Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 09:43 PM 2/14/2007, Tesla list wrote:
Original poster: "D.C. Cox" <resonance@xxxxxxxxxxxx>SCR's can be phased controlled in special circuits --- I'm presently working on solving these problems.Hopefully, our dual back to back SCRs with our unique trigger circuit will totally replace variacs in the near future.
Most industrial controllers using thyristors for controlling inductive loads use back to back SCRs rather than a single TRIAC (TRIACs are good for lightbulbs)... Back to back scrs gives the device that's on a whole half cycle to recover (i.e. waiting for the current to go through zero!)
There's a circuit in the old GE SCR handbook on how to do the triggering for back to back SCRs that's very clever, and works for inductive loads.
There's also a clever scheme (which I described, gosh, 10 years ago on the list?, and someone has subsquently tried and built) that uses a power FET inside a bridge rectifier in series to make a AC PWM controller. I was fooling with controlling AC motors in fans at the time, and needed a quiet way to reduce the voltage to replace a variac. Using a conventional SCR based controller resulted in a very "buzzy" sound at low speeds because of the short pulses. The challenge with implementing my idea is that the FET and its drive circuitry requires a "floating" power supply, which is expensive in a manufactured item.
Still, a work in progress. Dr. ResonanceGot first light tonite from my single 833a VTTC. It has a few problems. Still waiting on some parts but, It works pretty good. Notice the leak at the top of the feedback coil. I can only power up to about 50% on my variac now with that going on...Looking to rebuild on a compact base soon.I also had it running last night in a staccato mode of some sort using my computers parallel port, a 3055 transistor and a large SCR. This is why I ask the question about the SCR's. You remember, SCR's are either off or on? Correct or not? I then change over and was using a small LED blinking curcuit I have and a 9 Volt Battery.<http://www.youtube.com/watch?v=p0iwnxCC42w>http://www.youtube.com/watch?v=p0iwnxCC42w