How to determine specs of dead diode - Page 1
Forum section: electronics / beginners

author: giosif
author_rank: Frequent Contributor
timestamp: July 28, 2019, 03:03:20 pm
content: Hi,I'm repairing a (Gossen) ABB Metrahit 15 multimeter and, in the process, I found a dead (mini)MELF diode.As such, I am trying to determine the specs of that diode, so I can procure a replacement.Doing a bit of searching, I don't think that's possible based on the marking on the diode alone (it has a blue band, BTW), and one would need either the schematics or some understanding of the circuit where it was located.Now, with that latter part, I am trying to "decypher" that section of the circuit, but I'm yet to make any significant progress.In the meantime, I wanted to check if my above statement is correct (apart from the fact that the blue band most likely means it's a Zener).Thanks!

author: _Wim_
author_rank: Super Contributor
timestamp: July 29, 2019, 07:10:45 pm
content: Does it look anything like my gossen 22m posted here:https://www.eevblog.com/forum/testgear/gossen-metrahit-22m-26m-internal-photos-wanted!/msg2184389/#msg2184389If yes, maybe the diode in question can be tested on my meter.

author: giosif
author_rank: Frequent Contributor
timestamp: July 29, 2019, 08:42:47 pm
content: Thanks for the link and the offer, but the Metrahit 15 is quite a bit different from the 22M.The diode looks more like the one in this picture (upper middle, just above the rightmost green capacitor).Also, the diode is located in a different place on my meter (I'll look to provide a picture of the section in the coming days).Quote from: _Wim_ on July 29, 2019, 07:10:45 pm

author: giosif
author_rank: Frequent Contributor
timestamp: July 31, 2019, 07:03:17 am
content: And here is the promised picture, where I am showing the location of the diode, as well as it original orientation.I also tried to understand the part of circuitry the diode belongs to and I believe it is overvoltage protection for the meter power supply, I am guessing from any induced voltages (as the battery would not be able to create such an overvolatge condition; also, this meter does not have a DC power input jack, like other Gossen meters).If this is indeed the case (which really needs confirmation), I would be guessing then this is a 9V zener or thereabouts (the meter is powered from a 9V battery).Would you agree with the above?

author: _Wim_
author_rank: Super Contributor
timestamp: July 31, 2019, 06:02:07 pm
content: Does the meter work correctly when you remove the diode? If that is the case, then there is indeed a good chance the diode was for overvoltage protection only. 9V seems like a reasonable value in that case, but when the diode is removed while testing the functionality, just measure the max voltage across the pads, this way you will have a good idea what voltage it needs to withstand during "normal" operation.

author: giosif
author_rank: Frequent Contributor
timestamp: July 31, 2019, 07:52:40 pm
content: Yes, the meter seems to be fully functional with the diode removed.Good point about measuring the voltage across the pads.Thank you!

author: thedoc298
author_rank: Contributor
timestamp: July 31, 2019, 10:55:03 pm
content: Was the diode open or short.

author: giosif
author_rank: Frequent Contributor
timestamp: August 01, 2019, 07:22:44 am
content: Quote from: thedoc298 on July 31, 2019, 10:55:03 pmIt was short.

author: giosif
author_rank: Frequent Contributor
timestamp: August 02, 2019, 07:54:28 am
content: Quote from: _Wim_ on July 31, 2019, 06:02:07 pmJust an update on this: I measured the voltage across the pads with the meter turned on and it was about 3V.This is because, while one of the pads is directly connected to the positive battery terminal, the other pad is connected to COM input, not the negative battery terminal.Given this, I am not that sure of the voltage for the zener.Anyone any suggestions?

author: giosif
author_rank: Frequent Contributor
timestamp: August 02, 2019, 05:51:46 pm
content: Just to update everyone here that, in the end, I opted for a 3.3V zener.My decision was based on the following details:- the measured voltage across the two pads where the diode is supposed to be installed (i.e. a little over 3V)- one of the pads is always connected to the COM jack, while the other is connected to the V jack (via a 1.3kOhm resistor)onlywhen the rotary switch is set to Diode mode- whatever information I was able to find about a Gossen meter with a model number close to this one (i.e. 15s), it mentioned a maxmium no-load output voltage of 3.2V for Diode mode.I hope my judgement wasn't too far off.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: August 02, 2019, 05:58:26 pm
content: I found a couple of diode colour charts, here:http://www.marsport.org.uk/smd/sod80.htmAnd here:http://www.crystalradio.net/cal/diodeid.shtmlThe first one covers MELFs, but the blue band appears to have a generic meaning of 'zener' so not at all helpful!Your thread prompted me to do a bit of research on the subject. I was not aware that the colour bands on glass diodes could be read is a similar way to resistors, although it seems that the colour codes may depend on the manufacturer as well.Still, it seems that you have now made a determination based on your research.

author: _Wim_
author_rank: Super Contributor
timestamp: August 02, 2019, 06:34:08 pm
content: Quote from: giosif on August 02, 2019, 05:51:46 pmIf with the original shorted diode it was only the diode test function that was not working on the broken multimeter, than this seems like a good choice.

author: giosif
author_rank: Frequent Contributor
timestamp: August 03, 2019, 06:54:11 am
content: Quote from: _Wim_ on August 02, 2019, 06:34:08 pmWell, that wasn't really the case: the meter would turn on, but I remember it beeping repeatedly and various annunciators of the LCD being on, while they shouldn't have been.Since one side of the diode is directly connected to the positive terminal of the meter battery and, consequently, various power rails derived from that, I suspect the diode going short (to COM, not to battery negtaive terminal) was affecting the power rails which led to things like the microprocessor go haywire.I hope...

author: _Wim_
author_rank: Super Contributor
timestamp: August 03, 2019, 07:32:09 am
content: Ok, I was under the impression the diode was only connected on both sides when the switch was in diode mode, but it is the 1k3 resistor that is only in this mode in series with the diode, and the diode is always connected.I think the 3.2V zener is a good starting point, but there is a risk it will brake again under some measurement situations because it voltage was too low. But it is safer for the meter to start with a low voltage value and increase when necessary, than the other way around.

author: giosif
author_rank: Frequent Contributor
timestamp: August 04, 2019, 10:00:04 pm
content: Quote from: _Wim_ on August 03, 2019, 07:32:09 amThat's correct.Sorry, I didn't explain things that well from the beginning.Quote from: _Wim_ on August 03, 2019, 07:32:09 amAgree.I'll install the new zener and use the meter in various settings and input voltages (although, all will be below 35V).

author: giosif
author_rank: Frequent Contributor
timestamp: August 05, 2019, 10:07:18 am
content: Aaand it's the wrong value: in capacitance mode, the voltage across the pads increases to 5V.I should have checked all settings, not just some of them.Lesson learnt for next time.Now, I need to look for a zener going above 5V (yes, I now checked all settings and 5V is the highest reading I got across the diode pads).I guess either 5.1V or 5.6V (these seem to be the more popular values).I will probably go with 5.6V, for some margin.

author: _Wim_
author_rank: Super Contributor
timestamp: August 05, 2019, 07:26:12 pm
content: Does the voltage across the pads vary when the measured voltage/current goes up and down? If that is the case, then testing with only 35V will not be sufficient to see to max voltage, and maybe some additional margin is required. Otherwise I would go with 5.6V as you suggested

author: giosif
author_rank: Frequent Contributor
timestamp: August 05, 2019, 09:17:51 pm
content: Quote from: _Wim_ on August 05, 2019, 07:26:12 pmHmm... That I have not tried.I will run some tests and report back.

author: giosif
author_rank: Frequent Contributor
timestamp: July 01, 2020, 09:53:46 pm
content: Hi again,Sorry, it's been a while since looking at this, but just wanted to say a little birdie told me the Zener diode might be a 6.2V one.