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2 problems with phase control of a lamp

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Newbie level 6
Mar 15, 2011
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problems with phase control of a lamp

Hi - I am creating a lamp dimmer using a micro to do phase control. There are 2 issues that I am having problems understanding. I want isolation from mains to my micro so I am using an ILD620 as a zero cross detect. I am using a 50K resistor in series with the mains. This device works but the output "pulse" is really a sinewave with slow rise time so my micro is detecting the zero cross much earlier than it should because of the slow rise time. I am just polling an input pin to detect the pulse.

If I reduce the 50K resistor down to 10K I get a pulse with much faster rise time, but that gets me 1.44W of power on that resistor...should I be using a transformer to bring the mains down to 12 V before coming into the ILD620 or do I just have to get a high power resistor? Or is the correct technique to use a comparator circuit to detect the top of the pulse regardless of rise time?

Any help on this is much appreciated.

There's the concept of using a series capacitor to limit AC current.

A capacitor has reactance. Doesn't generate heat as a resistor does.

A few picofarads may do the job, depending on how much current you're drawing.

This technique is especially handy for driving LED's from house current. Try an internet search for sample circuits.

Of course using a capacitor will introduce phase change. Not sure if it will help you or hinder you in that regard.
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