e: [AMPS] AL1500

Paul Christensen paulc@mediaone.net
Fri, 7 Dec 2001 14:13:40 -0500

> Just tested that too. The circuit is a bipolar transistor sampling grid
> current, into a CMOS trigger that drives the cathode bias relay. There
> is a 1 millisecond delay on the input to the transistor, to prevent
> nuisance triggering, and the data sheet for the relay (nothing special,
> a regular Potter & Brumfield / Schrack RTE240xx 8 amp power relay) says
> the break time is 2 ms.

Quite impressive actually...even without an accelerating circuit.

> The important thing is that the delay of an electronic trip can be only
> a few milliseconds for *all* levels of overload. This is what an
> electronic trip can do, but a fuse can not!

Ian, very interesting results!

Another interesting possibility is the use of an opto-isolating photo-transistor (e.g., GE H11A2 or 4N32) to develop the required
isolation between the cathode sample point and the remaining portion of the trip/reset circuit.  In the Alpha 77 example, the
photo-diode side of the isolator could be coupled across the 3.3-ohm emitter resistor.   I believe the approximate isolation rating
for many of the garden-variety, low-end devices is ~ 7KV DC and cost less than 25-cents in quantities.

-Paul, W9AC

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