Glass Diode identification  - Page 1
Forum section: electronics / repair

author: candrian
author_rank: Regular Contributor
timestamp: August 01, 2019, 08:03:57 am
content: Hi all,I have teared down a CDI unit from an outboard engine removing with a lot of patience all of the epoxy resin. While i was removing the potting i broke 3 glass diodes. Two of them are diodes as they are marked on the PCB with the letter D and the 3rd one it seems to be a zener diode 1/2watt (marked with Z). Here comes my questions.The two diodes are made out of glass as common zeners with red color inside but instead of the black stripe they have one green stripe at the cathode side. Does anybody know what this mean? I searched all over the internet and the only close statement i have read is that they may be schottky diodes.edit: forward voltage for these two diodes is 0.6VThe zener diode cracked in the middle on the potting removal. So before i removed it from the PCB i tried to apply reverse voltage on it's leads and i measured 2.2Volts. Can i state that this is the reverse voltage of the zener or the crack of the glass enclosure may have affected the functionality as well as the reverse voltage of the zener?PS. There are no marks or numbers on all of the above diodes.Thanks a lot!

author: PKTKS
author_rank: Super Contributor
timestamp: August 01, 2019, 02:17:37 pm
content: A picture will help...as there are just too many glass models to consider.A gross ballpark figure is that GLASS DIODES are COLOR CODEDyou should have more than one band... green green yellow or magentasome thing like that indicating 1N4148 1N518 etc plain color codesPaul

author: Ian.M
author_rank: Super Contributor
timestamp: August 01, 2019, 02:51:38 pm
content: Many glass diodes used to be colour coded, but it was also fairly common to have markings printed on them which were always extremely vulnerable to wearing off due to handling unless the whole diode body was painted, and in more recent years, unmarked (apart from the cathode band) glass diodes have become common.2.2V seems rather low for a Zener. Isolate it without removing it from the PCB and put it on a curve tracer to see if you get sensible forward and reverse characteristics. If not, you've FUBARed it and are S.O.L. unless you've got another one to measure, or can reverse-engineer the design with sufficient understanding to be able to calculate a suitable value for it.

author: PKTKS
author_rank: Super Contributor
timestamp: August 01, 2019, 02:59:12 pm
content: indeed true ... those unmarked I just ditch in the trashThe marked ones I try the best to read codes with the microscopesome still readable some unreadable... trash can.The color bands are far more readable even those on aging ones

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 01, 2019, 03:10:42 pm
content: As other posters have mentioned, a good, properly FOCUSED photo would significantly increase the chances that we (as a group) can solve the puzzle

author: candrian
author_rank: Regular Contributor
timestamp: August 01, 2019, 04:48:50 pm
content: Thank you all for your fast responses.As i have already mentioned the circuit is a CDI unit of an outboard Engine.I have already reversed a schematic from the circuit that uses the zener diode and i hope that i'm not missing something as it's a double sided PCB with smd diodes in sot32 etc. there is still left some potting on the PCB which makes reverse engineering difficult. Attached is a photo of the schematic. In a more general view this is the circuit piece that takes the AC voltage of a coil called Charge Coil by the manufacturer and feeds it on a pin of the onboard MCU. As per the MCU datasheet (NEC PD78C11) this pin is characterised as interrupt 1 and in combination with the schematic i can imagine that this part of circuit is used to detect zero crossings? rpm? or something similar (in other forums with outboards i have got the info that the charge coil is used to charge the capacitor of the CDI but i haven't seen yet such circuit on my CDI. Maybe there is a trace from this circuit that goes also to the CDI capacitor charging). The broken zener diode is marked on the schematic asZ402. The Charge coil is connected onLandBrwires.Yrwire is on the air connected nowhere maybe it's there for debuging/testing the unit. According to the outboard manual thepeak voltageof the charge coil should be in open circuit at 110V and in connected 120V while cranking the engine or 160V at 1500rpm or 120V at 3500rpm.Attached you can see also some photos of other diodes similar to the two i have described in the previous thread. As you will see on your own there are no markings on them. The broken diode is the zener which also doesn't have any markings. I think there is no additional info from the photos.

author: candrian
author_rank: Regular Contributor
timestamp: August 01, 2019, 04:49:28 pm
content: Some photos of the diodes with the green stripe

author: goaty
author_rank: Regular Contributor
timestamp: August 02, 2019, 10:41:17 am
content: Wow that´s dirty.Needs some treatment with an Crest ultrasonic cleaner !

author: PKTKS
author_rank: Super Contributor
timestamp: August 02, 2019, 11:51:14 am
content: hopeless.. ultrasonic will wear out any chance of reading..what is left of it.

author: candrian
author_rank: Regular Contributor
timestamp: August 04, 2019, 06:39:33 pm
content: Well i have good news for me at least. I found that there is an onboard power supply which regulates the voltage to 5Volt which input comes from the zener diode i'm looking it's value. The power supply uses the TA8000S in a circuit similar to the attached photo.As per the datasheet TA8000S max input voltage is 40V. So i was thinking to add a zener diode to about 30Volt. Having in my mind to have a big enough voltage before the regulation to reduce the drawn current and have a voltage buffer. Otherwise i could use a smaller zener such as a 12VOn the other hand if there is a big voltage deference i don't know if there will be a big heat diffusionAlso something more worth noting, just after the zener there is an aluminium electrolytic capacitor rated to 50Volts so i was thinking if the zener was under 16 volts why they didn't use a capacitor of 16Volt rating or 25Volt accordingly, as there are already onboard other capacitors with less voltage rating as they are on the 5Volt side.What is your suggestions?https://images.app.goo.gl/7DmXFDgW7R4KQSN99Thanks

author: candrian
author_rank: Regular Contributor
timestamp: August 07, 2019, 08:19:49 pm
content: How would you translate the value of the zener in the attached photo? It is from the datasheet of the 5V regulator

author: fzabkar
author_rank: Super Contributor
timestamp: August 08, 2019, 03:29:57 am
content: Toshiba's 1Z30 is a 30V, 1W Zener.http://pdf.datasheetcatalog.com/datasheet/toshiba/3585.pdfhttp://biakom.com/pdf/TA8000S.pdf

author: candrian
author_rank: Regular Contributor
timestamp: August 08, 2019, 07:42:30 am
content: Thanks! So maybe my zener could also be the same as the datasheet. They could have used a capacitor of 35Volt instead of 50V next to the zener on my circuit if the zener was 30V but maybe they did it to be sure that the capacitor will last.I was thinking that as they did add a 50V capacitor next to the zener and the maximum input of the TA8000S is 40V the zener was about 33 to 35Volt.

author: fzabkar
author_rank: Super Contributor
timestamp: August 08, 2019, 03:09:12 pm
content: Since it's an automotive application, I would think that the maximum voltage would be 16V, if the alternator's regulator failed, but ICBW. I don't know what voltage to expect in a load dump situation, though.