I want a light bulb to light instead of a fuse blowing when the circuit is short-circuited.

What will happen if I put a 0 A light bulb in parallel with the 4 A fuse?

Annotated photo of PCB

  • 1
    \$\begingroup\$ What is a "0 A light bulb"? \$\endgroup\$
    – Andy aka
    Jan 7, 2023 at 21:20

1 Answer 1


If you put a light bulb in parallel with a fuse, nothing will happen until the fuse blows. A fuse is a piece of wire while a light bulb is more like a resistor.

Nearly all of the current will flow through the fuse - the device will function as normal and the light bulb will not light up.

If the fuse blows, then no current will flow through the fuse. Some current will flow through the bulb. It will light up.

You will need a 220VAC light bulb. A 220VAC light bulb that can pass 0.5A of current would be something like a 100W incandescent light bulb - rather large, and it will get quite hot if it is on for any length of time.

You might instead use a neon bulb indicator bulb (NE2) in parallel with the fuse. You'll need a large resistor (200k) in series with the NE2. The NE2 will light up when the fuse blows.

You'll not be able to have a light bulb that lights up instead of blowing the fuse.

  • \$\begingroup\$ this power supply is broken, and I want to test it under a load (AC220V). \$\endgroup\$ Jan 7, 2023 at 20:25
  • \$\begingroup\$ What if do I connect a bulb in series with 220V? \$\endgroup\$ Jan 7, 2023 at 20:26
  • \$\begingroup\$ You could connect an incandescent light bulb in series with the device. That would limit the current into the short circuit. You will need a 220VAC bulb of about the same power rating as the device. \$\endgroup\$
    – JRE
    Jan 7, 2023 at 20:44
  • \$\begingroup\$ Using an incandescent light bulb as a current-limiter is an old trick. Nearly everybody that works on vintage vacuum tube equipment does this ... I have one wired into my bench for this exact reason. Works on other equipment too but the user needs to assess if it's really suitable. instructables.com/Light-Bulb-Current-Limiter \$\endgroup\$
    – Kyle B
    Jan 7, 2023 at 22:30

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