This is where the rub comes in. I know what my PSU is but I don't know what they might plug in, it could have any rating of volts and amps. I am assuming it would be less than 2 or 3 amps on a 2.5mm plug and hopefully less than 16V or so but I don't know. I have to protect against anything.
At 12:54 PM 2/11/03 -0800, Jens Sch�nfeld wrote:
-BobAt 09:49 11.02.2003 -0800, Robert wrote: [...]None at all, because you are the one who knowscould supply up to a couple amps unregulated easily) to withstand the current before the ployfuse kicks in. An suggestions on power rating?
a) the power consumption of your device
b) the supply current of your PSU
Make sure that the (poly-) fuse does not open during everyday use, but also make sure that the PSU cannot blow the Zener. I'm sure you'll find the right values, it only takes some common sense :-)
You might want to take into account that on a "cold" power-up, there's a current spike to charge all the caps on your digital circuit. Not only the decoupling caps on your board, but also the capacitance of cables, a multilayer board and output pin states after reset. Depending on the speed of your digital circuit, you will already have taken good care about the ground returns for your critical signals, so you should have enough information to calculate the absolute maximum current that is drawn during power-up.
There's of course always the possibility to measure that spike if you have the proper equipment.
ciao,
--
Jens Sch�nfeld
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