A friend of mine has just finished his race car but is having fan issues. What I want to help him do is setup an LED that turns on when the fans have lost power. Often the fuse blows.

I would assume I can tap off the 12V supply that is after the fuse to send to the relay but how would you wire it up..?

  • 11
    \$\begingroup\$ If a fuse blows "often" then it's time to figure out why. \$\endgroup\$ Sep 2, 2014 at 0:08
  • 2
    \$\begingroup\$ Your question reads like you're assuming we have any idea what race car system you're talking about. Please provide way more information. Assume we know nothing about what you're looking at. \$\endgroup\$
    – Dan Laks
    Sep 2, 2014 at 0:13

2 Answers 2


If you don't want to bridge the fuse, you can add a relay in series with the fuse. Choose a relay that is closed when no voltage is applied to it (called SPST-NC for Single Pole, Single Throw, Normally Closed) and connect the LED to this relay.

When the fuse blows up, the relay is closed and the LED turns ON. This solution makes sure that your LED circuit is perfectly insulated from the fan circuit.

For example this relay can be used: http://uk.farnell.com/coto-technology/9001-05-02/reed-relay-spst-5vdc-0-5a-through/dp/1079820

Make sure the current and voltage ratings match those of your circuit, though.


I assume you want to ON LED when fuse is blown. It is quite simple to do that. Just add one resistor and LED series in parallel of Fuse. Fuse is having very low impedance hence normally LED will not be ON when Fuse is intact. When Fuse is blown then circuit will be completed by resistor and LED and LED will be ON.

But first to find out why fuse is blowing so frequently. :-)

  • \$\begingroup\$ But a fuse is meant to BREAK the circuit, because something is potentially wrong. I wouldn't consider it good practice to bridge a fuse. \$\endgroup\$
    – Rev
    Sep 2, 2014 at 6:59
  • \$\begingroup\$ @ Rev1.05: I agree but as far as i understood this is to Drive Fan. If i use <1mA current to Drive LED in parallel with 1.5A FAN drive with fuse, That is not a problem. This scheme will not work for nearby current conditions. \$\endgroup\$ Sep 2, 2014 at 12:40
  • \$\begingroup\$ I understand what you propose and I like it in a way, because its so easy. But I still would not advise to "bypass" a fuse. I know that the current would be very limited, but it would still source current to a potentially defective circuit. But maybe I am to fussy about it ;) \$\endgroup\$
    – Rev
    Sep 2, 2014 at 13:41
  • \$\begingroup\$ This answer also assumes that there's no UVLO circuit operating after the fuse. Not very likely in a fan circuit, but possible. \$\endgroup\$
    – horta
    Sep 2, 2014 at 15:18
  • \$\begingroup\$ There are commercial fuses with led failure indicators. Fuse blows, led lights. The concern of current sourcing is overblown (pun intended) \$\endgroup\$
    – Passerby
    Sep 2, 2014 at 16:31

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