Diode Question. Engine Run On..

Denny S. · #1 (**permalink**) 02-13-2002, 01:22 PM

My electric fan is feeding back causing run on... I went to Radio Shack to get a diode to fix this. THe guy at our Radio Shack doesn't know a diode from his A**. I Bought what I thought should work, but I'm not sure.. THey are labeled 1kV 2.5 A
Silicon Diode. Can I use this ??
Peak Inverse Voltage 1000V Forward Voltage Drop at 1f 1v
Forward current 2.5 A Maximum Surge Current (16ms) 80A
Reverse current at PIV 1uA I don't have a clue ......

wilf leek · #2 (**permalink**) 02-13-2002, 02:18 PM

Denny - I'm not an electronics expert, but I think you have to size the diode to allow for the full running current the fan draws, plus some for the start up surge?? That's assuming the diode is to be connected in series with the feed.
you don't need a particularly high inverse voltage rating - after all it will only see 12V, but it is going to have to be rated at 25 Amps minimum, unless I'm missing something??

Maybe the thing should actually be connected across the +ve and -ve terminals on the fan itself (obviously not so it shows up as a "short" to the feed, then it would only "short circuit" the fan motor when it acts as a generator?
Hmm - not sure I'm helping here, think I'll crawl back under my stone.

Bob Putnam · #3 (**permalink**) 02-13-2002, 03:40 PM

Are you using a relay with the fan? It seems to me that a properly designed circuit will interrupt any induced voltage.

Denny S. · #4 (**permalink**) 02-14-2002, 03:13 PM

I found the diode I was looking for at a small engine and generator repair shop... Thanks for the tips..

Jack21 · #5 (**permalink**) 02-14-2002, 03:47 PM

I can give you Radio Shack part numbers for diodes. You have to use one in the alternator energizer circuit to prevent ignition feedback and run-on.

There are a few other places they can be used to isolate circuits, but the fan is not. or should not be one of them. A relay, like any other high amperage circuit should control it.

Geary L. Sarno · #6 (**permalink**) 02-14-2002, 06:55 PM

I had the same problem with one of my cars. The cure is to simply hook up the positve feed of your fan as a switched accessory instead of having it wired on the ignition side of the ignition switch. When you turn of the switch your residual electrical feedback wil not make it to the ignition curcuit and the engine will stop. - Geary