There are some next generation theatrical dimmers that use IGBT
(insulated gate bipolar transistors) devices to control AC power.
Apparently, the dimming system works the opposite of SCRs and Triac dimmers,
in that the dimmer conducts at the start of the AC wave and shut off after
the appropriate amount of power has been delivered to the light. The system
is more resistant to dirty AC power, since it is not dependent on zero cross
detection. The dimmers use mathematical intergration to determine when the
correct amount of energy had been delivered.

   I was reading the lit (the company is Rosco - Entertainment Technologies)
and the IGBT method of dimming is much more efficient and produces much less
in the way of RF harmonics than does the SCR/Triac method. The dimmers also
apparently run much cooler than traditional models do.



At 06:01 PM 4/5/96 -0500, you wrote:
>I don't know if this is appropriate to this list, but the device that
>needs to do it is controlled by a PIC16C65...
>
>Does anyone know of an easy way to control the voltage on a 120VAC line? I
>need to vary the light from some lightbulbs, and I don't want to use the
>standard SCR solutions because they produce artifacts that seem to wear
>the bulbs out faster.
>
>I understand how to do D/A to get a variable DC voltage up to some small
>voltage. How can I get from there to the 120VAC?
>
>A cryptic remark from someone has led me to believe that there may be a
>solution involving a PWM signal and some power MOSFETs. I think I can see
>how that would work, but has anyone done anything like it who could
>reassure me that I'm not going to fry myself or my PIC?
>
**********************************************
* Andrew Yalowitz
* Theatrical Assistant
* University Unions and Student Activities
*
* ayalowit@vt.edu