Simple Blown Fuse detector circuit? take a look

Thread Starter


Joined Oct 5, 2006
I am wondering if the following circuit will function as a Blown Fuse detection system. The digital input needs max of 5V hence the voltage divider. I figure if the Fuse blows the digital input will be pulled to 0V via ground and if the fuse is intact the digital in will read 5V.
See attached picture.



Joined Sep 20, 2005
It will function,. but I would add low-pass filter to the input and a ~5.5V zener to the connection to the processor, because when the motor switches on and off it could do nasty things.

What is the wattage of the motor?


Joined Jan 10, 2006
There could be some nasty spikes there due to back EMF from the motor.
I'd be putting a 4.7v zener across the 5k resistor for a little piece of mind.

Thread Starter


Joined Oct 5, 2006
I forgot to mention there is a protection diode across the motor. It is a 12 V motor with max current draw of 8amps, 3 amps continuous. as far as the zener value, one value suggested was 4.7V, dont I want a hair above 5V since 5V will be the nominal value at the processor input most of the time? But if the voltage spikes above say 5.1~5.2 V I want it regulated? Thanks for the responses:)


Joined Aug 8, 2005
1) You have to consider tolerances. A 4.7V 10% zener may not break over until ~5.2V, and you don't want to drive a 5V logic pin higher than that. An input logic high is typically >2.0V, the 4.7V 10% zener would always be greater than 4.2V.

2) Add some capacitance (~ 0.22uF) across the 5K and zener, to help get rid of noise spikes.

3) Another (possibly simpler) method is to connect a low-current (2mA) LED with ballast resistor (~1K) across the fuse. The LED will light if the fuse blows because of the minimal LED current going through the motor to ground.