So this 12 VDC LED rated at 1.5 watts i.e. 125 mA, The LED is run by a computer circuit which sends 12 V power to LED for on and off, exactly like a car turn signal blinker.

Which value fast blow glass fuse should I put here?

I tried 100 mA 5x20 mm glass fuse and it works but the LED acts weird sometimes and just stops working until I power cycle everything, 100 mA fuse never blew since installation.

I think LED acting weird has to do something with fuse value.

  • 1
    \$\begingroup\$ Why there is a fuse needed? And even worse, why was a 100mA fuse selected for a load rated more than 100mA? \$\endgroup\$
    – Justme
    Jul 28 at 18:18
  • \$\begingroup\$ The 100mA fuse I'm looking at now has a cold resistance of 11 ohms. Depending on the circuit your LED is hooked into, that could be significant or not. I agree with @Justme on a fuse probably not being necessary here though. \$\endgroup\$
    – vir
    Jul 28 at 18:52
  • \$\begingroup\$ I need fuse because I burnt the computer system dash already once and it stopped sending blinking signal and it cost me 2000$ to replace it, the OEM do not put a fuse in there not sure why, the OEM blinkers are directly connected to computer dash output which is not good and the computer dash is ultrasonic sealed so if any smd component burns in that, you have to replace it \$\endgroup\$ Jul 29 at 15:47

1 Answer 1


A fuse melts first in the midpoint between the terminals. 125mA is probably not enough to "blow" the the fuse but may melt the very midpoint. Surface tension holds the metal droplet in place. At these temperatures the fuse resistance is high. This would cause some odd behaviour in the LEDs.

I had this happen once. The fuse would blow only when the apparatus was bumped. The fuse was in tact ecept for a small gap where the metal fell out. Usually when a fuse blows the metal is vapourized.

The fuse value needs to be greater than the operating current plus any power-on inrush current,say 250mA.

Milli-amp fuses do insert a resistance in series with the LEDs as indicated by vir in the comments. This may also affect the operation of the LEDs.

You will have to test.

  • \$\begingroup\$ i will order lowest OHM value fuse as possible, why should I put 250mA and not 160mA as I want it to blow as fast as possible with minimum of overcurrent. what types of tests you want me do, like measure the resistance of the LED? \$\endgroup\$ Jul 29 at 15:52

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