Many companies that manufacture lighting control systems for residential and
commercial applications use dual SCR's in their dimmers.  Many solid state
relays from companies like Crydom use dual SCR's.  Triac's don't always
conduct evenly on both sides of the sine wave. When used with low voltage
transformer type lighting (12-24 VAC, the uneven conduction can cause
transformers to run hotter as there is some dc component introduced.  In all
the testing I've done I've never seen this happen, but the engineers from
the companies who use the dual SCR approach spout this line.  My company,
Centralite, uses Crydom dual SCR solid state relays.

Lutron, the world leader in dimming uses TRIAC's. They are against dual
SCR's as in certain situations only one SCR can blow. This will cause a dc
voltage to go into transformers. They really push the safety side of TRIAC's
vs. SCR's. Recently on one of the homes we did locally, I found a crydom SSR
that had 1 SCR blown. When I turned the light off it only dimmed slightly.
It's the first time I've seen 1 of the SCR's blow. I think it was defective
from the factory.

Some countries are now banning Thyristor dimmers due to the noise the
radiate and conduct.  FET and IGBT dimming (reverse phase dimming) is
dropping in price and these circuits don't need chokes and snubbers and
other fitering circuits to work.

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Spehro Pefhany
Sent: Thursday, November 27, 2003 6:32 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC: ] Manipuling 220V light bulbs with PIC


At 07:31 AM 11/27/2003 -0500, you wrote:


>Don't use SCRs. They only conduct in one direction and are verbotten in
>lighting
>circuits in some countries.

SCRs are fine, you just have to use them right. For example, on the DC
side of a bridge rectifier.

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