Author Topic: Testing a Diode  (Read 968 times)

0 Members and 1 Guest are viewing this topic.

Offline cmccul002

  • Contributor
  • Posts: 9
  • Country: us
Testing a Diode
« on: November 14, 2016, 01:07:45 am »
Greetings all, fairly new to component level electronics repair. I

I have a scrap PCB board i was just poking around with a multimeter, testing components.

New to testing diodes, read up on it and was testing some on this PCB just to familiarize myself.

So i tested a few with my multimeter set to "diode" mode which is also a continuity tester.  So i tested a few, and got the normal readings with probes on the correct leads and reversed. 

During this testing, the continuity "Beep" did not come one,  then i came across this one diode where the reading was the same with the probes normal and reversed "0.660" and the continuity "beep" came one both ways.

I assume this diode is bad?
 

Online Rerouter

  • Super Contributor
  • ***
  • Posts: 4430
  • Country: au
  • Question Everything... Except This Statement
Re: Testing a Diode
« Reply #1 on: November 14, 2016, 01:14:08 am »
a diode drop can be between 0.25 (lowest i have ever seen them commercially) to 1.1V for some of the really cheap ones, so 0.66 seems reasonable,

The beep function you are hearing is called the continuity buzzer, generally used to check that 2 points are on the same wire, now the threshold where that buzzer will sound should be specced in the manual, it just means the voltage across the 2 points is under that threshold,

now as for a diode that reads the same way in both directions, it is likely caused by you measuring it "In-Circuit" and there may well be another diode in anti-parrellel somewhere else on the board, if you lift one of the diodes legs, you may find it only conducts in one direction.

It could even be a resistor in parrellel, as all the diode check function does is output a fixed current and report the voltage, a resistor could just as well drop that voltage for the test current before the diode gets a chance to turn on

In general a diode will fail either shorted (common in zeners), or open (common in conventional) and rarely is a difference of 0.72 vs 0.66 much of a concern
 

Online tautech

  • Super Contributor
  • ***
  • Posts: 16678
  • Country: nz
  • Taupaki Technologies Ltd. NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: Testing a Diode
« Reply #2 on: November 14, 2016, 01:44:22 am »
Greetings all, fairly new to component level electronics repair. I

I have a scrap PCB board i was just poking around with a multimeter, testing components.

New to testing diodes, read up on it and was testing some on this PCB just to familiarize myself.

So i tested a few with my multimeter set to "diode" mode which is also a continuity tester.  So i tested a few, and got the normal readings with probes on the correct leads and reversed. 

During this testing, the continuity "Beep" did not come one,  then i came across this one diode where the reading was the same with the probes normal and reversed "0.660" and the continuity "beep" came one both ways.

I assume this diode is bad?
Yep, you'll often get this when testing "in circuit".
When you do, the only way to be sure of the diode's real status is to get out the soldering iron and lift a leg.
Avid Rabid Hobbyist
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf