OT How to size a diode?

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.

What kind of fans are you using? 20W sounds like AC motors. If so, you do not want diodes. What you need is the right switch, or a two pole four postion rotary switch wired to give the function you want. Another way is the switch used in range hoods, or just a pair of SPST switches, one for each fan. You may need a metal flap to block the unused fan to keep the hot air from being pushed back through it.

I might be missing something here , but why not just use 2 spst switches and then you can run either fan , both fans, or none.

You don't neet a diode, you need a switch. Specifically, a DPDT Center-off switch.

Ground out both fans, and use the switch to switch the hot. Here is a picture of how you would wire it up:

As you didn't provide specifications for your fans, determining the correct ratings (and size and mounting hole) for your switch is up to you. Personally, I would order it online and get the switch you want, instead of buying whatever radio shack has laying around made by God only knows who.

Jon

IF he is running DC fans a center off double throw single pole switch will work with a diode. With the diode connected between the positive connections, and the negatives tied together, putting power to one side of the diode will run the one motor, with current to the other fan being blocked by the diode. Put power to the other side of the diode and the first fan will run through the diode, and the second fan will run full power. Not a bad way to do things - and the diode just needs to handle adequate voltage and current for one motor. If the fans are 12 volt, he needs a 25 voltpiv, 2 amp diode at the very minimum. I'd use a 50 volt 5 or 10 amp diode if I was doing it. The 3 amp barrel diode on the rat-shack site would do the job. It doesn't say on that site, but it is a 50 volt PIV diode. (some are actually 200PIV) - look up ecg-5800 for 50 volt,5801 is 100 volt, 5802 is 200 volt. A 6 amp diode would be a 5850, 5854, or 5855 and would be a stud-mount.(DO4 case)

snipped-for-privacy@c3net.net Inscribed thus:

Go grab an old computer PSU and strip one of the low voltage diodes out !

Often described in the online catalogs as a DPDT ON-OFF-ON.

Bob

Yes, that would work. I personally would not consider it a satisfactory solution because it introduces an extra component, additional mounting issues, additional repair considerations, and it drops at least 0.7 volts which is lost as heat.

With a switch you have a one component solution which follows the KISS principle, resulting in a simple and elegant solution. One switch and it's done.

Jon

Fan specs? If they're AC fans (as the 20W suggests) then forget it, just get a more appropriate switch eg. DPDT with center-off. Diodes will fry the fans.

is there any reason why you don't use a DPDT switch ? no diodes, works with AC or DC fans

He would need a dpdt center off switch. And exactly haw are you going to select ONE motor on one position, and 2 on the other with the motors in parallell? I can see if you tie the commons together, and tie both terninals of one side together, then feed the motors from the

2 terminals on one end of the switch - for 1 motor switch to the position that connects the common to the bridged terminals and the empty non-bridged terminal. For both to the position that connects the common to the occupied non-bridged and the bridged terminals. Other end of both motors tied together to the other (neuatral) line. Any other way?

effect.

OT, but worth mentioning.

Obama is on Fox on one TV and I'm recording Lynyrd Skynyrd singing God and Guns on PBS on the other one,

jsw

>> I am having difficulty trying to pick one - many of the diodes do not

first, I don't recall a mention of off, but yes, a center off switch is the way to go. Wire white to both motors. Wire black to both center terminals of the switch. Connect motor 1 to pole 1 terminal 1. Connect motor 2 to pole 2, both terminals.

How hard it this?

Well, I read your paragraph 3 times and still couldn't understand what you were saying.

Nor could I decide whether you were asking a real or rhetorical question.

In any case, this sort of thing begs for a picture or drawing.

Fixed pitch font.

0 is a switch terminal, / is a closed switch contact. Fan 1 is connected to two diagonally opposite terminals.

Fan 1 only: fan1--0 0 / / power-0--0

fan2--0 0--fan1

Both fans: fan1--0 0

power-0--0 \ \ fan2--0 0--fan1

jsw

That's why I linked to this picture when I replied: