It's pretty straightforward to use transistor as a switch while working with DC voltage. But I am faced with a situation to switch on/off an AC motor (single phase).

Is it absolutely necessary to use a TRIAC or SSR? A stupid question maybe, but what would happen if I were to just make use of a BJT? Would it burn out?

I am trying to build an Arduino project which uses a DC motor following the steps in this Instructables article, but happen to have several AC motors lying around without any use.

  • \$\begingroup\$ similar question answered \$\endgroup\$
    – User323693
    Jul 10, 2015 at 8:20
  • \$\begingroup\$ What kind of AC motors? If they have brushes they are likely to be "Universal" motors which also work on DC, and that opens up more options to control them. \$\endgroup\$ Jul 10, 2015 at 9:49
  • \$\begingroup\$ @brian, i dont know that. am talking about this(shopping.rediff.com/product/…) \$\endgroup\$
    – user221238
    Jul 10, 2015 at 13:07
  • \$\begingroup\$ Unlikely to be a brushed motor. Probably shaded pole, AC only, fixed speed. \$\endgroup\$ Jul 10, 2015 at 13:32

2 Answers 2


The capstan motor of one of my tape decks is speed controlled by a transistor that shorts a bridge rectifier - it should work for on-off control too.


simulate this circuit – Schematic created using CircuitLab

The tape deck uses a transformer to isolate the motor 9and the control circuit) from the AC mains, however, the same control could be achieved using an optocoupler to isolate the control circuit from the transistor and motor, which then can be connected directly to 220V.

  • 1
    \$\begingroup\$ Holy hacks Batman! \$\endgroup\$ Jul 12, 2015 at 14:12
  • \$\begingroup\$ @MarkKCowan Well, this is what is used in a Revox A77, seems to work just fine. \$\endgroup\$
    – Pentium100
    Jul 12, 2015 at 15:38
  • 1
    \$\begingroup\$ I meant "hack" in a complementing, "clever" sense :) \$\endgroup\$ Jul 12, 2015 at 15:39

The first problem that springs to mind is that you will damage the base-emitter region when the ac reverse biases the transistor. Base-emitter regions are normally specified as being able to handle a reverse voltage of between 5 and 10 V max.

Maybe you should use a dual back-to-back mosfet solution: -

enter image description here

This circuit is used inside solid-state-relays. But beware - the photo-voltaic opto-isolator hasn't got great switching speed so PWM doesn't work too well.


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.