RE: [SI-LIST] : Gnd plane in analog&digital circuits

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From: Doug (dmckean@gte.net)
Date: Sat Oct 28 2000 - 11:37:39 PDT


Not sure if the board is processing video, audio, or some
motor control. If the circuitry is partitioned correctly,
you can use the same ground plane. I've done heavy analog
(i.e. motor circuits) with digital on the same ground plane.
You want to minimize crossing return currents as much as possible.

UL will test the transformer at 135% the rating of the input
fuse. Let me give you an extreme example of how this can
ruin your whole day. Product nominal current draw was something
like 4 amps. Inrush currents required fusing at around let's
say 10 amps. UL tests the transformer by putting a load on
each output of the transformer and raising it until the input
current draw is 135% the rating of the input fuse. They also
put cheesecloth on the transformer to check if it burns.
Well, 13.5 amps current draw for about a minute was too much.
So the lesson is - be very careful about overfusing.

The 135% comes from they way American fuses (typically the
1/4" x 1 1/4" casing) are rated - to "open" a 135% overcurrent.
IEC compliant fuses (typically the 5mm x 20 mm casing) are
rated to "hold" at 150% overcurrent for (I think) one minute.
Two very different ratings. Check me on this. It's been a
while.

Beside that little war story, there's a host of information
regarding proper fuse selection in the fuse mfrs catalogues.
Disregarding inrush and temperature considerations, I start
with the fuse being 1/3 greater than the nominal current draw
of the device. I think that's pretty standard. Then, you go
on to include inrush and finally temperature from the curves
of the fuse supplied by mfr.

Regards, Doug McKean

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