Hi Matthias, Sorry to bother you again with this but the Triacs I found in Digi-Key (which I am pretty much reliant on for prototyping) have gate thresholds of 0.2V at 110C to 2.0V at 25C. Some are higher at 2.5V You mentioned a 1.0V threshold. Also, I am not sure what A1 and A2 are connected to. Is A2 ground and A1 my input voltage? Like I said, I know nothing about Triacs and have no literature on their operation. The data sheet is not a lot of help. Thanks,
At 01:25 PM 2/11/03 -0800, Matthias Weingart wrote:
On Tue, Feb 11, 2003 at 12:39:12PM -0800, Robert Ritchey wrote:-Bob
> Thanks for the feedback on my options. I am not familiar with the
> Triac/zener circuit. In fact, I have never used a Triac. Is it just
> as you describe it below, Polyswitch in, Triac with a 4.5V (or close)
> zener on the gate with a 1nF cap?
Right. (Low power) Zener between A2 and gate, 1nF between A1 and gate. Very
simple. Your circuit would never get killed; but however the wrong power
supply could be cooked.
Dimensioning for 5 volts +-10% is difficult. Regulated power supplies
delivers 4.5 to 5.5 volts; choosing 5.1 zener +0.2...1.3 volts Triac
threshold will result in 5.3...6.4 volts for the triac getting fired. So
your device could be switched off from your normal supply too. I would
choose a 5.6 zener. (check the datasheets and the tolerances of zener and
triac).
M.
>
> At 12:14 PM 2/11/03 -0800, Matthias Weingart wrote:
> >On Tue, Feb 11, 2003 at 07:59:27AM -0800, Robert Ritchey wrote:
> >
> >My favourite solution is a combination of Polyswitch, Triac and Zener at the
> >gate of the Triac. This will protect from reverse power (<1.5V) and
> >overvoltage (zener voltage + 1V triac gate threshold). Do not forget a
> >capacitor (1nF?) from gate to ground, to protect from firing because of fast
> >rising input voltages or very short spikes. In case of fault, the current
> >flowing is only at the rated level of the Polyswitch and the power in the
> >triac is very low (1.5volts * polyswitch current).
> >
> >I remember there was a component called TRISIL that worked in this way.
> >But I think this is no longer available (why?).
> >
> >However I would always use a regulator on the board for the 5 volts +-10%
> >for a wide input voltage range and use a unregulated wall socket. That also
> >solves problems with the sometime quite high voltage drop over the poor
> >poor DC plugs.
> >
> > Matthias
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