wrote:
I am fabricating a small box that will house a couple of "muffin" fans
to direct air behind a fireplace insert. I have a 3 position switch
(0, 1 , 2) so that I can run either one or two fans. I need a "check
valve" (diode?) to prevent both of the fans from running when the
switch is set for a single fan. I am having a bit of trouble with
selecting a diode. I did a quick google search that leads me to
believe that the diode needs to be specified based on reverse current
to be blocked and the voltage. The fans draw 20W each. For such a
cheap simple part, I wish to buy it at Radio Shack but looking at
their web page: http://www.radioshack.com/family/ind...goryId=2032269
I am having difficulty trying to pick one - many of the diodes do not
even show a spec sheet. Recommendations please. Once I complete this
task I can get down to forming the sheetmetal. Thanks

You didn't say how many volts your source is.

In any case, we have to assume you are running
the fans off of DC, prolly 12 volts else the
diode trick won't work. Since we are using DC,
reverse voltage doesn't exist and isn't an issue.
That leaves current rating. At 12 volts and 20W,
your fan will draw 1.67 amperes. So to be safe,
you need at least a 2 ampere diode.

The diode will drop .8 volts across it during
operation, and 1.67 amperes times .8 volts is
1.3 watts of power that the diode will have to
dissipate. That's a reasonable amount in free air.

This should work fine.

http://www.radioshack.com/product/in...77#tabsetBasic