blueovalz Posted July 12, 2001 Share Posted July 12, 2001 Question for any EE's out there. I finally wired up my radiator fans and found that every time I switch the fans off, the tach pegs out. I thought about using a high current diode to prevent this, but even a 70amp diode gets hot fast. Being these fans are only drawing about 20amps, this diode, I would have thought, should be able to handle 20amps DC, shouldn't it? (or are they rated at RMS and at only for 1/2 cycle???). Anyway, I'm trying to keep from ruining a good tach. Terry Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted July 13, 2001 Share Posted July 13, 2001 Sorry, Terry, don't know the answer, but a couple of questions perhaps to help stir someone else's brain: what style is your tacho, i.e. is it like the original one, that has the loop thru the ignition circuit, or is it an electronic one, counting pulses, tapped between the coil and the points (or whatever ignition trigger method you have)? Also, I'd guess you have relays on your fans; where is the low ampage side triggered from? Reason for asking the second question is that I got lazy and have the low-amp side of the relay feed for my fans from the ignition circuit on the coil (but I don't have the problem you describe). And my tacho's of the latter style, not the original Nissan one. I figure you've probably thought of all this, being a guy who plays with megavolt Tesla coils, but thought I'd chuck in my $0.02 (or $AU 0.04). [ July 12, 2001: Message edited by: zedskid ] Quote Link to comment Share on other sites More sharing options...
blueovalz Posted July 13, 2001 Author Share Posted July 13, 2001 The tach is a typical aftermarket one that counts pulses. No relays, in that I chose instead to use the high amp toggles I had for my coiling. Anyway, I figure the DC motors on the fans are acting like generators when they spin when no current is applied, and backing back into the ignition circuit. Forturnately, when the engine is running, everything works fine and no hick-ups with the ignitions system. Quote Link to comment Share on other sites More sharing options...
TimZ Posted July 13, 2001 Share Posted July 13, 2001 How did you have the diode hooked up? Should be reverse biased, in parallel with the fans. The striped side of the diode should be on the +12v side of the fans. This will keep the voltage from spiking when the fan relay shuts off, and since it's reverse biased when the fans are on (no current flow), it should never get hot. You shouldn't need anywhere near 70A for this. A 20A diode is probably overkill, but should work fine. Quote Link to comment Share on other sites More sharing options...
Guest MM_280Z Posted July 16, 2001 Share Posted July 16, 2001 Motors are extremely inductive loads. Inductors store energy in the magnetic field surrounding their coils when in operation. When you switch them off, this energy is dumped back into the system in the form of a high voltage pulse in the opposite direction of the driving voltage of the motor. The clamping diodes connected properly will prevent the voltage from spiking. You probably have them on backwards. Quote Link to comment Share on other sites More sharing options...
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