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> need help with RFI and triac light dimmer
_N_
Posted: May 04, 2005 11:03 am
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Hi

I'm designing a light dimmer that controls the light with a triac.

I see that the triac switching on and off causes a lot of noise which needs to be filtered. The RFI can go up to MHz frequencies.

Manufactuers recommend that I have a inductor in series with the triac and a capacitor in parallel with the both. Capacitor removes high frequency noise, inductor the lower frequency noise.
I see that the capacitor should an X type RFI filtering capacitor.

The problem I'm having is choosing an inductor.

The inductor should be around 200 uH, and be able to carry up to 4A. The theory is that with the inductor in series, the switching on and off of the triac will be slower, say 100 slower, resulting in a smaller dV/dt, therefore reducing noise.

I had a look for inductors, and the cheapest I can find (probably more expensive than all the other components on the circuit smile.gif ) is a common mode suppression choke, that reduces RFI.

The thing I can't understand is this.

They recommend that I have an inductor in series with the triac. But the one I'm looking at has 4 connections. It is a ferrite ring with 2 windings. So, am I suppose to just use one of the windings on the choke and not worry about the other?

An inductor which only has 2 connections costs double the one with 2 windings, so I would like to use this one that looks like a transformer - they call them open former??

Also, the noise created by the triac travels through the wiring in the house and that acts as an antenna and then broadcasts the noise, so, is it a wise idea to have the inductor in series with the triac, or should I have the choke on the Negative and Phase as they come into my circuit??

If someone can explain to me how to get rid of the RFI, like where to have the inductor I will be really happy, smile.gif

Thanks, wacko.gif unsure.gif huh.gif
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Sch3mat1c
Posted: May 04, 2005 03:27 pm
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A common mode choke is one where two windings are used, one for each line (hot and neutral), connected in phase, so that differential signals (like the 60Hz line that you don't want any loss on) are not attenuated, while for common mode signals, they act in parallel and look like an inductor, attenuating it.

Not easy to imagine it working without a big electronics background...

Tim


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_N_
Posted: May 05, 2005 02:22 am
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Ok, I can kind of understand that,

so, a differential mode choke is just and inductor, with 2 connections.

where as, common mode choke is like a transformer, with 4 connections on it?

If I use a differential mode inductor in series with the triac, would that eliminate the noise, or would the noise still travel through the wiring,

Is it better to use a common mode choke? - with appropriate capacitors.
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EdwardM
Posted: May 05, 2005 01:24 pm
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Hi _N_

normally, you would design the ferrite ring inductor with a single winding but in this case you have two...

It's feasible to wire the windings in series, making sure that you join the end of winding 1 to the start of winding 2 (ie make sure that the coils are truly in the correct series sense) If you then have sufficient inductance AND the core material is suitable for this application you should find that it works OK.

CODE
Also, the noise created by the triac travels through the wiring in the house and that acts as an antenna and then broadcasts the noise, so, is it a wise idea to have the inductor in series with the triac, or should I have the choke on the Negative and Phase as they come into my circuit??


The triac is acting as a very efficient switch that cuts off power at a particular part of the AC waveform. It's the cut-off action (step function, lots of harmonics) that causes the problem of RFI. An inductor in series with the triac essentially changes the step into a more gently changing slope with consequent reduction in RFI.

When using a ferrite ring inductor, the magnetic field is contained almost completely in the ferrite, ie no external field generated.

Best of Luck

Ed
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Sch3mat1c
Posted: May 05, 2005 07:23 pm
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The difference between a common-mode and differential-mode choke is exactly: switch two pins.

Consider the two inductors of a given inductance as a 1:1 transformer. If you put a voltage on one winding, it also appears (equally) on the other. Say your circuit is bouncing up and down, getting rowdy, too rowdy for your radio reception. If both lines are moving in parallel (i.e., common mode), the noise voltage appearing across a capacitor placed across them is zero, hence no filtering is possible with a capacitor. Oh no! If you add the inductors in series, the powerline voltage (which you need) is going up in one and down in the other, at any given moment. The current drawn by the noisy device causes a voltage drop across each winding inductance (say, 20mH, 3A and 60Hz, that's 23V drop!), but since the flux is mutually coupled, they both induce the same inverse voltage drop that they are creating, and thus no 60Hz power is wasted. As for the noise, the inductors act mutually in parallel, causing a springyness which attenuates the noise.

Likewise, if you mix up the leads on one, the flux of both windings will sum, causing the 60Hz power to be squishy and allowing noise straight through!

Compare this to a single series choke (or two, one on each line, isolated so no flux is shared), which attenuates all signals as a low-pass filter, and you'll see it is much better - at least for rejecting common mode noise!

Tim


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_N_
Posted: May 06, 2005 07:19 am
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ok, thanks for that info, smile.gif
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Søren
Posted: May 07, 2005 04:19 pm
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Hi,

QUOTE (EdwardM)
The triac is acting as a very efficient switch that cuts off power at a particular part of the AC waveform.  It's the cut-off action (step function, lots of harmonics) that causes the problem of RFI.
Not at all !
In phase control, you cut off at the zero cross (of the current), or rather, the TRIAC does this automatically. You control the phase angle it kicks in and the most noise is generated when turning it on at 90°, since the amplitude will be the largest.

QUOTE
When using a ferrite ring inductor, the magnetic field is contained almost completely in the ferrite, ie no external field generated.
That's not the reason, it's the dI/dt limiting that reduces noise !

A powdered iron core from a PC supply will be fine for this, just fill it up with AWG15 or better (for 4A) in a single layer and you're all set smile.gif


Regards,

Soeren


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EdwardM
Posted: May 07, 2005 05:50 pm
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Hi

thanks for helpful comments...

in first quote was thinking about modern IC's which switch on at zero crossing...

QUOTE
An inductor in series with the triac essentially changes the step into a more gently changing slope with consequent reduction in RFI.


Can't understand why you referred to (ferrite ring inductor) as it's obvious from my quote that a 'gently changing slope' is dI/dt limiting.

thumbsup.gif Have an even nicer day thumbsup.gif

Ed


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