Diode mode testing - how does it work? - Page 1
Forum section: electronics / beginners

author: aneng
author_rank: Regular Contributor
timestamp: March 20, 2021, 11:42:12 am
content: 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?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 20, 2021, 12:43:06 pm
content: 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.

author: ogden
author_rank: Super Contributor
timestamp: March 20, 2021, 01:11:06 pm
content: Quote from: aneng on March 20, 2021, 11:42:12 amWhat if board contains single resistor? Then as @Kleinstein said - resistance mode indeed is betterDiode mode is better for testingdigital 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.

author: aneng
author_rank: Regular Contributor
timestamp: March 20, 2021, 04:12:34 pm
content: Sorry guys... I'm still not getting it. Can you explain in simpler terms please ?

author: ogden
author_rank: Super Contributor
timestamp: March 20, 2021, 04:26:29 pm
content: Quote from: aneng on March 20, 2021, 04:12:34 pmToo vague. More details please. Describe what exactly you did not understand, ask questions.

author: WattsThat
author_rank: Frequent Contributor
timestamp: March 20, 2021, 04:29:24 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: March 20, 2021, 05:02:58 pm
content: 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.

author: kripton2035
author_rank: Super Contributor
timestamp: March 20, 2021, 05:55:02 pm
content: https://electronics.stackexchange.com/questions/426442/how-the-diode-mode-of-a-multimeter-works

author: aneng
author_rank: Regular Contributor
timestamp: March 21, 2021, 12:10:52 pm
content: 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 ?

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2021, 12:18:53 pm
content: Quote from: aneng on March 21, 2021, 12:10:52 pmForshortfinding low ohms range is better than diode mode.

author: aneng
author_rank: Regular Contributor
timestamp: March 21, 2021, 12:53:37 pm
content: 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).

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2021, 01:42:38 pm
content: Quote from: aneng on March 21, 2021, 12:53:37 pmIt'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.

author: bdunham7
author_rank: Super Contributor
timestamp: March 21, 2021, 02:34:06 pm
content: Quote from: aneng on March 21, 2021, 12:53:37 pmWithout 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?

author: ogden
author_rank: Super Contributor
timestamp: March 21, 2021, 03:29:52 pm
content: Quote from: bdunham7 on March 21, 2021, 02:34:06 pmNo. 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.

author: Paul T
author_rank: Regular Contributor
timestamp: February 08, 2025, 01:24:30 pm
content: Quote from: aneng on March 20, 2021, 11:42:12 amThis statement is generally accurate, and is also accurate of resistance measurements. In fact, some multimeters such as the A830L specify their 2K resistance setting as the diode mode setting too.

author: CaptDon
author_rank: Super Contributor
timestamp: February 08, 2025, 02:28:21 pm
content: Take most You-Bube videos as here say. Ten different youtube videos yields eleven different opinions. As to your original question, several perfect explainations have been given. It is up to you to grab a circuit board, a good one or a bad one, and use your meter in the different modes and discover for yourself which is more useful to you. The technical details have been presented to you here as well as how the meter may react to different situations. If you don't 'get it' that is too bad. It is time for you to see actual measurements for yourself by experimenting. When I test a problematic circuit board I may use both modes to compare results.

author: BillyO
author_rank: Super Contributor
timestamp: February 09, 2025, 07:42:03 pm
content: On a cheap meter that does not have a good low ohms range, the diode mode might be a little more sensitive as it's giving a voltage and even most cheap meters will measure down to mV and some into the uV range.

author: mwb1100
author_rank: Frequent Contributor
timestamp: February 09, 2025, 08:24:17 pm
content: Quote from: aneng on March 21, 2021, 12:53:37 pmI'm curious about this - can you point to a specific video or two where this is stated? Even better if you can give an approximate time.

author: BillyO
author_rank: Super Contributor
timestamp: February 09, 2025, 08:39:20 pm
content: Quote from: mwb1100 on February 09, 2025, 08:24:17 pmI gave the answer above.

author: mwb1100
author_rank: Frequent Contributor
timestamp: February 09, 2025, 10:28:17 pm
content: I'm missing something (knowledge, or simply smarts).How does being able to measure down to mV or uV range in diode mode make it better than using the resistance mode for checking continuity? Generally meters with a continuity buzzer will buzz when the resistance is below a threshold somewhere in the 30-100 Ohms range (give or take). Surely even a crummy meter can tell you whether the resistance between two points is above or below 100 Ohms?

author: BillyO
author_rank: Super Contributor
timestamp: February 09, 2025, 11:13:22 pm
content: Quote from: mwb1100 on February 09, 2025, 10:28:17 pmYeah, it will tell you if there is a short, but it will not be able to pinpoint where the short is.For instance, on my BM786 it can measure down to 1/100 of an Ω, but that's not always good enough to tell WICH tantalum capacitor across the 5V rail is shorted. However, in diode mode it can measure down to 100uV. That is good enough to pinpoint the bad cap (or solder whisker, or copper whisker, etc.). The effect is even more pronounced for cheaper meters. Most DMMs are far more sensitive measuring volts than ohms.

author: ledtester
author_rank: Super Contributor
timestamp: February 09, 2025, 11:49:15 pm
content: Quote from: mwb1100 on February 09, 2025, 10:28:17 pmYou don't always use diode mode to check for continuity.In this video:https://youtu.be/-1HokBhP_64Sorin shows how resistance mode suggests to you that part of a circuit is a 1K resistor but diode mode reveals that it is PN junction.In this video:https://youtu.be/pRpyHF2dfq0diode mode is used to compare a bad board against a good board to find where the fault is.Modern ICs usualy have protection diodes on their input pins and a diode test can tell if they have been damaged (and thus the input pin might have suffered damage).

author: BillyO
author_rank: Super Contributor
timestamp: February 09, 2025, 11:58:49 pm
content: Quote from: ledtester on February 09, 2025, 11:49:15 pmOf course not, but the OP had asked why diode mode can be better for finding shorts than ohms mode. I assume he was talking about a regular DMM without a good low ohms mode. As you say, It's handy for other things too.

author: bdunham7
author_rank: Super Contributor
timestamp: February 10, 2025, 12:58:38 am
content: Quote from: BillyO on February 09, 2025, 11:13:22 pmI wouldn't say "most" as I can think of plenty of counterexamples. On those that use a precision constant test current, the only reason that the diode range would be more sensitive is if the test current was higher than the lowest ohms mode. The same generally goes for meters with a simpler design and a somewhat variable test current. I think the BM786 is one of the latter and does use a higher test current in diode mode. IDK for sure, the specs don't list the test current for the lower ohms ranges--perhaps you could check that? I'd be very curious as I'd expect a meter with a 600.00 ohms range and a lowest DCV range of 600.00mV to simply use a 1mA test current. Perhaps that's too easy.However, I noticed that you say that the diode mode measures to 0.1mV, so that seems to indicate that it is using the 6.0000range, not it's most sensitive 600.00mVDC. The manual says that the test current is ~0.35mA, so that would be the equivalent resistance resolution of only 0.28R. Perhaps I've made a math error somewhere here.

author: JoeyG
author_rank: Regular Contributor
timestamp: February 10, 2025, 06:44:57 am
content: to test anything electronic - typically you need to apply a voltage or a current and then measure a voltage or a current.If you apply a know (limited) current a voltage may be developed across the two terminals.If you apply a voltage with out limitation then you may overstress the electrical component under test.V/I curve traces are one example on in circuit component testers.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 10, 2025, 08:01:21 pm
content: Quote from: bdunham7 on February 10, 2025, 12:58:38 amThis is basically the answer. Diode mode can work better for finding shorts not because of anything relating to diodes or resistors, but simply because onmost models of DMM (especially those lacking a special Lo-Z resistance mode)the diode mode uses a higher test current/voltage than resistance mode. This means that the measurement (whether it's in terms of volts, amps, resistance, whatever) will be more accurate when applied to very low resistances.Of course this isn't universal, and it's entirely possible that some DMM models use the same or higher test current in resistance mode. Also if your test leads are crappy and have high resistance themselves that will cripple low resistance measurements regardless of what mode you use. So take the youtuber advice with a grain of salt and test it for yourself.

author: bdunham7
author_rank: Super Contributor
timestamp: February 10, 2025, 09:21:58 pm
content: Quote from: mtwieg on February 10, 2025, 08:01:21 pmThe other factor, what voltage range the meter uses for diode check vs resistance, is also important. Most DMMs will use a single voltage range for diodes and that generally won't be the most sensitive one. The most extreme example I have at hand is the Fluke 8842A bench meter where it uses a 1mA test current for the lower ohms ranges and the 2k range doubles as the diode check range. So it's the same test current, but there are 200R and 20R ranges with 10X and 100x gain, meaning the best resistance range is 100X as sensitive as the diode range. Since we're discussing the relative advantages of diode mode, why not do some checking and see what the numbers actually are?I'll start with 5 meters I have. Since there's no assurance that meters all use a constant test current (many don't) I'm measuring the current using a Fluke 289 50mA range which uses a 1R shunt which when combined with the fuse, etc yields a total resistance of about 1.5 ohms. I've recorded the meter type, the range, the current measured and the resistance reading shown. Because I had to use slightly different test leads in the setups no the actual resistance measurements have no relevance and are shown only to demonstrate the resolution.Fluke 27 (CAT III version) R: 300R / 0.29mA / 1.8R Diode: 0.385mA / .001 3.200V rangeHF DT830 (100% crap) R: 200R / 1.8mA / 1.9R Diode: 1.173mA / 002 2.000V rangeCen-Tech (HF) P37772 R: 200R / 0.238mA / 2.3R Diode: 1.312mA / .002 2.000V rangeFluke 189 R: 500R / 1.008mA / 2.12R Diode: 1.008mA / .0021 5.0000V rangeFluke 116 R: 600R / 0.300mA / 2.4R Diode: 0.854mA / 0.002 6.000V rangeSo out of 5 meters, three had higher test currents in diode mode, one had less and one the same. But in all cases, the lowest resistance mode had better resolution apparently because the voltage range used was lower. 4 out of 5 meters here are going to be pretty worthless for looking at anything less than a few ohms in diode range. Perhaps there's some other advantage to diode mode or maybe my collection of meters is not representative of the rest of the DMM world. So, diode-mode proponents--show us your diode mode!

author: BillyO
author_rank: Super Contributor
timestamp: February 11, 2025, 09:57:03 pm
content: Quote from: bdunham7 on February 10, 2025, 12:58:38 amYou're right. The Brymen uses: (burden of the measuring meter might affect values)0.37 mA for the 600Ω range0.18 mA for the 6KΩ range26 uA for the 60KΩ range2.5 uA for the 600kΩ range0.25 uA for the 6MΩ range0.03 uA for the 60MΩ rangeand0.51 mA for the diode range.In fact, I have only one meter (Kaiweets KM100 - cheap, but not quite as bad as your DT830) where the diode range is the same sensitivity as the lowest ohms range.I don't remember where, or how long ago I read that, but I should have fact checked it.Thanks for pointing it out. It's good to clear the crap from one's brain.

author: TheoB
author_rank: Regular Contributor
timestamp: February 11, 2025, 10:48:54 pm
content: Resistance mode is mostly using a low burden voltage below 0.5V to measure a current. At such low voltages most silicium diodes don't conduct. A multimeter with autorange will quickly range up to a higher range until OL is displayed. The diode mode uses a test voltage well above the junction voltage.When searching for faults on a circuit board both modes can be used. A hard short will give a low voltage in diode mode or a low resistance in the Ohm mode. More interesting is to measure a pin on a connector. The pin on the connector should be connected to a pin of a chip that you cannot probe. In that case only the diode mode will show the presence of the chip's input pin ESD protection. The Ohm mode would only show an open.When semiconductors are tested in productions each pin of the device is tested for proper contact using this method even before the first actual test starts. Just make sure that all pins make good contact.

author: bdunham7
author_rank: Super Contributor
timestamp: February 12, 2025, 12:43:19 am
content: Quote from: TheoB on February 11, 2025, 10:48:54 pmThat's a good explanation and reason to use diode mode. But is it true?Here are the open circuit voltages of my 5 meters in the lowest resistance range and in diode mode:F27: 320R 2.00V Diode 2.54VDT830: 200R 2.93V Diode 2.93VP37772: 200R 0.54V Diode 2.62VF189: 500R 5.17V Diode 5.17VF116: 600R 0.52V Diode 2.52VAs you can see, only two out of five sort of match your explanation. Of those, one (the P37772) is manual ranging and indeed reads a standard 1N4001 diode as open in the 200R range and 0.579 in diode mode. The other, the F116, actually does read the diode as a high resistance and if you manual range it, it give a wildly different resistance in each range. So diode mode is good for looking for diodes, for shorts maybe not so much.