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xxguitarist 26th March 2009 05:18 AM

Use of transistor for switching 120AC
Hoping someone could give me hand with this concept.
I need to switch AC to a load based upon a small voltage input of variable amplitude.

My idea was to use a FET, with the input at the gate, load(consider it a lightbulb) between 120AC and the drain, and the source straight to ground/ the dead half of a household AC plug.

I'm concerned about transistor selection, and the potential for reverse biasing on the negative swing of the 120AC.

Any input is appreciated.

Bone 26th March 2009 09:39 AM

Why not use a triac - its what they're for

xxguitarist 26th March 2009 12:46 PM


Originally posted by Bone
Why not use a triac - its what they're for

Wasn't aware of this component. Looks perfect.

sreten 26th March 2009 02:35 PM


Regarding the mains voltage supply. If you do not know what you are
doing leave the mains alone before you kill yourself or someone else.


xxguitarist 26th March 2009 04:43 PM

I appreciate your concern, I have a good background in electronics, my coursework is just lacking in 120AC coverage. I have looked at datasheets on the triacs since, and it looks like it will work very well.

sreten 26th March 2009 05:17 PM


There is a good reason mains circuitry is not usually covered.
Proper insulation practise must be followed and you'll need
something like an optical-isolator for the control input.


dangus 27th March 2009 07:02 AM

If you want an incandescent light to flicker to the music, look for "color organ" schematics. Old electronics hobby magazines, or books with titles like 1001 electronic projects in the library may have circuits.

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