It Glows when it Blows™! [add obligatory Michael Scott line here]

(I’m sorry. I couldn’t help myself.)

Okay, now that the lowbrow humor has been dealt with, I had to replace a car fuse because of a shorted 12-volt power socket. Luckily, I was able to replace the fuse without the circuit blowing again; however, I had used the only spare fuse in the fuse box and needed to buy some more in case the fault was to recur. Browsing my local Canadian Tire, I stumbled upon a pack of fuses that allowed for a visual check for blown fuses by simply turning on the ignition: the Littelfuse Smart Glow fuse. A 36-pack of these fuses cost about $35 Canadian, making them a bit pricier than their non-illuminated counterparts.

Construction

Closeup of fuse, LED and resistor

The Smart Glow fuse is comprised of three main components: the actual fuse (which is really just a regular automotive fuse), a 360-ohm resistor, and a dual red LED package with the diodes in inverse parallel to allow for the fuse to glow regardless of orientation. The LEDs and resistors are affixed to the fuse body using various epoxies: an opaque red epoxy to glue the components down, a conductive silver-filled epoxy to provide an electrical connection without soldering, and a clear epoxy to protect the components from damage; the fuse amperage is re-printed on top of the protective epoxy coating since the resistor and LED obscure the original fuse’s markings.

Schematic of Littelfuse Smart Glow fuse

Performance

Simply put, this acts like any other automotive fuse would. The only difference is that the LED will illuminate if the fuse is blown, and sufficient load is still present in the circuit to provide enough current for the LED to act as a fault indicator.

Fuse blown and LED indicator lit with 5 volts

When testing the fuse’s brightness, I found it to be quite noticeable at 5 volts and almost blindingly bright when run at 14.4 volts (the approximate charging voltage for a 12-volt car battery).

Simulation of LED indicator

Running this circuit through a simulator, the LED has almost 35 mA of current running through it. Given how LEDs are typically rated for a maximum of 20 mA, this LED is not going to last long; that said, it shouldn’t need to run for a long time as the LED’s only purpose is to notify the user that the fuse needs to be replaced (and at that point the fuse and its indicator will be disposed of anyway).

Conclusion

Yes, it glows when it blows; I have nothing more to add.

(The same could be said for Rudolph the Red-Nosed Reindeer, but he’s a non-electronic entity and is therefore outside the scope of this blog. :P)