diode mode  test for short to ground - Page 1
Forum section: electronics / beginners

author: ewcc
author_rank: Contributor
timestamp: March 28, 2023, 05:40:34 am
content: Hi,Having watched Louis Rossmann's video on diode-mode short to ground test (where you put the red probe onground and black probe on the part of the circuit you are testing), I'm still not understanding this. I'm nowfeeling 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 detectsif there is a direct (for lack of a better word) path between the two probes and gives a beep. (I thinkthis 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 diodeis bad, or some component (between the diode and ground) is bad.Am I correct in my understanding?ThanksEwx

author: EPAIII
author_rank: Super Contributor
timestamp: March 28, 2023, 06:13:53 am
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 28, 2023, 06:47:51 am
content: Quote from: EPAIII on March 28, 2023, 06:13:53 amMe, 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.A resistance range will show what the resistance of the "short" actually is.

author: MathWizard
author_rank: Super Contributor
timestamp: March 28, 2023, 11:52:02 pm
content: 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.

author: ewcc
author_rank: Contributor
timestamp: March 29, 2023, 02:53:35 am
content: QuoteI did some testing with the short that the diode-mode test showed, and indeed, the resistance range betweenthe '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.

author: MathWizard
author_rank: Super Contributor
timestamp: March 29, 2023, 08:04:57 am
content: Yeah I can't remember for sure what Louis says on that, but assuming nothing they are probing should measure under a few hundred ohms, I guess it's just quicker or easier to have the diode/continuity mode beep at you if there's a problem, than wait for a resistance reading.But yeah I'd have to watch those video's.Yeah a lot of DMM's continuity beeper's are set for dozens of Ohms. I can adjust it on my Siglent meter, but it never stores the settings.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 29, 2023, 08:20:41 am
content: The diode test mode, continuity test and low ohm ranges are not that different. The hardware part can even be the same, just a different way to show the result, either as voltage, the beep and either voltage or resistance and the voltage divided by the test current to get the resistance.The diode mode has not auto-rangeing, which makes it faster than the ohms mode with autoranging.To really localize a dead short (e.g. bad MLCC) the diode more is not really helping. This is more thing of using a lab supply to drive a more sizable current (e.g. 100 mA) and than use a sensitive voltage range.

author: m k
author_rank: Super Contributor
timestamp: March 29, 2023, 10:42:48 am
content: I'd say that diode and continuity tests are two very different things.Diode test can be used as a continuity test but beep is just a level too low and must be rechecked from screen.And either test by beep or single LED indication only is pretty useless.

author: xavier60
author_rank: Super Contributor
timestamp: March 29, 2023, 10:51:06 am
content: I have used the black probe with the red probe grounded on diode test range to find possible damaged inputs on logic ICs. On good inputs, the DMM beeps and reads the low-side input protection diode's voltage drop.

author: m k
author_rank: Super Contributor
timestamp: March 29, 2023, 12:54:26 pm
content: Possible damaged connections are nasty.How much variations are accepted, what level is the wrong side.

author: Michal Mytnik
author_rank: Newbie
timestamp: March 29, 2023, 07:24:44 pm
content: as someone said above it's pretty much the same, what differes is the ohm vs volatage. But why they do it, and not only Louise, is the speed (no autoranging in diode mode) and also becuase when probing he doesn't need to contantly looking at the display to see if no beep means there is some resistance but maybe be small. He just put a probe in many placeses and listen for a beep. It speeds up troubleshooting very much.Usually when they find coulprit they use ohm range to see how "big" is the short.They also usy it to check voltage drop when they find short. But primarly it's for speed.

author: EPAIII
author_rank: Super Contributor
timestamp: March 30, 2023, 07:22:00 am
content: I have done a lot of troubleshooting and, far from speeding it up, shortcuts like that are the best way to jump past actual problems. So you wind up taking much more time, not less.Just my $0.02s worth.

author: Michal Mytnik
author_rank: Newbie
timestamp: March 31, 2023, 06:07:28 pm
content: I tend to agree with you, it might be misleading at times. But I imagine Louise and others working on these boards every single day, pretty much know what might be wrong and just checking quickly a few spots here and there to localize the problem. As said, they are doing more thorough checking later and even then they have run right into the rabbit hole many times.In their business time is money so they want to check things as quickly as possible. And they have said many times that they are just listing for a short or long beep:), not that much checking for voltage drop.Also, when checking caps over power lines, the short beep is very important, cuse it tells them that they have put the probe tip correctly and there is a connection with solder joint. In resistance mode/continuinty mode you don't get that feedback, you need to check your meter whether it shows something on the screen (when you don't get any beep, that is).

author: floobydust
author_rank: Super Contributor
timestamp: March 31, 2023, 07:26:54 pm
content: Multimeter Diode-Test has a much higher test current and higher compliance-voltage, compared to the Ohms function.Old multimeters had the "low power ohms" feature. The ohms compliance-voltage is below a semiconductor junction Vf. So you could poke around accurately reading just resistance, without any semiconductor forward-biased junctions getting in the way. It's a necessary feature that fizzled out and is old knowledge.You have to know your multimeter- measure what it puts out on Diode-Test and Ohms for voltage and current.Typically I find Diode-Test is almost 2mA test current, and compliance-voltage at the battery limit or over 3V. Although one old bench multimeter I have puts out 15V on Diode and Ohms, that was a surprise. Great for dirty electrical or high power work, but death to IC's.Compare to a 100Ω/200Ω Ohms range which would be ~0.4-0.8mA depending on the brand of multimeter, half that of Diode-Test. Ohms compliance-voltage is usually clamped to 0.3-0.5V but still a problem with some IC's low Vf as Schottky junction will still mess up that reading.

author: EPAIII
author_rank: Super Contributor
timestamp: April 01, 2023, 09:32:28 am
content: My troubleshooting experience was on one of a kind problems on different pieces of equipment. I guess I didn't consider doing problems on identical boards, over and over. Sounds terribly boring.

author: ewcc
author_rank: Contributor
timestamp: April 03, 2023, 02:11:52 am
content: I want to thank everyone for the info on the diode mode checking for short. This diode mode check for short is a tricky thing to use and I was too caught up in that. I turned to checking using resistance mode.That said, I was overcomplicating myself. I knew for a certain that the board that I was looking at (a power board from a BENQ monitor) had a bad cap(470uF 16V). Replacing it actually fixed the monitor.What confused me the most was both the anode and the cathode of a diode was beeping in diode mode. Then Inoticed that the anode of the diode was connected to a transformer which traced to a ground plane. I thoughtI was tracing it wrong so I looked up the schematic and indeed it was. Which confused me even more sinceI had always thought that the cathode was supposed to be connected to the ground.Anyways, still learning.Thanks to this site, I have managed to fix two things. A JVC television and a BENQ monitor.Thanks all.ewcc