Help needed for component identification in a small Power Supply. - Page 1
Forum section: electronics / repair

author: paul8f
author_rank: Regular Contributor
timestamp: May 18, 2019, 10:04:30 am
content: Hi,I'm fixing a 24Vdc (@350mA) wall-wart style adapter at the moment. There is no O/P voltage whatsoever from the SMPS.I've discovered that a component connected directly between the 220v Live input pin and the bridge rectifier measures as an open circuit (even on the highest DMM range). I desoldered this component from the PCB and hooked it up to an LCR meter. Here it only measured as -0.2pF.The code on the body is10RJ,so I'm guessing its a 10 ohm wire-wound resistor. However there is asmall doton the end connected to the mains. I wouldn't have thought this was polarization sensitive?See photo attached for the logo on the body. I'm guessing that's a manufacturer logo and not a component symbol.Any ideas? ....Also, is there anyway to tell what wattage rating this component is?

author: kripton2035
author_rank: Super Contributor
timestamp: May 18, 2019, 10:17:41 am
content: it is a 10 ohms resistor, but if it fried, there should be something else after it that is shorted.look at the diodes, mosfet, for a short. look at the mains capacitor (the 400v one) for esr > 1 ohmand change the resistor.

author: paul8f
author_rank: Regular Contributor
timestamp: May 18, 2019, 12:29:50 pm
content: @kripton... Thanks for helping. This device has a multiple issues now....On first opening up the housing, there was a broken track (from what looks like an impact/drop) and a blackened resistor near the Opto-Isolator, (making it impossible to read its original value, although it did measure about 1k). Voltage drops for all 4 diodes in the bridge rectifier measured ok, and there was no short across the 24v output line. Gently warming up the circuitry had no effect.Unfortunately, before I read your reply, I went ahead and applied power to the cct. via a 30 ohm wirewound resistor as a temporary substitute part! This was a higher wattage than the original 10 ohm resistor, so a large current was able to flow. This caused further damage straight away. There are two 400v rated 6.8uFRubyconelectrolytics in parallel for the bulk storage. Out-of-circuit, one has ESR of >40 ohms and measures only 2.2uF, the other is 23 ohm ESR and a value of 6uF. I'm not sure if I caused this fault, or if they were like this before I started.Applying power also made a diode in the bridge rectifier go open-circuit, put over-current through another resistor, and cracked the TH2028.5A IC (which I'm guessing is the PWM controller chip).I removed the SiemensBUZ78FET from the board. Drain-Source resistance measures as0.6 ohm. Again, I'm not sure if this was my fault or not.This adapter is definitely for the recycling bin, but it was a good learning experience for me! I might still try fix it just it to see if it's possible, but I would cut the o/p cable and throw it out straight afterwards.Don't suppose you have any idea what the original fault may have been?

author: kripton2035
author_rank: Super Contributor
timestamp: May 18, 2019, 01:50:31 pm
content: I think too much esr on the 400v caps. made the device draw too much current. so the 10 ohms "fuse" resistor blows.then with your replaced resistor, still too much esr, so too much current in the diode bridge that made one blow. etc ...pretty good example of a catastrophic failure ...the BUZ78 still seems ok.the TH2028.5A seems to be a chinese-only-no-more-available-except-in-5000qty circuit.so yes, to the bin !cheapest option :https://fr.aliexpress.com/item/Free-shipping-1pcs-EU-input-100-240v-ac-24vdc-24-volt-0-35-amp-Power-Supply/32796605677.html

author: paul8f
author_rank: Regular Contributor
timestamp: May 18, 2019, 09:16:36 pm
content: Yeah, it's highly likely the out of spec caps were the root cause of the failure.Was that original 10resistor definitely the "fusible resistor" type, and is the photo I've attached the correct schematic symbol for this?Is this resistance for the BUZ78 transistor very low, I thought this value indicated some damage?I will try to find a substitute part online for the TH2028.5A. I think Dave in one of his EEVblog youtube videos detailed some good resources for odd-ball Asian parts.Yes, this P/S will be binned eventually, but for now I'm using it as a training exercise!

author: kripton2035
author_rank: Super Contributor
timestamp: May 19, 2019, 07:08:13 am
content: or check it with a classic transistor tester

author: paul8f
author_rank: Regular Contributor
timestamp: May 22, 2019, 06:38:04 pm
content: Thanks for posting the YouTube link. (Not sure how ESD-friendly this FET test is though!)