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Dimmer noise suppression

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kybert

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Hi,


I have a dimmer circuit that works by controlling the phase of the AC lamp, this is done with a triac.

I have read that mains supression is required, and i acheive this via a series toroidal dimmer supression choke in series with the lamp, and a X2 capacitor.

Some circuits i have seen use a 100uH choke, and a 150nF cap, others use 1mH chokes and 100nF cap.

How do i work out what value choke and cap i need ?
 

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I can't see anything wrong with that.
 
Guess work is not really design. What makes the values "correct" ?

There must be a formula, or somthing to decide on the values for the choke and the cap.
 
kybert said:
Guess work is not really design. What makes the values "correct" ?

There must be a formula, or somthing to decide on the values for the choke and the cap.

Why must there be a formula?.

It's simply to filter harmonics generated by the switching during every half cycle - the values depend where you want to start filtering.
 
Well what are good values and why.

Do i really need supression for the dimmer? I see circuits with 100uH and others with 2-3mH, thats a huge difference!
 
kybert said:
Well what are good values and why.

Do i really need supression for the dimmer? I see circuits with 100uH and others with 2-3mH, thats a huge difference!

Use what you have, you DO need suppression, as it sends RF back up the mains. So pick what inductor you have, and go from that.
 
Nigel Goodwin said:
Why must there be a formula?.
Because that's what we're taught in college.

It's simply to filter harmonics generated by the switching during every half cycle - the values depend where you want to start filtering.
I know, let's look at the waveform and do a Fourier transform to figure out the frequency range we need to filter.

Oh no, my calculations show that there are harmonics going up to infinity!

I'll need s special inductor with amazing core characteristics to filter anything above 100kHz to gamma rays. I've contacted NASA and they sell one for two million pounds each and the college won't provide the funding for my project!
 
I was joking.

The component values you're using are fine.
 
kybert - my thoughts on this - that there are several answers (yours and theirs) and they are all correct - within limits. This applies to many of the posts here.

You are not wrong to think that there is some science and methodology that applies to the design in your situation. The problem - you need to define the inputs and what you expect as an output in order to "calculate" the values. You probably can't precisely define the waveform at the filter input - and it's unlikely you know how much attenuation must be offered by the filter.

As implied - just about any combination of components will provide some suppression. What you might do is try a combination based on what you have or what you can afford. I think it's likely that manufacturers of commercial equipment design their filters based in part on experience and calculations but also based on experimentation.
 
The only thing I see is that the 100uh choke will no doubt carry more current than the 1mh. This is the general rule I know and you can get the opposite.
I have both types in my general stock and it is without a doubt the 100uh is wound with a larger wire size.
 
Well, they could both have the same current rating, it's just that the 1mH choke will be 10 times bigger than the 100:mu:H choke.
 
Happy new year everybody!

OK, i see what you guys are saying now. I'll use the values in the circuit.

Can someone tell me why R9 and R10 are the values that they are, and what they do in the circuit?
 
It's all about rise-time

There will be a minimum L x C combination to give EMI suppression.
But higher values of L will give a higher rise-time eg 200 microsec for professional dimmers.
Just Google "dimmer chokes" and you will find dimmer choke manufacturers who offer this.
 
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