Film-Tech Forum ARCHIVE: Consumer SCR or Triac Dimmers -- Automated?

This is topic Consumer SCR or Triac Dimmers -- Automated? in forum The Afterlife at Film-Tech Forum ARCHIVE.

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Posted by Frank Angel (Member # 248) on 08-10-2010, 12:04 PM:

As I move along with rebuilding our home theatre; I need to automate a simple dimming system. For many decades I have been using standard off-the-shelf SCR dimmers -- the household SCR units you can get for under 10 bucks -- to dim the house lights, curtain warmers, down lights and accent lights. I would dim or raise lights manually with the 4 SCRs mounted in a 4 gang box in the rack.

With this new setup, I want to start the movie and automate the lights and curtains. My first solution was to go with X10 controller modules instead of the SCR dimmer. The X10 has a remote interface that lets me dim remotely and with a single button push, but then I ran into a big negative with this design. The X10 dimming modules only have FIXED dimming rates and it's way to fast. I want each of the four channels to dim at different speeds.

My thought was, short of looking for an actual theatrical pro dimmer system which would be way overkill, I was hoping I could use an FRS ramp generator; these are simple modules smaller than a paperback that can are fed 12vdc and by shorting to ground one of two pins, the output is a voltage that can be raised or lowered from 0 to 12vdc out at a speed you set with pot, anywhere from .1 sec to 5 min. Perfect.

BUT -- and here's my question: if I use standard household SRC or Triac dimmers, shouldn't I be able to break them opened and with some buffering, feed the variable 12vdc ramp voltage in place of whatever the SCR dimmer pot is varing? In other words, eliminate the SCR potentiometer and substitute the ramp voltage so that the SCR becomes the dimmer controlled by a variable speed FRS ramp generator. I push a button and the SCR dims up or down at whatever speed I've preset on the ramp gen. The X10 macro module will let me set up a "cue" where I push a button and the house lights begin to dim say at a slow 40 second count and then at 20 second the curtain and accent lights dim at a 25 second count, then at 10 seconds the curtain motor triggers to open. All with the push of button on X10s key chain transmitter bob.

Thing is, where do I find a schematic for these SCR dimmers (or a Triac may be a better type of dimmer?) so I can modify the dimmers to accept a ramp voltage? What kind of buffer circuit will I need to protect the FRS ramp gen? Should I just experiment and read what voltage the pot is varing and then try to match that with the ramp generator output?

If I can get the dimmer to respond to a varible 12vdc (or modified to whatever the SCR need to see), then I've got exactly what I need without spending thousands on a commercial theatre dimmer unit.

Anyone have intimate knowledge of how these simple SCRs work and if it is possible to safely buffer and external voltage? I'd rather not blow up more FRS ramp generators than I have to. [Eek!]

Posted by Bruce Hansen (Member # 281) on 08-10-2010, 06:07 PM:

There is AC on the pot in those home type dimmers, not DC. The wiper of the pot feeds a capacitor and resistor, then the gate of the TRAIC. As the pot is advanced, there is more and more of the AC waveform applied to the cap, and the voltage across the cap reaches the trigger voltage of the TRAIC earlier and earlier in the AC cycle. This turns the TRAIC on earlier and earlier in the AC cycle, and more of the AC cycle is allowed to go to the load (the light). This will not work with DC. The TRAIC must be able to turn off when the AC cycle passes through zero volts.

Some years ago, I built some dimmers that used a photo resistance cell in place of the pot, and glued an LED to the photo cell. By varring the current through the LED I could work the dimmer. This worked OK, but the photo cells would drift.

Don't forget, this is all HOT. There is no transformer to isolate anything from the AC line.

Posted by Robert Minichino (Member # 3495) on 08-10-2010, 07:04 PM:

How about something like this:

http://www.smarthome.com/2486DIV6/KeypadLinc-Dimmer-INSTEON-6-Button-Scene-Control-Keypad-with-Dimmer-Ivory/p.aspx

It works like X10, but better.

Posted by Frank Angel (Member # 248) on 08-10-2010, 10:41 PM:

Thanks guys.

Bruce, I was all ready to start experimenting, ripping apart a few household dimmers now that I am armed with the detailed information you posted, but then I went to the link Robert posted and it seems, if I understand their description of what the Insteon dimmer system does, that product is perfect for what I need it to do. It allows control of the ramping time -- just what the X10s don't. For its low price, I think I will forego the risk of electrocution and go with the Insteon unit. [thumbsup]

Posted by Robert Minichino (Member # 3495) on 08-11-2010, 10:37 AM:

And if you still feel like tempting fate with DIY:

http://sound.westhost.com/project62c.htm

[Big Grin]