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Posted by D Sherman [72.47.153.24] on Thursday, June 07, 2012 at 11:43:36 :
In Reply to: Diodes are cheap, 2/ 90¢ @ Radio Shack. posted by chris case [75.36.32.85] on Thursday, June 07, 2012 at 08:46:14 :
Very true. I'm an EE so I have approximately a gazillion assorted diodes of all sizes in my parts bins. I just have a personal reluctance to put anything solid-state in a pre-1960 vehicle. I will admit to having done it occasionally to solve a problem I didn't want to solve the right way. The electrical environment in a vehicle, especially an older one, is pretty tough. It's easy to get 100 volt spikes, temperatures and vibration are high, and you need it to survive even if someone attempts to jump-start it backward or disconnects the battery while the engine is running. Much solid state "automotive" stuff will fry if you try that, but anything I design has to survive it. A seriously over-sized diode (I usually use a 1N4007, rated at 1000 volts, 1 amp, and cost 5 cents if I had to buy any, which I don't) will probably survive most abuse if you don't need much current, but relays, despite the moving parts, tend to be more rugged in electrically-abusive or high-current environments. Lastly, there's the matter of nuclear electromagnetic pulse, and gamma ray burst from a nearby nuclear detonation. That stuff is hell on silicon crystals. When the bombs start going off, don't you want your PW to keep on driving while everybody else's new cars stop?