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Using transistors to control an AC voltage

CallumA

Jul 22, 2013
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Hi!

I've been battling with this one all day!
What I'd like to do is make a kind of automated dimmer for mains AC (240V here) so I thought it would make sense to use transistors.

The issues I have here is:
1. I can't adjust the live voltage - I'd prefer to add the transistor on the live side so I can shut off the live voltage completely (or close enough to completely) for bulb changes. The problem with this is I'm using a 5V switching voltage and that's never going to get enough current from base to emitter to switch the transistor.
2. On the neutral side I've managed to get the positive part of the wave adjusted but not the negative (a diode switches to an NPN between live (via bulb) and neutral for the positive art of the wave and that works).

As I said in the first issue, I don't really want to go with the transistor on the neutral side, I also don't want to have to use a stepper motor and a pot :D

I'm still quite new to logic gates really but I'm trying to learn.

I haven't tried anything in practise, I've just tried stuff out in an electronics simulator.

Attached is a copy of the schematic I've been trying. There's a space left after D2 where I haven't gotten anything to work. Again, I'd prefer to completely scrap this for a design on the live side.

Thanks in advance!
 

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BobK

Jan 5, 2010
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A transistor is not good for controlling AC. Use a triac. And if the AC is mains voltage, use an opto-isolated triac. Or just get a dimmer switch for a couple of bucks, which is the same thing.

Bob
 

CallumA

Jul 22, 2013
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A transistor is not good for controlling AC. Use a triac. And if the AC is mains voltage, use an opto-isolated triac. Or just get a dimmer switch for a couple of bucks, which is the same thing.

Bob

I was going to use opto-isolated transistors but I was not aware that traics were analogue, I thought they were just a matter of on-off.

The idea for the light is that it's remote controlled so a dimmer switch isn't quite what I'm looking for.

Thanks for your help.
Callum
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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You are correct, triacs are either on or off.

However they can be used to make a light dimmer for lights powered by AC because you can turn them on after a delay from the start of each half cycle.

A short delay means the light gets almost all of the half-cycle. A longer delay means it gets less.

This is a form of pulse width modulation.

Our eye doesn't see the flicker because (a) an incandescent filament doesn't cool completely between half-cycles, and (b) because being turned on and off 100 or 120 times per second is way faster than we can normally perceive.

If you used an analogue device to simply reduce power to the light, it would need to dissipate the excess energy as heat. That is both wasteful, undesirable, and more costly to manufacture.
 
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