At 12:02 PM 7/29/02 -0400, Chris Loiacono wrote:
>Here's a two part posting:
>The background: I am having trouble with my PIC controlled phase angle
>controller when using it on a 480V AC line. The design works great on 240,
>and I have a number of these in the field running happily. The gate firing
>h/w is fairly simple, taking the appropriate line thru a current limiting
>resistor, then an opto-isolator to the gate. The PIC derives the firing
>angle from various inputs. The weak point in this firing scheme is that the
>gate voltage will vary with the firing angle, according to the level of the
>AC wave at the time the PIC outputs via the optocoupler. The trouble appears
>when gating the thyristors at phase angles that correspond to low sine wave
>voltages, when gating occurs, but turn-on of the devices is not consistent.

Chris - why not try using an external gate driver just to try a couple of
things out.  I routinely drive the gates of SCRs and triacs *very* hard:
600-800 mA pulses for small triacs and SCRs, 3-10A pulses for larger
SCRs.  My gate driver power supply ranges 12 thru 20 Vdc (depending upon
project) and I use a picket fence type pulse stream: 6 us pulses at 100 -
130 us intervals.  Note that I use separate gate drive power supplies in my
designs just so that I can avoid the problems of not enough gate drive near
the zero crossing point - the gate voltage and current is independent of
the anode - cathode voltage.

My thought is that if you can get reliable gate triggering with something
similar, you should be able to adapt your existing drive circuit into
delivering the same result.  But I think that the key is to use something
external just to allow you easy changes of pulse width, rep rate, voltage
and current.

dwayne

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Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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