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Old 02-21-2021, 12:05 PM   #1
Askic
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Default Testing for short circuit in diode mode

Hello electronics experts,

On a few occasions I notice experienced technicians inspect for short circuit in diode mode on multimeter. This is OK and in some way equivalent to resistance mode, but more specifically I noticed that they put red probe on ground and black probe (connected to COM input on multimeter) is placed on the rail or pin.
I wonder why is that. Why not place black probe of multimeter on the ground and red probe on the rail which is tested for short?

Can you clarify this? When I test if something is short I use diode mode First and then double check by measuring actual resistance against the ground.

One more thing, if multimeter shows me on one place (in diode mode) 380 and on some other place 680 to the ground, what I should conclude from this reading?

I know that in both cases there is no short to ground, but what this 2x number in second case should tell me?

Thank you!
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Old 02-21-2021, 01:13 PM   #2
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Default Re: Testing for short circuit in diode mode

your talking about Rossmann.

i wouldnt advise what he does - most meters diode-test at over 2v and sometimes as much as 4v or more.
checking digital circuits that may be running as low as 1.8v could do some damage
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Old 02-21-2021, 01:21 PM   #3
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Default Re: Testing for short circuit in diode mode

Quote:
Originally Posted by stj View Post
your talking about Rossmann.
Exactly, I have watched dozens of his videos and I noticed that.

Do you recommend only resistance mode on multimeter? What would be recommended practice here?

Does this depends on the multimeter type itself? I notice that most repair guys use Fluke.
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Old 02-21-2021, 01:48 PM   #4
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Default Re: Testing for short circuit in diode mode

fluke only has one model even close to suitable for electronics,
they are overpriced and targetting electricians.

to know if you should use resistance or diode mode you need to check the manual for the meter.
if you cant find one - find a datasheet for the chip in it.

diode test can light an led - i consider that risky.
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Old 02-22-2021, 12:32 AM   #5
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Default Re: Testing for short circuit in diode mode

Older digital multimeters had "low power ohms" that apply under 0.4V which keeps diodes and most semiconductors off, allowing you to make ohms measurements in-circuit.

You would use both with semi's: First diode-test (both polarities) and if a suspicious/low V is measured, then check on ohms. Because a shorted or leaky semi no longer has the barrier voltage and will give a reading on low power ohms.
If in doubt, lift a pin/lead and remeasure.
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Old 02-22-2021, 02:22 AM   #6
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Default Re: Testing for short circuit in diode mode

Most DMMs (at least I have yet to find a DMM, even old ones) do not supply enough current in ohms mode to damage semiconductors. They may still turn them on which may affect your readings depending on whether you forward bias or not. Yes, the low power mode was intended to keep semiconductors off in older DMMs which got obsoleted with the low voltage newer DMMs.

This is not true with analog VOMs, but it depends on the particular meter. I've found pretty much all battery powered passive VOMs in Rx1 and a Heathkit VTVOM in Rx1, Rx10 to be able to fry sensitive semiconductors. I would suspect a lot of old VTVOMs to be the same way.

However I have a old analog FETVOM that does not supply enough current in Rx1. Unfortunately the sample size = 1 so I don't know if all analog FETVOMs are ok or not.
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Old 02-23-2021, 05:01 PM   #7
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Default Re: Testing for short circuit in diode mode

Quote:
Originally Posted by redwire View Post
older digital multimeters had "low power ohms" that apply under 0.4v which keeps diodes and most semiconductors off, allowing you to make ohms measurements in-circuit.

You would use both with semi's: First diode-test (both polarities) and if a suspicious/low v is measured, then check on ohms. Because a shorted or leaky semi no longer has the barrier voltage and will give a reading on low power ohms.
If in doubt, lift a pin/lead and remeasure.
+1
Diode mode does not show resistance reading, it shows Voltage drops reading of the device under test.
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Old 02-24-2021, 12:26 AM   #8
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Default Re: Testing for short circuit in diode mode

Ok, I know that if I test diode and I see 605 on my multimeter, that these are mV. But what about practice to place red probe on the ground and black (common multimeter) on point of interest. Is this some kind of preventive practice (reduced risk of damaging components)?
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