Zener's burning out - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 19, 2019, 03:09:37 pm
content: I was using a 5V1 1/4W (surely it's not 1/8W?) in a linear PSU I made, and it would pass up to 20mA sometimes, maybe even 30mA. LTSpice said it would barely pass 1mA, and that was at 120Hz peaks.So I tried a 2v0 and 3v0, and they burned out too, during a test of about 15V at 300mA of the PSU. So now I have a 1/2W 5V1, if thtis burns out, I'll try a TL431.The schematic used a 1/2W zener, and I guess it will get pretty hot. 1W zeners and expensiveI guess there's no question here, I'll try and see if 1/2W zener survives.

author: Vgkid
author_rank: Super Contributor
timestamp: July 19, 2019, 03:46:47 pm
content: Schematic, what value of current limiting resistor are you using?

author: james_s
author_rank: Super Contributor
timestamp: July 19, 2019, 04:14:13 pm
content: Without posting the schematic, how can you expect anyone to help you?

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 19, 2019, 04:44:08 pm
content: Well, 20mA through a 5.1V zener works out to around 0.1W of Pd, so if a 0.25W zener is failing then either the current measurement is in error or the zener isn't actually capable of dissipating even 0.1W... If you bought said zener from some shady ebay seller for really cheap then no prizes for guessing which cause is more likely.That said, post a schematic and/or LTSpice .asc file if you actually want some help.

author: tggzzz
author_rank: Super Contributor
timestamp: July 19, 2019, 05:23:03 pm
content: Quote from: james_s on July 19, 2019, 04:14:13 pmI refer you to a thread started by the OP yesterday - which was locked 4 hours later.https://www.eevblog.com/forum/chat/is-science-allowed-on-eevblog-if-it-contradicts-the-offical-version/msg2554770/#msg2554770Currently he's at 911(!) posts. I wonder whether he will reach 1000.

author: Gyro
author_rank: Super Contributor
timestamp: July 19, 2019, 06:04:12 pm
content: Quote from: tggzzz on July 19, 2019, 05:23:03 pm...but he has been thanked a whole 33 times! (Two of them were from Treez though)

author: ebastler
author_rank: Super Contributor
timestamp: July 19, 2019, 07:08:46 pm
content: Quote from: tggzzz on July 19, 2019, 05:23:03 pmAhh, now I see! This mysteriously burning Zener must be the proof of the Science-which-cannot-be-named about which the OP mumbled in yesterday's thread...

author: james_s
author_rank: Super Contributor
timestamp: July 19, 2019, 10:40:45 pm
content: Quote from: tggzzz on July 19, 2019, 05:23:03 pmHm I'm reminded a bit of the strange emails I started getting from a former friend. Later I found out he had gotten into methamphetamine.

author: OwO
author_rank: Super Contributor
timestamp: July 20, 2019, 05:50:24 am
content: Quote from: lordvader88 on July 19, 2019, 03:09:37 pmOk, maybe the first step should be to figure out why the circuit is behaving differently from simulation.Quote from: lordvader88 on July 19, 2019, 03:09:37 pmHuh what? That's a bit of a non-sequiteur; zener passing more current than it should => I should replace it with a lower voltage zener! If that burns out too => a TL431 should work better?Quote from: lordvader88 on July 19, 2019, 03:09:37 pm5.1V * 30mA = 0.153W, so I'm not sure how the original zener burned out, but you should probably double check your measurements and make sure you are using the correct probing techniques.Quote from: lordvader88 on July 19, 2019, 03:09:37 pmIf a 1/4W zener "burned out", it's probably dissipating way more than the datasheet absolute maximum rated power, so it's likely not in spec for a 1/2W zener either. I would suggest getting the simulation not matching reality sorted first.

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 20, 2019, 06:47:30 am
content: Quote from: Vgkid on July 19, 2019, 03:46:47 pmIt's on page 11/12http://www.introni.it/pdf/Farnell%20-%20L%20series%20Bench%20Power%20SuppliesThe 470 ohm resistor passes around 32mAZ4, the 5V1 is what burns out. One time, P1 also burnt out. It normally is locked around 1mA, I admit IDK how the feedback loop toasted it.

author: mikerj
author_rank: Super Contributor
timestamp: July 20, 2019, 10:20:54 am
content: Quote from: lordvader88 on July 20, 2019, 06:47:30 amWhere are you pulling these numbers from? Given your recent thread I can take a pretty good guess...Z4 is a 1N823 6.2V Zener to start with, and the current through the 470 ohm resistor is set by Z3, a 5.1v zener on the other side of the current mirror: 5.1/470 ~ 10mA. This is the maximum that should be passing through Z4, since some of the current will be going into P101/P102.

author: bd139
author_rank: Super Contributor
timestamp: July 20, 2019, 10:34:04 am
content: I see the problem here:1. Circuit copied verbatim without understanding it.2. Circuit something from the middle ages to start with (Farnell power supplies usually are - decent supplies however).3. Cargo cult developing.Solution:1. Go back to basics and understand the problem.2. Get art of electronics and the learning the art of electronics book (both are on library genesis if you're a cheap arse)3. Work through the whole damn thing until after lab 11.4. Consider oneself hit with the clue stick.Short cut as I know it'll be taken:1. Get an LM723 in and use a circuit off the datasheet.2. Build the supply around that and an external pass transistor.3. Admit that it's a cargo cult.Some advice: building a bench power supply is much much harder than it looks. Even all the semi sexy looking designs on here and a lot of commercial units are full of stupid flaws. Overshoot, spontanous oscillation, failure prone (try charging a car battery up with that Farnell supply and then the line power fails!), all sorts. Even an LM317 can oscillate like hell if you happen to have a couple of metres of test leads hanging off it.

author: thinkfat
author_rank: Unknown
timestamp: July 20, 2019, 10:57:24 am
content: LM723 is actually not such a bad choice. You can make quite decent power supplies with it if you combine it with some power transistors.Gesendet von meinem Nokia 6.1 mit Tapatalk

author: bd139
author_rank: Super Contributor
timestamp: July 20, 2019, 11:05:23 am
content: Yeah it's a VERY nice IC despite being 50 years old this year. It's still going strong.

author: tggzzz
author_rank: Super Contributor
timestamp: July 20, 2019, 12:07:02 pm
content: Snipped a reasonable analysis of PEBCAK...Quote from: bd139 on July 20, 2019, 10:34:04 amFirst time I have bumbled into this:QuoteSuppose I'll have to investigate how to avoid that, presuming they aren't using deep packet inspection.

author: xavier60
author_rank: Super Contributor
timestamp: July 20, 2019, 12:07:10 pm
content: Quote from: lordvader88 on July 20, 2019, 06:47:30 amI wouldn't like to be debugging that overly complicated monster.

author: bd139
author_rank: Super Contributor
timestamp: July 20, 2019, 12:08:46 pm
content: Quote from: tggzzz on July 20, 2019, 12:07:02 pmhttp://libgen.unblockall.org/I suggest you move to Zen or Andrews and Arnold if you can. No blocking.Edit: of course, the best option...I only keep the electronic copies as well as they are easier to read on thetoiletthinking throne

author: tggzzz
author_rank: Super Contributor
timestamp: July 20, 2019, 12:20:15 pm
content: Quote from: bd139 on July 20, 2019, 12:08:46 pmSame problem, and I can't reasonably. I'm on VogonMuddia cable, and the ADSL broadband checkers indicate "up to 17Mb/s estimated 6Mb/s". That's consistent with what my neighbour used to get.QuoteWell, you wouldn't want someone to be, um,confusedon the toilet.I have paper TAoE 1, plus both formats of TAoE 3. Continuing to (very selectively) read usenet ishighlybeneficial sometimes!

author: bd139
author_rank: Super Contributor
timestamp: July 20, 2019, 12:23:28 pm
content: Yes I've got 1st edition (black one) on the shelf above that as well. I prefer that one if I'm honest unless I need some detailed information.

author: tggzzz
author_rank: Super Contributor
timestamp: July 20, 2019, 12:50:21 pm
content: Quote from: bd139 on July 20, 2019, 12:23:28 pmYes, it is swings and roundabouts. I'm keeping both

author: james_s
author_rank: Super Contributor
timestamp: July 20, 2019, 06:29:49 pm
content: Quote from: bd139 on July 20, 2019, 10:34:04 amThis is one of the things I like about it though. A power supply is simple in concept but you can really dive deep into the details and learn a tremendous amount if you want to. It doesn't have to be complicated, when I was about 12 I built a simple fixed 12V power supply using three 7812 regulators in parallel (yeah, I know that's bad practice but it worked) and I used that thing heavily for around 15 years. A basic zener referenced voltage regulator with a pass transistor can teach you all kinds of basic concepts and in the end you get something useful, it's hard to have too many power supplies. These days I use a commercially made bench PSU but when I was starting out I couldn't afford something like that and I'm glad I was building my own because it taught me a lot.