Trying to learn electronics.

I am thinking about trying to create a circuit that has 2 DC fans with independent power sources. Only 1 will be working at any one time.

It will sit on my desk and cool me and my coworkers down on a warm day :). And if I pull the power from FAN 1, FAN2 should kick in and be powered.

I tried with 2 cheapy 12 V fans from aliexpress... I tried with a relay and an l9110 but am not sure how to implement the 'if this one fails, that one gets power'.

I don't need to do it with these parts, it's just the parts I tried and have. So I am willing to purchase new components.

Can you point me in the direction of the right components?

  • 1
    \$\begingroup\$ Please update your question with more information. What is the fan voltage? What is the fan current? Depending on the answers, this probably can be done with one relay and no electronics; certainly not a motor driver IC. \$\endgroup\$
    – AnalogKid
    Commented Dec 28, 2023 at 23:17
  • 1
    \$\begingroup\$ You will get more help if you show exactly what you have tried already. Include at least a wiring diagram or a schematic of your idea. Links to the parts you've selected would also be very helpful. (A free schematic application is available in this forum, (see the small schematic icon). \$\endgroup\$
    – Nedd
    Commented Dec 28, 2023 at 23:22
  • \$\begingroup\$ @AnalogKid Thank you for the reply. I updated with the voltage. I don't know the current but could measure it once in front of the fan again.. It's OK if I need to buy stuff. I hope this sets some parameters. \$\endgroup\$
    – jouell
    Commented Dec 29, 2023 at 0:40
  • \$\begingroup\$ Post any information on the relay you tried. \$\endgroup\$
    – AnalogKid
    Commented Dec 29, 2023 at 0:57

1 Answer 1


If you have two independent fans and two independent 12 V sources, then try this:

Source #1 powers both fan #1 and the relay.

The relay NO (Normally Closed) contacts go between source #2 and fan #2.

When Fan #1 is up and running, the relay coil is energized, holding the NC contacts open. When source #1 is off, the NC contacts close, completing the circuit from source #2 to fan #2.


if I pull the power from FAN 1, FAN2 should kick in and be powered.

Here is a schematic of my reading of your requirements. Turning off or unplugging Source #1 enables power to Fan #2.

enter image description here

  • \$\begingroup\$ Thanks. I believe this does answer the question so I accepted it. I was not specific enough I think. In this design pulling out the power for fan #1 does not trigger fan #2 to go on. Pulling the power to the relay does. \$\endgroup\$
    – jouell
    Commented Dec 29, 2023 at 3:56
  • \$\begingroup\$ See the Update. \$\endgroup\$
    – AnalogKid
    Commented Dec 29, 2023 at 4:10
  • \$\begingroup\$ Thanks. Looks great! \$\endgroup\$
    – jouell
    Commented Dec 29, 2023 at 14:55

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