Diode mode measurement question

[caphair]

When using diode mode to find a short in a circuit (not just across a diode) I've read to use red probe on ground and black on the pins of an ic, component.

Why red on ground and not black probe on ground?

[budm]

Re: Diode mode measurement question

When I look for shorts I use Ohm mode since shorted Semiconductor will no longer acts like semiconductor any longer, and you also see how low the resistance of what you are measuring.

[caphair]

Re: Diode mode measurement question

Here's an excerpt from the method I'm talking about found on another website except it doesn't explain why to use red probe on ground

"Testing Chips.
The DMM's diode test mode can also be used to check most logic chips. It's best to test chips out of circuit, but frankly this just isn't possible most of the time.

To test chips, the ground leg of the chip in question will need to be known (that's why we went through the "short course on logic chips" above). Then put the RED lead of the DMM on this ground leg. Yes I know, it sounds weird. "Why the RED lead on the ground leg? Isn't that backwards?" Backwards or not, that's how to do it. With the red lead on the ground pin, check all the other pins (except for VCC, which is the chip power pin) with the black lead of the DMM. Again .4 to .6 volts should be seen for each leg. If a different value is seen, chances are the chip is bad. The biggest indicator of a bad chip would be a value less than .2 (probably a short)."

[Longbow]

Re: Diode mode measurement question

You are wasting your time reading material from this source. There is no such thing as a magic reverse diode ic test.

[flocko]

Re: Diode mode measurement question

the confusion here is with testing of a digital Logic ICs (Nand/And/Or/Nor gate ) that have only two states (Hi+Low) and Linear (Analog) Semiconductors ICs. By definition a short circuit would be a low Ohm resistance between two points of a circuit, and should be tested with your meter set to show low Ohms. I hope this is of help Flocko

[Longbow]

Re: Diode mode measurement question

No, I don't think there is any confusion. The question is whether probing around with a diode tester is a productive troubleshooting method and the answer is no. Faulty discreet logic IC's are generally discovered by observing state changes with an oscilloscope or taking voltage measurements along the cicruit path. Simple short-circuit failures of any component are even easier and don't require diode testing to confirm.

[cpt.charlie]

Re: Diode mode measurement question

I guess that polarity inversion is done to to avoid the unwanted activation of other components in circuit like diodes, this way if something is shorted you will have a low reading, but keep in mind that if you are working with complex chips (like CPU/GPU chips) you will get a low reading, the bigger the chip, the lower the reading will be.

[jiroy]

Re: Diode mode measurement question

When you're out of a schema and an Oscilloscope , things are tough and no method is excellent enough to catch the error . But this method is far from reality and i can even say , out of logic .. If forced , i use 2 very effective methods :

1 - the first method is very easy and can apply to beginners as well as professionals . Locate Zener diodes and regulators . Zener Diodes are obvious to give the same voltage as they are labeled , and so do regulators like , for an example a 7805 (positive ) or a 7905 (negative) . A small example , a non powered regulator lead me to diagnose a faulty transformer on a Health Dryer machine in less than 10 minutes .

2 - The second method is only for professionals , it needs a combination of strong logic and long experience ... Sounds like a Doctor , listening to the patient heart and feeling the pain he's into . Searching for bleed , oops , a rust or humidity , a blown component amid dark brownish areas . A small example , One Aiwa Stereo with only some led lit when powered . The Digital screen is dark and so anything else .
Well , go directly to the problem , a B1370 (2sb1370) , and it's shorted all the the ways .

[caphair]

Re: Diode mode measurement question

I guess that polarity inversion is done to to avoid the unwanted activation of other components in circuit like diodes, this way if something is shorted you will have a low reading, but keep in mind that if you are working with complex chips (like CPU/GPU chips) you will get a low reading, the bigger the chip, the lower the reading will be.

I see so it's almost like testing a diode in reverse polarity looking for an infinite reading as oppose to its voltage drop?

[cpt.charlie]

Re: Diode mode measurement question

That's it.