triggered, MT2 tracks MT1, so the voltage drop from the V(G->MT2) is much lower.

DickCappels answer makes a lot more sense to me, but perhaps I'm misunderstanding.
 

DickCappels

Joined Aug 21, 2008
10,152
Are you using an oscilloscope to take those measurements? Voltmeter readings would be very difficult to interpret correctly. Impossible might be a better term.
 
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