How to detect if fuse has blown?

Question

I want to detect with a microcontroller if a fuse has blown. The circuit with the fuse is 12V when on (it is not always on, so I need to distinguish between a blown fuse and the circuit not running). The microcontroller can only handle max. 3.3V.

I thought about using two optocouplers like this:

Is there a better solution you would use?

Answer 1

If you can live with a bit of leakage through a blown fuse, and there is always a reasonably low value load, put one optoisolator LED across the fuse (with a resistor in series, of course, and an inverse diode across the LED wouldn't hurt).

Otherwise you could do something like this:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

You need a way to differentiate between 3 current levels:

• 0 when +12 is faulty
• I/10 when fuse is blown
• I when fuse is OK

eg use two different pullup resistors, or use an ADC input

The key design issue is that there has to be a large ratio between the two currents as you have to allow for variation in +12V, Opto Current Transfer ratio from unit to unit, and over temperature, and port pin threshold.

^{simulate this circuit – Schematic created using CircuitLab}

Of course this is assuming you need isolation. If they have a common ground then its trivial...

^{simulate this circuit}