0
\$\begingroup\$

I'm using a cheap Chinese commercial triac speed controller to control a universal motor that pulls considerable amperage (where "considerable" is no more than 15 amps at 117 volts).

I would like to modify the controller so that it will cut all current to the motor, via a small low current micro switch and associated wiring. I do not know the exact circuit and the device would be difficult to preemptively disassemble. I do have easy access to all three triac terminals, however.

It was suggested to me that shorting A1 to G would disable the triac – it does not.

  • Could I just switch the gate with the micro switch to accomplish what I want?
  • Is significant current flowing through the gate at any time?
\$\endgroup\$
  • \$\begingroup\$ Once triggered, SCRs and TRIACs continue to conduct, even if the gate current ceases, until the main current drops below a certain level called the holding current; this should normally happen once every 1/(60 Hz) (if 60 Hz is your grid frequency). \$\endgroup\$ – Marcus Müller Dec 17 '18 at 18:15
  • 1
    \$\begingroup\$ @Marcus meant to say "this should normally happen twice every 1/(60 Hz)". \$\endgroup\$ – Transistor Dec 17 '18 at 19:46
  • \$\begingroup\$ yes, yes I did. \$\endgroup\$ – Marcus Müller Dec 17 '18 at 20:26
0
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Two ways of holding the gate off. One works.

It was suggested to me that shorting A1 to G would disable the triac – it does not.

If 'A1' is the lower terminal as shown on my schematics then Figure 1 (a) should work. If it doesn't then I suggest that you have mixed up A1 and A2 or that the gate drive current is somehow strong enough to raise the voltage across the switch contact resistance enough to trigger the triac. The first option is the most likely.

Is significant current flowing through the gate at any time?

It may be significant as far as your switch is concerned. It may also be high voltage and if your switch isn't rated for this it may not last long.


Update:

Triacs can be triggered if the dv/dt is very high. In other words, if something else up stream is switching the voltage very suddenly. I can't think what this would be in your case but we know nothing about the controller.

\$\endgroup\$
  • \$\begingroup\$ The triac is a BTA26. The outer pins are A1 and G. Even when these two pins are directly shorted and confirmed with a meter, the pot on the unit continues to control the output level. \$\endgroup\$ – p9993 Dec 17 '18 at 22:48
  • \$\begingroup\$ See the update. \$\endgroup\$ – Transistor Dec 17 '18 at 23:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.