Project: diode-in-automotive-relay tester

Thread Starter


Joined Oct 12, 2009
Hi All,
this is the first time for me to start the thread, I apologize for all mistakes.
this project started when i was still working in heavy equipment dealer. some problems happened with electronic devices in machine due to unknown reason. as I learned that all inductive electrical devices create back Electromotive Force (EMF) that can harm electronic circuits in the same installation. one of them is relay. that's why relay has internal quenching diode inside it, to 'drain internally' the back EMF.
but sometimes the diode failed to work or broken. since the automotive relay is enclosed with metal case, it's hard to see it. and after doing some test using ohmmeter on terminal 86 and 85 on good relay, i found no different reading since the diode is in parallel with the coil.
then I made this simple device called Diode-in-automotive-relay tester. i used LED as a detector. I used one 9V dry battery to energize the coil (whether 12V or 24V relay works), when the switch is pressed, current will flow through the coil, after 1 second, release the switch and see if the LED flash. if the LED flashes, the diode inside the relay is broken.
component :
1 9V battery
1 Resistor 10K 1/2 watt
1 LED (any color)
1 push button switch
have a try!