You don't want to put the two diodes in series, in general. Ideally,
it works fine, but the real world is far from ideal. It should be
fine in this case, as the ammount of power is far below the rated
maximum of both diodes, but the load across the diodes will be
unbalanced. Adding a high value resistor (which carries about the leak
current of the diodes) in paralell to each diode will balance it
somewhat.
I think you're confusing things.
In the old days, when semiconductor diodes were still new and tubes
were still common, diodes were often used in series because there
weren't any diodes capable of the higher voltages needed by the tubes.
So they'd put the diodes in series so the string could handle the voltage.
But that caused problems, since each would see a different voltage. That's
where the equalizing resistors came in. The parallel resistors were there
to equalize the reverse voltage drop.
When higher voltage semiconductor diodes became available, you stopped
seeing serial diodes in power supplies, because the scheme was no longer
needed.
But this voltage drop thing isn't about AC voltage, which the equalizing
resistors was about.
The diodes will never see a reverse voltage. They are being used to
drop the voltage, and if one diode doesn't provide enough, then add
another one. And it's easy to get diodes that can more than handle
the voltages seen and the current handled, so getting close to ratings
is not an issue either.
You could also use a simple voltage regulator. Minimal support
circutry and high tolerance to overvoltage.
ANd that's already been dealt with in this thread. He needs a volt
or two voltage drop, and doesn't need regulation. The diodes do
it simply. And as has been pointed out, the unit may not even need
exactly five volts, but the diodes just add a bit of security.
Michael
