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I am trying to stop power feedback on a 12 V car lighting circuit, actually the 2 indicators that feed back from one side of the flasher unit.

My question is: what diode do I need for this? I have 1N4001s, but I think the 1 A rating won't work for long, as one indicator bulb is 21 W which means 1.75 A, so two would be 3.5 A. Is this correct?

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Sounds correct, provided that the two blubs flash at the same time. If they alternate (and you trust the circuit that they always will) you will get by with a lower rated diode, maybe even a 1N4001 would survive.

So find yourself a bigger diode from your favourite supplier. I would opt for a large margin, maybe a 5A diode. Be aware that diodes for such currents can have a significant voltage drop, which might cause your flasher to flash somewhat dimmer.
You can limit the voltage drop by using a Schottky diode; the SB560 is rated at 5A and drops only 670mV, so even at 3.5A dissipation will be less than 2.5W.

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  • \$\begingroup\$ Thanks for the answer, that's what I figured but I thought I would check with an expert :) \$\endgroup\$
    – user5846
    Sep 23, 2011 at 10:41

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