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.
>
> Part 1. The thyristor manufacturer tells me that this is because I am
> exceeding the max recommended gate voltage, which is almost always the case.
> I *assumed* this was OK because of the short duration of the gate pulse as
> compared to old analog designs long pulses & back porches.


You may have combination of factors. What is the
minimum load? Do you have enough holding (through)
current at the low conduction angles? If using PIC
gate drive and optocouplers why only use a very
short gate pulse? The pulse may occur at a point
corresponding with a mains voltage anomaly (spike,
lag etc) where the holding current may not be enough
espcially at low angles which are lower anode currents.

It would be better to keep gate voltage within the
proper levels, and a nice constant voltage, and use
a full-length PIC pulse from the start of conduction
to the end of the half-cycle.
-Roman

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