This is famous circuit from the MOC308x datasheet which many people use and refer to:
I have recently asked related question on the usage of the BT136.
I would like to understand how this circuit works before processing to "optimize" it and make it more compact and efficient with modern parts (MOC chip is dated back to 2000).
My load will be fully resistive with 0.1 A current @ 230 VAC.
What I learned so far:
- BT136 is 4Q triac, and requires snubber. There're more recent 3Q triacs like BTA201, claiming very attractive advantages. Would be super cool replacing big 4A BT136 with 1A BTA201.
- The circuit above, as said here (citing this), opens triac in quadrants I and III only because polarities match, thus it is a good question if feature being "3Q" makes any sense here as these devices miss quadrant IV.
- Why this circuit above has snubber at all, I have read somewhere (can't find reference any more) that with only quadrants I and III snubber is not needed at all.
- The same source says
Resistor R (shown in Figure 7) is not mandatory when RL is a resistive load since the current is limited by the gate trigger current (IGT) of the power triac.
Igt of the BT136 datasheet states 2.5 to 10 mA (max) current, and BTA201 5 to 50 mA, does it mean that maximal current flowing through the "R" and the gate to the T1 (bottom terminal on the circuit) will be limited to 50 mA (for BTA201), therefore power on 360 Ohm resistor for this (short) period of time will be 0.9 W (BTA 201) and 0.03 W (BT136)? 0.9 W is kind of big resistor, what's the point of having so high max current?
In addition, datasheet has Igm and Vgm peak values, with Igm for BT136 being 2A. If there's already an Igt of max 10 mA, how there could be Igm of 2A?
- As an addition to previous point, when MOC's output detector is on, there's additional current flowing through the R (360 Ohm) and Rg (330 Ohm) on the circuit. I would solve the equation with currents, but having difficulties understanding voltage on the gate relative to T1.
I am sorry, it seems the more I research on the subject the more I get confused. In particular:
- 360 + 330 Ohms of course are passing the current for short period of time, but only if triac is operating properly. How short is this short, and how to ensure resistors are not being burned at specific power rating?
- what is an advantage of considering 3Q triac here, and why this circuit is having the snubber when gate current is in phase with main current? Is it here compensating from the phase shift due to inductive/capacitive load (depends on the right part of circuit) rather than the property of the triac and left part of the circuit? Why then 3Q triacs state (advertize!) they do not need snubbers?
