I came across the circuit suggested
by MichaelN sometime back and decided to try it out.
I used the only MOSFET available at RadioShack. For some reason they only carry an N-channel, type IRF510. A P-channel blocking the high side would have been my choice, but I tried what was available. The circuit couldn't be simpler - I soldered it together on literally a scrap of perf board with a terminal block. I tested it mainly at 5 volts, with a few measurements at 10 volts, and measured the voltage drop across it vs. load current. I used 50 mA steps from 50 mA to 300 mA and 100 mA steps from there up to a total of 1 amp. It looks like I dry-labbed this, but the plot below shows my results.
The voltage drop for small currents is very low. Above about 400 mA with this part, a series Schottky diode might result in the same performance. With careful selection of the MOSFET, the useful operating range could probably be extended.
And by the way, yes, the circuit is effective at blocking voltage when connected backwards.
The data sheet
shows the same data I measured in Figure 1. The second line from the bottom is for a 5 volt supply.
This is definitely recommended for applications with a current draw below a few hundred mA. A search of data sheets may show even better options.
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