Diode mode testing - how does it work?

[aneng]

I've watched loads of videos on how testing boards is better using diode mode but none of them have explained the actual theory behind it.  As far as I have been able to tell, diode mode puts a constant current between the electrodes and measures the voltage drop across them (?).  Is that right?

If so, apart from speed, why is it better to use diode mode for testing boards?  Is it purely down to this mode forward-biasing any p-n junctions, in the path to ground that you are measuring?  If so, why is that better?

[Kleinstein]

The diode mode is not that different from a lower resistance mode.

As written the diode test mode shows a voltage, usually up to some 2 V maybe even a little more. The lowest ohms modes use a comparable current (may very well be the same source), but a lower voltage range (e.g. 200 mV) and give the result as resistance with an additional scale factor. So the low ohms modes will usually show a forward PN juntion as open.
It depends what one looks for: for testing diodes the diode mode is better, for in circuit resistor measurements the resistance mode is better.

[ogden]

If so, apart from speed, why is it better to use diode mode for testing boards?

What if board contains single resistor? Then as @Kleinstein said - resistance mode indeed is better Diode mode is better for testing digital circuts, and obviously transistors and diodes. Take microcontroller board (eg Arduino) you have on hand and go through it's pins using resistance mode (pin->gnd), then diode mode, then again resistance mode. Using resistance mode you may have unconclusive results, you will see pull-ups and pull-downs not knowing - trace is connected to IC or not. Digital input may show as pure disconnect in resistance mode. Using diode mode you will measure junction(s) of the IC and you will see which trace most likely is connected to healthy pin of the IC and not just pull-up resistor.

[aneng]

Sorry guys...  I'm still not getting it.  Can you explain in simpler terms please ?

[ogden]

Sorry guys...  I'm still not getting it.  Can you explain in simpler terms please ?

Too vague. More details please. Describe what exactly you did not understand, ask questions.

[WattsThat]

Testing boards for what? You never explain.

The simplest explanation is hopefully this: Resistance is displaying ohms, diode test is displaying volts.

They are two entirely different functions. One is used for measuring resistors and looking for shorts, one is used to test diodes and transistors.

Resistance uses a constant current source. Apply a constant current across an unknown resistance and you get a voltage across the test point. That voltage is proportional to the current supplied and the resistance under test. Different resistance ranges use different currents.

Diode test is a voltage test. The meter places 2-3 volts on the probes at a small limited current. When the probes are connected to a semiconductor junction that is forward biased, the device conducts and you’ll read the voltage drop across the junction, typically 0.7 volts for silicon, 0.3 volts for germanium (not that you’ll find many of those these days). A reversed based device will read INF or open, a damaged or shorted device will read zero or very close to zero volts.

Unfortunately, a large percentage of the videos on YouTube are utter garbage. Provide a link if you want people to understand what you’ve watched and perhaps they can dispel any bad information from the fountains of mis-information known as the World Wide Web.

[bdunham7]

It's hard to give generic advice, as troubleshooting is usually a more specific endeavor.

FWIW, the difference between 'diode mode' and 'ohms mode' is also a meter specific thing.  In at least some cases, the diode mode and one of the ohms ranges, typically the 2K range, are exactly the same thing electrically. 

[kripton2035]

https://electronics.stackexchange.com/questions/426442/how-the-diode-mode-of-a-multimeter-works

[aneng]

Sorry -  I wasn't specific enough in my original question.  What I'm trying to get my head around is why diode mode is better for finding shorts on computer motherboards than ohms measurement.

Why can diode mode find shorts when ohms mode often cannot ?

[ogden]

Sorry -  I wasn't specific enough in my original question.  What I'm trying to get my head around is why diode mode is better for finding shorts on computer motherboards than ohms measurement.

Why can diode mode find shorts when ohms mode often cannot ?

For short finding low ohms range is better than diode mode.

[aneng]

So why do all the popular YouTube techs extol the virtues of diode mode for this task?  Louis Rossman, Northridge Fix, Sorin, probably even Dave (though I haven't actually checked his channel).

[ogden]

So why do all the popular YouTube techs extol the virtues of diode mode for this task?  Louis Rossman, Northridge Fix, Sorin, probably even Dave (though I haven't actually checked his channel).

It's because there are other measurements which are not short finding. For such diode mode in general is more useful than resistance mode. It is explained here in this thread already.

[bdunham7]

So why do all the popular YouTube techs extol the virtues of diode mode for this task?  Louis Rossman, Northridge Fix, Sorin, probably even Dave (though I haven't actually checked his channel).

Without specifying the particular meter you are using the discussion seems pointless to me.  I don't know why they make that recommendation--perhaps they don't know how their meters actually work? 

[ogden]

I don't know why they make that recommendation--perhaps they don't know how their meters actually work?

No. They recommend it because many don't know how useful diode mode actually is for certain board troubleshooting measurements, don't know when and how to use it, how to interpret results. I did watch episode where Louis Rosmann talks about his "discovery" of diode mode. I think he explains it quite well.