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While working on a switching setup for a automotive cooling fan, I came up with a question. Can diodes be used instead of a relay when dealing with two switching sources? The main source would be the cars accessory line that is on when the key is turned on. The other source is a dash mounted switch that would let the fan be used when the key os off. Here are the two wiring diagrams.

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  • \$\begingroup\$ Two/three extra remarks - You'll want to add a flyback diode for the motor as well - I'ld recommend that the fuse protects the battery entirely, that is: put it in series with the battery so that it also protects the switch. By putting it in the ground path to the batteries, you can protect both batteries with a single fuse and protect against potential damage from the currents that the motor could generate. \$\endgroup\$
    – le_top
    Commented Sep 5, 2018 at 7:23
  • \$\begingroup\$ Your relay probably has an integrated flyback diode for its coil - check that and you can save that component - the flyback diode for the motor needs be capable of dissipating enough power - the DC motor might also have a flyback diode integrated. \$\endgroup\$
    – le_top
    Commented Sep 5, 2018 at 7:31

2 Answers 2

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Logically, yes, but in your circuit with no relays, note the diodes are in series with the motor, so that 1) the motor will see 12V minus the diode's voltage drop (say half a volt), and 2) that voltage drop multiplied by the motor's current (2A is it?) will generate heat (say 1 watt) within the diode and 3) 1N-series diodes are only rated for about one amp (at cooler temps).

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  • \$\begingroup\$ So if the power source for SW1 was moved to Battery1 would the same voltage drop and heat issue occur? \$\endgroup\$
    – Red Sonic
    Commented Sep 4, 2018 at 20:19
  • \$\begingroup\$ Do you think the voltage drop of SW1's diode disappears? \$\endgroup\$
    – mike65535
    Commented Sep 5, 2018 at 15:46
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Yes, diodes can select between two sources, but they will select with an OR function. That means in the circuit below if the SW1 is on OR SW2 is on the fan relay will be activated.

This is no different than the logic of the top circuit, which means you'll get the same functionality.

You may want to use 0.2V schottky diodes as the heat dissipated in the diodes will be lower, and you'll get more voltage to the fan. Dropping the voltage to the fan will most likely cause it to run slower if it is a BLDC fan.

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