> I read the threads about polarity reversal and overvoltage protection with
interest . In my situation, I've got cheap access to 2Amp unregulated  5
Volt DC power supplies at about USD $3 a piece. I have a bunch of
unprotected equipment that take 5 Volt input needing currents from around
500mA to 1A. I'd like to provide protection for these circuits. Here's what
I've come up with based on what I've read so far.


1.    > http://www.geocities.com/roinesreenig/simple_pwr_protection.JPG

The zener isn't going to do much at all because of the high value series
resistor.
if you intedn the zener to clamp while the fuse blows then you want no
resistance at all. Maybe that R was thtrere as part of thinking about your
crowbar circuit.

You want to use a higher wattage zener unless price/size utterly prevent it.
Philips have some glass bead zeners rated at ?5 watts that can be relied on
to fuse hard short if pushed too far. .

Test a few of the zeners of your choice to see what their preferred failure
mode is.

Note that the FET will not act as a really useful current limiter as the
body diode is conducting when the battery polarity is correct.

2.    > http://www.geocities.com/roinesreenig/pwr_reg.JPG

You need a "low dropout" regulator. No linear regulator will give you zero
volts drop. However, if the supplies are rated at 5 volts and 2 amps
unregulated, they will make more than 5 volts at lesser loads. It's highly
likely that you have 0.5 volts and quite possibly over 1a volt extra
available at 1 amp. If so then many LDOs would work OK.
egrettably, these atre usually substantially dearer than the 7805.
LT1086 rings a bell - Googles - yep -
    http://www.linear.com/prod/datasheet.html?datasheet=233
under 1v drop TYPICAL at up to about an amp at 25 C. Hotter gives lower
dropout voltage fwiw.

The LM2931 is another LDO reglator and has a number of protection methods
built in. They are almost bulletproof.
Check dropout at current of interest to see if they suit your need.

You can make an LDO yourself with a lower voltage voltage regulator (eg
LM317) and a pass transistor. Not overly complex and probably as good dropot
wise as you buy at a reasonable price. .

You could also use a buck-boost reguilator to give you true 5v in 5v out,
but that's mega overkill in this case.



        Russell McMahon

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