An unclear situation with a Schottky diode. - Page 1
Forum section: electronics / projects

author: iet
author_rank: Frequent Contributor
timestamp: July 30, 2024, 01:50:19 pm
content: An unclear situation with Schottky diodes 1N5819. There is one removed from the board and a new diode. After measuring Fv, it became clear that there is a fairly significant difference between 0.153 V for the one removed from the board and 0.213 V for the new one. I also noticed that the dimensions of the new one are two times smaller than those removed from the board. Who has dealt with such diodes in their practice??The measurements were taken with two multimeters and tweezers.

author: moffy
author_rank: Super Contributor
timestamp: July 30, 2024, 02:25:51 pm
content: The data sheet is found at:https://www.onsemi.com/download/data-sheet/pdf/1n5817-d.pdfYou should be able to answer your questions from that.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 30, 2024, 03:08:35 pm
content: Just like all other electronic components Schottky diodes also come in different forms. Differences in handling current, leakage, maximum reverse voltage, forward voltage drop, size, power dissipation (correlated to size) etc.

author: iet
author_rank: Frequent Contributor
timestamp: July 30, 2024, 03:12:25 pm
content: moffyThanksThe datasheet gives the maximum value for 1N5817,18,19. Fv-0.32,0.33,0.34v. The real value, as we can see, can differ significantly.

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 30, 2024, 04:09:08 pm
content: Quote from: iet on July 30, 2024, 03:12:25 pmIt may differ, but assuming equal test current and temperature (and equally working part) it should not bethatdifferent. Anyway those 0.3v values are for 0.1A IF, meters often reads somewhat lower than that simply due to the lower test current.

author: iet
author_rank: Frequent Contributor
timestamp: July 30, 2024, 07:02:34 pm
content: Quote from: ArdWar on July 30, 2024, 04:09:08 pmThis is very interesting in general in relation to Schottky diodes. Three meters give the same result on two diodes.

author: iet
author_rank: Frequent Contributor
timestamp: July 30, 2024, 07:13:32 pm
content: Quote from: Doctorandus_P on July 30, 2024, 03:08:35 pmI encountered this problem for the first time. What is the correct way to measure?

author: dietert1
author_rank: Super Contributor
timestamp: July 30, 2024, 07:28:11 pm
content: As you can see in the datasheet, forward voltage correlates with maximum reverse voltage spec. Different schottky diodes do exist! For lower forward voltage one could get a 3 A or 5 A 20 V schottky, or two of the smaller diodes in parallel.Regards, Dieter

author: Phil1977
author_rank: Frequent Contributor
timestamp: July 30, 2024, 07:36:26 pm
content: 1N5819 is a very common type that´s available from many sources in different housings - so it´s no big surprise to have some variation.In case you have doubts about the quality of the new parts you probably have to do some tests under load - measure Vf at 1A and check for allowed dissipation. You can also check the Vr-reverse-breakdown voltage with a voltage source limited to e.g. 0.5mA. It´s a little more effort to check recovery time.In case these tests pass everything should be okay. It would be no good circuit design if the application requires a 1N5819 that´s better than specified in the datasheet.In case these tests fail then it´s probably a fake. Nothing is to cheap to be faked

author: iet
author_rank: Frequent Contributor
timestamp: July 30, 2024, 08:23:45 pm
content: Quote from: Phil1977 on July 30, 2024, 07:36:26 pmI don't have such opportunities.

author: Seekonk
author_rank: Super Contributor
timestamp: July 30, 2024, 09:03:07 pm
content: I have some 15A 15SQ045 commonly used as bypass diodes in solar panels. I was given some higher current diodes of the same flavor. I forget the number and the data sheet had little info. The forward voltage of the higher current diode at room temp was higher and that surprised me. Putting both diodes in series with a thermal break I ran them up to temperature with current only about 5A. At temperature the higher current diode then had the lower forward voltage. Didn't expect that.

author: iet
author_rank: Frequent Contributor
timestamp: July 30, 2024, 09:22:24 pm
content: Sometimes I wonder why Schottky invented these diodes. Just kidding.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2024, 02:20:18 pm
content: Quote from: ArdWar on July 30, 2024, 04:09:08 pmI measured about one third of that difference with a set of 9 1N5819s, but they were likely all from the same order, so 60 millivolts would not surprise me at all between two different batches or two different manufacturers.

author: iet
author_rank: Frequent Contributor
timestamp: August 01, 2024, 11:17:07 am
content: Quote from: David Hess on July 31, 2024, 02:20:18 pmAt what current did you make your measurements?

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2024, 01:47:42 pm
content: Quote from: iet on August 01, 2024, 11:17:07 amI just used my multimeter diode test function, like the original poster.

author: iet
author_rank: Frequent Contributor
timestamp: August 01, 2024, 02:14:37 pm
content: Understood. There may be a significant discrepancy in the measurement results, depending on the manufacturer and the production batch.

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 01, 2024, 02:28:11 pm
content: That´s especially true for the DMM-test with quite low current.You can try it out by yourself: If you switch a - e.g. 1kOhm - resistor parallel to the diode then you ostensibly lower the forward voltage of the diode quite significantly. But is this diode in any way better than without the leakage resistor? for sure not!In other words: What the DMM is showing can vary quite much and it´s not even clear if lower values are good or bad. The DMM is best suitable if you just want to know if the semiconductor is still alive and to test for polarity