Just a quick note about what was said before about protection diodes
on solenoids.  The diode current rating is important, but is the
voltage rating is even more so.

Earlier it was said that the voltage rating should be twice that of the
operating voltage.  This is not quite right.  When the magnetic field
of a low voltage coil collapses (the solenoid turns off), the
"inductive kick" can produce a brief voltage of several hundred volts.
For most low voltage coils (up to about 48 volts or so) a 1000 volt
diode is more appropriate.  A good diode for this purpose is a 1N4007.
This is a 1000 volt, 1 ampere diode and is very inexpensive, about
$0.20.  Also a 1N5408 will work as well.  This is a 1000 volt, 3 amp
diode.

Concerning protection and all, understand that the diode will _not_
protect the solenoid but rather will "eat" the inductive kick and
protect the transistor driving the solenoid from over-voltage.

My two cents,

Ray Finch
Albuquerque, New Mexico (USA)

 [ A series diode would 'see' the high voltage spike, but normally
 [ the diode is connected in parallel with the magnet coil so that
 [ it conducts when the inductive kick occurs.  In this circuit
 [ the diode is exposed only to the power supply voltage.  However,
 [ it is nonetheless a good idea to install a power diode rated for
 [ 800 or 1000 volts, simply because nowadays any diode in the
 [ 1N4000 family which won't withstand 800 volts is from a bad
 [ production lot and so is more prone to failure.  (This I learned
 [ from a firm that tests barrels full of components to find the
 [ few that will withstand 1600 volts!)  -- Robbie