Multimeter and Diode Testing

Thread Starter


Joined Sep 26, 2018
Hi, I recently took up electronics as hobby and I am still a beginner. Someone gave me an old LCD screen that was not working. I discharged the killer capacitor. I started taking some components out of circuit and testing them. I came across two diodes. When I measured the one, an SR5150, in diode mode it gave me a reading of 0.00000 but didn't produce a tone; reverse it reads OL as expected. I put it aside . I then measured the next diode, a SR506, and it again gave me a reading of 0.00000 but produce a tone; reverse it reads OL as expected. I thought these readings were odd and so I went into my parts bin and took out an new SB560 I bought from RS a year ago and measured it, and i got 0.00000 with a tone; reverse it reads OL as expected. I am very suspicious about the forward readings.

Anyway, then thought if I apply a voltage across the diode i can measure the Voltage drop in the forward direction. I hooked up the SB560 diode to the 12V rail of an old PC power supply I use as a bench supply. Almost immediately as I connected the diode, the led on the PSU went really dim and some smoke came out the crocodile clip on the negative lead of the diode and the cable to the clip extremely hot. I very quickly disconnected the crocodile clip. The light on the PSU came back on.

  1. With much smaller diodes the multi-meter reads Vf just fine. Is the issue above a short coming of the multi-meter or do I maybe need a new battery?
  2. If i connect a diode to a power source without any other circuitry will it just try to pull as much current as it can or did I maybe connect it incorrectly?


Joined Jun 4, 2014
Yes, a diode straight across a power supply in the forward conducting direction will draw a LOT of current.
Assuming your meter reads the forward voltage of some diodes correctly then I can offer no explanation of the zero readings you have seen.


Joined Oct 2, 2009
If you connect a diode in forward bias across a 12V PC power supply you will most likely destroy the diode.

If you want to test a diode across a power supply, place a 1kΩ resistor in series with the diode.

With that resistor in place, the maximum test current the diode will experience is the supply voltage divided by 1kΩ.

Thus, if the supply voltage is 12V, the max test current is 12V/1kΩ = 12mA.


Joined Aug 10, 2008
This reminds me of a colleague, an EE, who proclaimed that any diode can be "light emitting... at least once!"

Follow what MrChips describes above. What you did was apply a low impedance voltage supply across your diode with no means of limiting the current. With too much current through the diode you likely destroyed it.


Joined Mar 10, 2018

As you can see once you exceed the threshold voltage the current skyrockets,
especially if you have no limiting as prior poster suggested.

Not also very little flows when reversed biased until it hits the breakdown V
and then current again skyrockets.

More than you want to know about diodes -

Regards, Dana.