Electronics > Beginners
diode mode test for short to ground
ewcc:
Hi,
Having watched Louis Rossmann's video on diode-mode short to ground test (where you put the red probe on
ground and black probe on the part of the circuit you are testing), I'm still not understanding this. I'm now
feeling this might even be due to my lack of understanding of how a DMM works in diode mode.
As I understand it, in diode mode, a small current is passed between the two probes and the dmm detects
if there is a direct (for lack of a better word) path between the two probes and gives a beep. (I think
this is similar to the continuity mode).
So in 'theory', if both ends of a component, say a diode, beeps, it should mean that either the diode
is bad, or some component (between the diode and ground) is bad.
Am I correct in my understanding?
Thanks
Ewx
EPAIII:
You seem to be describing what would be continuity mode on most DMMs. It beeps if there is a short.
Diode test on the DMMs I have used passes a small current through the component and measures/displays the Voltage drop across that component. So a silicone diode, when the test leads are connected to forward bias it, would provide a reading of about 0.7 V or the junction Voltage of a silicone diode junction. When the test leads are connected for reverse bias the reading would be very high or an open condition would be displayed.
Germanium and Shockley diodes would show around 0.3V or 0.4V. You can also read the junction Voltages of bi-polar transistors in diode mode.
I have not seen the video you are talking about, but I would just use a low (but not necessarily the lowest) resistance range to check for shorts unless there was some particular reason for using the diode test function. Perhaps one such reason may be to limit the amount of current that the meter would inject into a particularly sensitive circuit. But I have been at it for a long, long time and have never damaged a circuit by taking resistance readings with either digital or analog multimeters. I would not hesitate to do so.
vk6zgo:
--- Quote from: EPAIII on March 28, 2023, 06:13:53 am ---You seem to be describing what would be continuity mode on most DMMs. It beeps if there is a short.
Diode test on the DMMs I have used passes a small current through the component and measures/displays the Voltage drop across that component. So a silicone diode, when the test leads are connected to forward bias it, would provide a reading of about 0.7 V or the junction Voltage of a silicone diode junction. When the test leads are connected for reverse bias the reading would be very high or an open condition would be displayed.
Germanium and Shockley diodes would show around 0.3V or 0.4V. You can also read the junction Voltages of bi-polar transistors in diode mode.
I have not seen the video you are talking about, but I would just use a low (but not necessarily the lowest) resistance range to check for shorts unless there was some particular reason for using the diode test function. Perhaps one such reason may be to limit the amount of current that the meter would inject into a particularly sensitive circuit. But I have been at it for a long, long time and have never damaged a circuit by taking resistance readings with either digital or analog multimeters. I would not hesitate to do so.
--- End quote ---
Me, too!
I am often bemused by the trouble people go to, so they can use "continuity mode" to chase shorts, relying on the "beep".
In the DMMs I have used which have this feature, the beep kicks in when the resistance is around 50-70 \$\Omega\$.
A resistance range will show what the resistance of the "short" actually is.
MathWizard:
I added a low ohms/continuity tester to my old bench DMM's, with a 100k between 5V, and the hi probe tip, low probe was GND still, so making a voltage divider with the DUT. And I used an op-amp as a comparator, to compare to a reference divider of 100k over a few ohms, that I can set. And then that triggered a multivibrator circuit that buzzed a peizo-disc buzzer. Plus a couple of protection diodes.
So for a very low resistance between the probes, you get a very low voltage, and lower than the set reference, and that triggers the buzzer. And the max current was really low, so nothing in the DMM would care, or the DUT.
I shoul;d redo it thio, but just use the Ohms settings, and just add the comparator +buzzer to that, and then get a real reading as well.
ewcc:
--- Quote ---I am often bemused by the trouble people go to, so they can use "continuity mode" to chase shorts, relying on the "beep".
In the DMMs I have used which have this feature, the beep kicks in when the resistance is around 50-70 \$\Omega\$.
A resistance range will show what the resistance of the "short" actually is.
--- End quote ---
I did some testing with the short that the diode-mode test showed, and indeed, the resistance range between
the 'short' was in the 10k range (so not a short). Which confuses me.
What is the purpose of the diode mode test that a few people swear by? (Louis Rossmann, and the guy at "Learning Electronics Repair" YT. ) though I suspect that I misunderstood all their intention.
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