AC/DC Module  - Page 1
Forum section: electronics / beginners

author: hpibmx
author_rank: Regular Contributor
timestamp: January 09, 2022, 03:24:29 pm
content: Hi,What's the purpose of this resistor in series?it's on input line before bridge rectifier, is it acting as fuse? if yes why not placing fuse itself?learning about circuits,Thank you.

author: Ian.M
author_rank: Super Contributor
timestamp: January 09, 2022, 04:11:24 pm
content: Inrush surge limiting. It *may* also be a flameproof resistor intended as a fuse.For purposes of learning - most low cost SMPSU*(aka SMPS) that aren't designed by a first-world company and manufactured to strict quality standards are *NOT* good to learn from except maybe how *NOT* to do stuff!For curiosity, please peel the remaining silicone off the IC and give us the part number.Flip the board over and show us the track side and we'll tell you whether it looks to have been designed competently, or if its another piece of 'monkey see, monkey do' dangerous sh!t out of the lower reaches of the Chinese Shanzhai ecosystem.N.B. we cant tell you if it (or any other SMPSU) is *SAFE* without forensically dissecting its switching transformer to check insulation thicknesses and creepage distances, which renders the information moot as it certainly wont be repairable afterwards, but if the PCB and circuit look OK to an experienced eye, and it passes a 1KV Hi-Pot test from input side to output side, its *probably* safe.* SMPSU:SwitchedModePowerSupplyUnit

author: hpibmx
author_rank: Regular Contributor
timestamp: January 10, 2022, 10:08:43 am
content: Thank you for your advice!So I assume circuit would be working fine without that resistor am I correct?I manage to take these picture as well,I also found that thezener diode D2is beeping on diode mode reading on both directions.That means it's shorted am I right?

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2022, 08:48:01 am
content: Quote from: hpibmx on January 10, 2022, 10:08:43 amThat's unlikely.QuoteProbably, but it depends on what's in parallel with it. Which side is it on: the primary, or secondary?It's generally not worth repairing small switched mode power supplies, but don't let that put you off trying, at least for educational purposes.

author: Ian.M
author_rank: Super Contributor
timestamp: January 11, 2022, 09:40:26 am
content: Definitely not - with the resistor broken the rest of the board isn't getting any current and if you shorted it out (*BAD* idea) the inrush surge would sooner or later take out one or more of the diodes in the bridge causing a short across the mains supply. I'm with Zero999 on the diode: It depends . . .Recommended reading:'Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies'by 'Silicon Sam' Goldwasserand your SMPSU's controller IC datasheet:TEA152x SMPS ICs for low-power systemsfor its pinout, theory of operation and a sample schematic which may or may not closely resemble your board, but if printed out large, and attacked with Tippex, and a pen, at least is a good starting point for tracing the actual circuit.

author: hpibmx
author_rank: Regular Contributor
timestamp: January 13, 2022, 08:03:56 am
content: thank you for helping,yes just for learning purpose only.I can't read what is paralleled with it's a blown resistor as well,but it's okay I'm going to put it aside.

author: hpibmx
author_rank: Regular Contributor
timestamp: January 13, 2022, 08:05:19 am
content: Thank you for the recommendation.

author: Zero999
author_rank: Super Contributor
timestamp: January 14, 2022, 11:08:45 am
content: Have you checked the bridge rectifier diodes? It should be possible to test them in circuit.I tried looking for a datasheet for the IC but didn't have any luck. If it's bad, then there's nothing you can do, but buy a new PSU.

author: Ian.M
author_rank: Super Contributor
timestamp: January 14, 2022, 05:33:36 pm
content: @Zero999: Errrrr....... I linked the datasheet for the IC: TEA1521P in reply#4.

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2022, 02:17:26 pm
content: Quote from: Ian.M on January 14, 2022, 05:33:36 pmSorry I missed it. I did Google but I misread the part number in photograph. I thought it was TBA.The schematic is probably very similar to the example on the data sheet.Check the bridge rectifier first, as I mentioned previously. If it's not that, then check the other diodes (refer to the datasheet), then the electrolytic capacitors, both for the correct value and ESR. If the aforementioned is fine, it's most likely the IC.

author: Ian.M
author_rank: Super Contributor
timestamp: January 15, 2022, 06:26:58 pm
content: Yes, I also misread it as 'TBA....', but I also saw the Philips logo, so when nothing showed up, I knew it was very unlikely to be that obscure so I must have misread it and I went back and peered at it some more then tried 'TEA....'