LED if fuse is blown

[okw]

I have 12V in -> a slide switch -> and a fuse. I want to have a red and green LED, so the green is lit when the fuse is intact, and red when the fuse is blown.
Question is, will this MOSFET (or simple transistor) work as intended? And is it safe, since a failed MOSFET will bridge fused and unfused side? I do not wanna damage my brand new motorcycle.

[coromonadalix]

from google

[themadhippy]

I want to have a red and green LED, so the green is lit when the fuse is intact, and red when the fuse is blown.

why complicate things? A led and resistor in parallel with the fuse will light when the fuse is blown

[ataradov]

BigClive just did a video on that in the last couple weeks.

[floobydust]

You can also buy automotive blade fuses with the built-in LED. They cost a bit more and still reasonable.
https://www.littelfuse.com/products/fuses/automotive-aftermarket-products/indicator-fuses/mini-smart-glow.aspx

I would avoid the cheap china (and LED) fuses, they are found to be terrible in not blowing, or far above their rating. Dangerous, many youtube videos showing they are unsafe crap.
If the motorcycle's load is a problem with shorts, you could use a PTC polyfuse as well as the fuse.

[bostonman]

Would adding a fuse in series with the LED and resistor be safer? If the LED happens to short then that could be dangerous.

[ataradov]

If used for relatively high currents, then LED won't remain shorted for long.

[bostonman]

If used for relatively high currents, then LED won't remain shorted for long.

I agree, but thought it was worth mentioning.

[Andy Chee]

I do not wanna damage my brand new motorcycle.

The existing fuse-LEDs available from your automotive parts store should be suitable for your motorcycle.  No need to design a complicated MOSFET circuit.

[SteveThackery]

I agree with the previous posters.

I advise that you forget the red/green thing and just have one light which comes on when the fuse is blown. Back in the day I fitted a "grain of rice" tungsten bulb across the fuse (it was all low voltage stuff, typically 12V, so no safety issues). Nowadays you can use a bidirectional LED (plus series resistor if it's not built in).

Bearing in mind that it will probably never blow the fuse anyway, anything more complicated would be way over the top.

[Zero999]

from google

The circuit won't work, unless the positions of the red and green LEDs are exchanged, but then the red LED will light if the fuse is good or the green one will light if it's bad.


Adding a couple of diodes (one might be enough, it depends on the forward voltage of the green LED, one will probably suffice for yellowish-green GaP, but you'll need two for pure green InGaN) in series with the red LED, will make the circuit work, as the original poster wants.

R1 isn't needed, unless the intention is for the LED to light, when the supply voltage exceeds a certain threshold.


EDIT:
Another option is to use the original circuit and, rather than a real red LED, a phosphor converted LED, i.e. pink or white, if you can't get red (add a red filter if needed) and an old yellowish-green GaP green LED, which will have a lower forward votlage.

[TaylorD93]

I was going to post a similar response, ive used that circuit (i.e. adding some extra 1n4148's in series with the red LED) in a few projects, nice, simple and cant really go wrong.

[magic]

It can go wrong, if the fuse blows and some inductive load on the output manages to overvolt, overload and short the green LED.