Electronics > Projects, Designs, and Technical Stuff

Triac control - 1 or 2 resistors?

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ricko_uk:
Hi,
with reference to the attached pictures, to enable the TRIAC some circuits use only one resistor between gate and one of the terminals and other circuits use two resistors (one between gate and one terminal and another between gate and the other terminal. Why do some use one and other use 2 resistors? Any pros and cons to each solution?

Which is correct?

Also I assume that T1 and T2 are fully swappable correct?

Thank you :)

Zero999:
You forgot the attachments.

ricko_uk:
Oops, attached now :)

Doctorandus_P:
Wat is an T1 and T2
The A1 and A2 terminals of a triac are NOT interchangeable.
An image search reveald they're somtimes also called "MT1" and "MT2", maybe other names too. But still, they are not interchangable.


In the schematic with the MOC3021 and the BT136 you only have a resistor to limit the trigger current.

In the schematic with the MOC3061 and the BTA41 you also have a resistor to lower the sensitivity of the triac.

Why do you think (assume?) these different components would work in the same schematic?
The values of the snubber network are also different between these schematics.

When looking at some pictures I saw this:
http://www.penguintutor.com/

It shows clear differences between the MT1 and MT2 terminal.

Ah, the resistor...
Whether you need (or are wise to) use the extra resistor depends on a lot of things.
Simple answer is, sometimes it's needed, and it almost never hurts to put it in.
Main factors are the leakage of the Opto Triac, minimum sensitivity of the big Triac, dU/dt stuff, leakage over (old dirty) PCB's and probably more.

Zero999:
Both of the schematics are posted are correct, as mentioned above the gate-T1 resistor is there to reduce the TRIAC's sensitivity, thus making false triggering from any leakage current through the opto-TRIAC unlikely.

T1 and T2 aren't interchangeable. The TRIAC's gate is referenced to T1. If you connect the gate to T2, via a resistor, it will always be on, whilst if the gate is connected to T1, it will always be off. The TRIAC is triggered when the gate is more positive/negative, than T1.

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