Will a 120vac dimmer (rheostat) work with 12vac power input?

Thread Starter

DocK

Joined Apr 7, 2022
26
I would like to dim 12vac landscape lights with a 50w max load. Will a 120v knob controlled dimmer work?

I see many 12vdc and 120vac dimmers. The only 12vac dimmer is >$100.

Thanks,

Michael
 

Ian0

Joined Aug 7, 2020
5,504
A mains voltage dimmer will dim low-voltage filament lamps supplied via a transformer. That's a proper copper-iron transformer not the so-called "electronic" transformer.
 

Thread Starter

DocK

Joined Apr 7, 2022
26
A mains voltage dimmer will dim low-voltage filament lamps supplied via a transformer. That's a proper copper-iron transformer not the so-called "electronic" transformer.
Yes, I'm using a copper-iron transformer.
 

DickCappels

Joined Aug 21, 2008
8,812
It is unlikely to work using a commercial because of one got-ya': All of them that I have seen use a diac that triggers the triac when there is 30 to 36 volts DC across the timing capacitor. With a 12 VAC (RMS) input voltage you cannot get enough voltage for the diac to avalanche and trigger the triac (or other thyristor).

With such a low AC voltage it would be difficult or more presicely impossible to find a diac or pair of transistors to substitute as a diac with a breakdown voltage low enough so that you would still have plenty of dimming range.

Last month a person in a developing country asked me to show how to make a lamp dimmer using the 12 volt VAC winding on a transformer to dim a 12 VAC tungsten bulb. The circuit generates a trigger pulse for the NE555. By adjusting R7 the fraction of each half cycle waveform can be adjusted. Yeah, this uses a lot of transistors and other discreet components because very few integrated circuits are available where he lives. When I checked back with him he said it works "perfectly". Electric power where he lives in 50 Hz. Just a thought.

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Thread Starter

DocK

Joined Apr 7, 2022
26
It is unlikely to work using a commercial because of one got-ya': All of them that I have seen use a diac that triggers the triac when there is 30 to 36 volts DC across the timing capacitor. With a 12 VAC (RMS) input voltage you cannot get enough voltage for the diac to avalanche and trigger the triac (or other thyristor).

With such a low AC voltage it would be difficult or more presicely impossible to find a diac or pair of transistors to substitute as a diac with a breakdown voltage low enough so that you would still have plenty of dimming range.

Last month a person in a developing country asked me to show how to make a lamp dimmer using the 12 volt VAC winding on a transformer to dim a 12 VAC tungsten bulb. The circuit generates a trigger pulse for the NE555. By adjusting R7 the fraction of each half cycle waveform can be adjusted. Yeah, this uses a lot of transistors and other discreet components because very few integrated circuits are available where he lives. When I checked back with him he said it works "perfectly". Electric power where he lives in 50 Hz. Just a thought.

View attachment 265526
Thank you for the details. I am a novice when it comes to electricity but what I hear you say is a commercial dimmer won't work well because there is not enough power in the 12vac input to permit the dimming. Thanks for the reply.
 

Ian0

Joined Aug 7, 2020
5,504
Just to clarify what I said in post #2. You connect the primary of the transformer to the output of the dimmer and the lamps to the secondary. The dimmer is supplied by mains voltage.
 

Thread Starter

DocK

Joined Apr 7, 2022
26
Just to clarify what I said in post #2. You connect the primary of the transformer to the output of the dimmer and the lamps to the secondary. The dimmer is supplied by mains voltage.
I see... I didn't understand that. I'll have to think about it. That arrangement may not work since I have multiple 12v circuits on the low voltage side of the transformer.
 
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