Mains PWM Light Dimmer

[davepusey]

I need to build a PWM Light Dimmer that dims my mains (244V 50Hz) lights to a fixed setting of 15.8% brightness (when logic input is 1) and 100% brightness (when logic input is 0). Can anyone help me with a circuit please.

 

[Nigel Goodwin]

I need to build a PWM Light Dimmer that dims my mains (244V 50Hz) lights to a fixed setting of 15.8% brightness (when logic input is 1) and 100% brightness (when logic input is 0). Can anyone help me with a circuit please.

You don't use PWM for dimming AC, that's for DC power control. You use use 'phase control', this consists of detecting the zero-crossing point of the mains and then turning the triac (or SCR with a bridge rectifier) ON at the point in the mains cycle to give the brightness you require. This fast switching can cause considerable interference, and it's important to filter the mains into the dimmer.

 

[davepusey]

Ok then. So how do i do all this?

EDIT: Even better would be a circuit for a 38.5V AC supply as that will dim my lights to 15.8% brightness just the same.

 

[grrr_arrghh]

I googled "light dimmer circuit" and the first hit had several circuits and other relevant info. Try it.

 

[Exo]

Microchip has an application note about a dimmer with a pic12c508...

 

[davepusey]

What about the circuit for a 38.5V AC supply i mentioned earlier?

 

[Nigel Goodwin]

What about the circuit for a 38.5V AC supply i mentioned earlier?

It's called a mains transformer 8)

 

[Sebi]

Yes. But if You need to hold the exact brightness, need some feedback from light (with LDR or photodiode), so the voltage must be variable, depending from mains voltage and the really brightness. You can do this with Nigel's method or a transductor. (this was before the semiconductor solutions...)

 

[davepusey]

It doesnt matter as long as it is 38.5V AC.

 

[Exo]

Why does it have to be 38.5V AC?. Any special reason ?

 

[davepusey]

Because that is what my multimeter reads when i have the lights dimmed to the required level.

 

[Exo]

well, then i agree with nigel, just get a transformer...
you could use a relay to choose between the transformer, or feed the lamp directly (for your 0 - 1 input)

 

[davepusey]

well, then i agree with nigel, just get a transformer...

Yes, but is isnt a transformer that i know of that takes 240V in and give 38.5V out.

 

[gerty]

How about a Vari-ac (variable transformer) you can dial in whatever voltage you want.

 

[Nigel Goodwin]

well, then i agree with nigel, just get a transformer...

Yes, but is isnt a transformer that i know of that takes 240V in and give 38.5V out.

40V or 35V would be plenty accurate enough!.

Or just buy a light dimmer, they cost very little and are freely available at any DIY store - simple switch it in and out of circuit, and preset it to the brightness you want.

How did you dim the light to get the 38.5V reading - I'm assuming you used a dimmer for that?. It's also fairly likely that your meter read completely wrong - unless it's a true RMS meter, in which it will be clearly marked as such. Conventional meters read the 'average' voltage, with the scale calibrated for a sinewave input, anything else will read incorrectly.

 

[davepusey]

How about a Vari-ac (variable transformer) you can dial in whatever voltage you want.

Yeah but they are really expensive any really chunky. See **broken link removed**