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?
At 12:14 PM 2/11/03 -0800, Matthias Weingart wrote:
On Tue, Feb 11, 2003 at 07:59:27AM -0800, Robert Ritchey wrote:
The solutions with zeners and polyswitch are not the good way. I tested it
with different currents and there is a big range of current that can flow
and dissipate too much power in the zener. The zener will blow and convert
in most cases to a "permanent crow bar"; you need to replace the zener. The
solution with a fuse and zener is better, but the user/service also have to
exchange the fuse.
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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