At 13:16 11.02.2003 -0800, Robert wrote:
If it's just about "protect against anything", specify the Zener to withstand anything your power plug is rated for.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.
If it comes to "must work on anything between 5V and 16V", I'd start looking into DC-DC converters. Actually, it might be cheaper to migrate your digital design to 3.3V and regulate down with an LM317 on your board. This will work fine on a regulated 5V supply, as well as on a 16V supply. The limit is just the amount of power that is converted into heat by the LM317 - larger heatsink, more idiot-proof, easy as that. An LM317 only needs two resistors to work, maybe some caps to stabilize voltages. Alltogether a solution of about 35 cents plus heatsink.
Migrating to 3.3V might also help EM emissions, as most 3.3V digital semiconductors draw less current than their 5V brothers. Replace 74LSxx with 74HCxx, replace EPROMS with low-voltage Flash, replace Xtal oscillators with resonator circuits and so on. Less current equals less magnetic field, which equals less emissions, but I'm drifting away from the topic.. (am I too paranoid, or did I spend too much time on the spectrum analyzer lately? ;-))
ciao,
--
Jens Sch�nfeld
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