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PWM control of 120v AC lights

I have a PIC PWM chip that has built in programs for modulating three channels (RGB) at +5v levels; Works great with a MOSFET on each channel driving an RGB LED array. But now I would like to be able to drive a R, G, and B incandescant light bulb (more specifically, individual R, G, and B Xmas light sets).

I have done some searching on similar topics, but am a little confused how to PWM switch 120v AC. Can I just use a opto-isolated triac on each channel? Is this the same thing as a Solid-State Relay? The PWM switching speed looks to be around 350 Hz, so it would not be synched w/ the line (60 Hz) frequency. I don't know if this would cause problems/extra heating/triac burn-out if it did not switch at the zero crossing.

Or should I investigate means to convert the 120v AC to 120v DC through appropriate switch?

Thanks for any help.

Kevin

Reply to
speedytahiti
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You could probably chop the AC line with a high voltage MOSFET, a suitable driver, and a bridge rectifier, but there are safety and possibly EMI issues.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

yup. Thats the usual and cheapest approach.

No, not the same. Triacs stay on once fired, until the mains i zero crosses. So to dim without flicker you do need to synch your control to the line frequency.

It is possible to get triacs to switch off with a load on them, by using 2, one direct and one thru a capacitor, but this isnt such a cost efficient option.

NT

Reply to
meow2222

In article , wrote: [...]

This is usually refered to as "phase control". You delay the turn on to some time after the zero crossing. The triac is turned on an latches on until the line current hits zero.

You need to opto or transformer isolate the PIC for safety reasons.

The power supply for the PIC can use a small transformer and full wave rectifier. If you detect the zero crossing at the transformer's windings, this will be close enough to what is seen in the high powered part of the circuit.

You should put EMI filters on both the input and output sides of the circuits. These sorts of things tend to make lots of RF.

OR:

"Saturable Reactors" can be fun too. Unfortunately, they are kind of large. You can home brew one like this:

T1 T2 Line ----------)))))))))------------)))))))))--------- Load ========= ========= ))))))))) ))))))))) ---+---! ! ! ! ! ! ! ! Control === C1 -------------------- ! ! ---+------------------------

T1 and T2 are 2 identical transformers. Since the control side is wired in series bucking, there will normally be no AC current in C1.

If you feed a control current into the circuit, the cores in T1 and T2 will alternately saturate. The one that saturates will natually provide a low impedance to the AC from the line.

C1, ACwise hooks the unsaturated transformer to the saturated one.

C1 has to be largish and AC rated to handle the current.

Since the control voltage only has to overcome the IR drop in the windings, a quite small amount of power can control a quite large load this way.

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Reply to
Ken Smith

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