Diode with wrong voltage drop (answered) - Page 1
Forum section: electronics / beginners

author: IsmAvatar
author_rank: Contributor
timestamp: February 18, 2022, 02:55:33 am
content: I bought a strip of 0.6v SMD diodes and used many of them in a project. After a little while, the project started behaving weird, so upon inspection, I found one of the diodes now reads 1.5v (both in and out of circuit). I replaced it and the project works perfectly again.I was wondering what causes the diode's forward voltage to permanently change like that. I was also wondering if this diode is still usable anywhere I might need to drop 1.5v (low current of course) or if it's just a tiny paperweight now.

author: MK14
author_rank: Super Contributor
timestamp: February 18, 2022, 03:10:57 am
content: Assuming it's a silicon small signal diode. It should read around 0.6V to 0.7V (EDIT: Correction, see posts below. At a low current of around 1ma or a multimeter diode test mode).It sounds like that diode has gone faulty, and would probably get worse if used in a circuit now and/or is possibly leaky as well (reverse bias current much higher than it should be).Cause probably means it was abused when soldered into position, too much heat and/or high temperature on soldering iron ?Or was overloaded/overvoltaged in circuit, or (very rare) went faulty all by itself.Alternatively, if they were very cheap, Chinese Ebay/Aliexpress/etc ones. They could be of sub-par quality.

author: fourfathom
author_rank: Super Contributor
timestamp: February 18, 2022, 03:15:09 am
content: How much current are you running through these diodes? What are the diode specs, or part numbers?

author: IsmAvatar
author_rank: Contributor
timestamp: February 18, 2022, 03:48:46 am
content: I'm not sure of the current, but it's driven by an Atmega, so should be 40 to 50 mA. The diodes are marked T4, which I believe is 1N4148 SOD-123.And yes MK14, I suspect I abused it with the soldering iron, cooking one of the legs a little too long. I didn't think about current leak - that would probably make it less than useful in a circuit. Perhaps it started off at 0.6v and because of the, erm, prior cooking, it just continued to deteriorate and drift over time. I think that answers both of my questions. Thank you very much.

author: tooki
author_rank: Super Contributor
timestamp: February 18, 2022, 07:33:10 am
content: Quote from: IsmAvatar on February 18, 2022, 03:48:46 amFYI, for common MCUs like the Atmega328p, 40mA is the absolute maximum rating per pin, and the absolute maximums aren’t intended to be normal operating conditions. 10-20mA should be your design targets.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2022, 08:09:51 am
content: The forward voltage of a diode is not fixed but depends on the current. The tendency is to have a higher voltage drop with smaller diodes and also low leakage diodes have a higher voltage drop. 1.5 V would be very high for a silicon diode: even the low leagage BAV199 should not go much above 1.2 V or 900 mV at 1 mA. For high voltage there are a few types that internally have multiple diodes in series - though I would not expect these to be available in SOD123.The measurement with the µC may be off.

author: MK14
author_rank: Super Contributor
timestamp: February 18, 2022, 10:06:27 am
content: Quote from: Kleinstein on February 18, 2022, 08:09:51 amFor my answers, because they were referring to 'reading' (of a strip of components), I was assuming they were using a multimeters diode test mode. With reasonably fixed test characteristics, rather than a wildly varying current load.But, I should have been clearer in my answer(s) and/or clarified how they were reading the voltage, with the OP.On the other hand, 1.5V sounded way too high, to be a working normal diode, especially as they seemed to say it read 0.6V originally.I've now somewhat corrected my original post.