I am replacing the single-speed electric fan on my street rod with a two-speed fan. I have an indicator lamp on the dashboard that tells me when the fan is on. The new setup will have two separate relays for the two-speed motor. I want to use the one indicator lamp to indicate when the fan is on regardless of which speed it is running (switches on the dashboard control which speed is running by grounding one of the two relays).

I need two diodes to put in the lamp circuit so that the current to the lamp from the energized motor circuit will not back-feed to the de-energized motor circuit. In other words, two alternate sources feed the lamp, but I need to block back-feed from one source to the other through the lamp circuit. The lamp is a miniature which probably draws less than one amp. The voltage is 12 volts.

Can you help me specify a diode so I can buy two of them? As you know, there are a gazillion diodes out there. It would be helpful if you could specify a Radio Shack part number, or some other supplier part number.

Incidentally, I made a similar setup before, and I used two diodes that are way over what is needed -- they had heavy wire leads, like about 16AWG. I just picked them up by guesswork from a local supplier that is out of business, so I have no way of duplicating them.

I sure would appreciate some help.

Olephredd (Old Fred")

 

The 1N5402 will do the job, a little over the top but nothing wrong with that when cost doesn't really come into it?

 

Thanks, Paul.

Olephredd

 

...this sounds an awful lot like an automotive modification....
http://www.allaboutcircuits.com/l_tos.html