Dead Power Supply - Page 1
Forum section: electronics / beginners

author: Polybus
author_rank: Contributor
timestamp: April 07, 2017, 03:58:19 am
content: Howdy - not sure if this should go here or in Repair - but I'm a beginner so I'll probably be asking beginner questions - so here seems good.I am trying to repair the controller for my garage door. I have attached some photos:Working one day - dead the next - not as a result of our recent storms here in Sydney.There used to be a red LED that came on with Power inside the silver power supply, but that doesn't come on any more - so I figure its the Power Supply module.I've had a good read of EEV Blog - and came to the conclusion I should check the electrolytic capacitors first - they all look good - no swelling/bulging. I took them out of the board (except the small one at about 6 o'clock in the photo) and tested them, they all test good. Tested with DMM.So then I checked - the Rectifier - the Transistor - the Rectifier/Diode - all seem to test OK.I can measure 240v coming into the board from the mains - but the output confuses me - I was expecting to get about 26V DC out (from looking at the cover shown in the photos), but I actually get about 12v AC out...... that is what made me check the Rectifier - but as I said, that seems to test OK.I'm confused!!! Any ideas??Thanks.(Sorry - attachments should now be working.......)

author: Polybus
author_rank: Contributor
timestamp: April 07, 2017, 05:25:04 am
content: Great News!!!I think I have fixed it!!!I am such a ChuckleHead!!The answer was written in my own post - quote:QuoteBecause I was so stumped I started thinking to myself - Why did I test all the electrolytic capacitors except for the small one at 6 o'clock - and I had no answer except that it was smaller than all the others, and hence harder for me with my, ahem.... *average* soldering/desoldering skills, to get out. Dave Jones was quite emphatic about this in his Capacitors video - saying electrolytics are crap and if your power supply has blown up check all the electrolytics first - but of course I checked them all except one didn't I??I thought I'd bite the bullet and get the only one I didn't check out of the board and test it. 100uf 25v.It tested at 8uF on my DMM!!! Now I was getting all excited - could that be the problem??Dig out a new 100uF 25v, solder it in and HEY PRESTO!!! the little red LED now comes on!! Have I fixed it??? I think so.THANKS DAVE!!!! (By the way - we're virtually neighbours - you just live a few doors around the corner from me....)So now I will put it all back together and see if it will actually power the Garage Door.Capacitor cost me 50 cents - Garage door repairer said it couldn't be fixed and I needed to buy a whole new controller/motor system at $650!!!I'll let you know how it goes when hooked up to the door.Cheers guys.

author: capt bullshot
author_rank: Super Contributor
timestamp: April 07, 2017, 07:06:28 am
content: Good job on finding the faulty cap!The "small" one in this kind of PSU is usually the one that is used to start up the converter. It's full capacity isn't needed when the PSU is running, but next time when you turn it on. So these caps can degrade over time while the PSU is working and no one notices the failure - until you turn it off and on again.Quite a common failure on SMPS. All the big caps are OK, but the innocently looking small startup cap failed.

author: Zero999
author_rank: Super Contributor
timestamp: April 07, 2017, 08:59:13 am
content: Congratulations on repairing it.Did you test the capacitors on an ESR meter?It's quite common for bad capacitors to have normal capacitance but much higher ESR, than what's acceptable.

author: james_s
author_rank: Super Contributor
timestamp: April 08, 2017, 05:08:39 pm
content: In this case it was apparently far enough gone that a multimeter was sufficient. It's not uncommon though for a bad capacitor to look perfectly fine and test within spec on a multimeter but have ESR that is too high for it to work in a switchmode power supply. It's often the smallest ones that go bad first too, while the bigger ones like the bulk filter cap on the primary side are rarely bad.

author: cvanc
author_rank: Frequent Contributor
timestamp: April 09, 2017, 08:21:35 pm
content: Nice work!To add one more data point - I too have found it's often one of the little electrolytics in a SMPS that's causing you trouble. Check them all; every time.

author: james_s
author_rank: Super Contributor
timestamp: April 10, 2017, 01:52:24 am
content: Another indicator that the problem is bad electrolytics is when something is working fine, then you shut it off or it loses power for a bit and then it won't turn on again. Most people blame a power surge but what really happens is a capacitor in the startup circuit cools off, the ESR rises and then it won't start up again.

author: Polybus
author_rank: Contributor
timestamp: April 10, 2017, 05:31:13 am
content: Hi Guys - Thank you all for your inputAnd now the good news - the power supply is back in the Garage Door opening unit - and all is working fine - I know this has not been what most of you guys would think of as a major repair - but hey - I am feeling pretty pleased with myself - I feel good every time I open/close the door.As for your comments:HERO999 - Not tested with an ESR Meter - (only because I can't find it......I've got one here somewhere - I built it from a kit - just can't find it), but with a DMM - the faulty cap was rated at 100uF, but it tested at 8uF - so as noted by James_S - it was pretty far gone and easy to pick up with my DMM - but as you both say - in the future I will make sure to test with my ESR meter.LESSON (CVANC) - Always check ALL the Capacitors - EVERY time - and start with the smaller ones.I have now replaced all the electrolytics with Low ESR versions (you can see the green one at about 3 o'clock in one of the photos) so hopefully it will give the unit a new lease of life - or at lease extend its life.Just another learning point for me in this whole process.....Can I ALWAYS replace a standard electrolytic capacitor with a Low ESR variant?? I have done a lot of reading and it seems they are quite good for SMPSs - but is there any instance where it would be wrong to replace std with Low ESR?? Given that they cost pretty much the same - maybe a couple of cents different - why/when would I not choose Low ESR where an Electrolytic is being used? I realise a couple of cents adds up over huge production runs - but for me - using them a couple at a time - can I do any damage using Low ESR??Thanks guys.

author: kripton2035
author_rank: Super Contributor
timestamp: April 10, 2017, 05:58:44 am
content: this small capacitor is often the cause of all the problems on an smps ... I always check them first !by the way you should invest in an esr meter, there are plenty to build or buy on my esr repository (web site below)kudo for your repair !

author: ProBang2
author_rank: Frequent Contributor
timestamp: April 10, 2017, 06:09:59 am
content: I have no knowledge about a circuit,not even only a single one, where a minimum ESR is specified for an electrolytic capacitor.(The other way around is possible: Sometimes are electrolytic capacitors with low ESR necessary. E.g. for avoiding of heating.)Therefore: You can´t damage anything with the change from "standard electrolytic capacitors" to electrolytic capacitors with low ESR.

author: ludzinc
author_rank: Unknown
timestamp: April 10, 2017, 06:19:26 am
content: MateYou learnt something, and saved $650.Even if you could have earned $650 in the time it took you, YOU STILL LEARNED SOMETHING!Top stuff.

author: Ghydda
author_rank: Regular Contributor
timestamp: April 10, 2017, 06:54:00 am
content: Quote from: ProBang2 on April 10, 2017, 06:09:59 amWell it is common for smps circuits of any of the forward topologies to require a minimum ESR in the output filter caps, to ensure loop regulation stability across all operating conditions.The same is true for some LDO voltage regulators:TI app noteThe reason is that the ESR creates a zero higher up in frequency and it helps the circuit to maintain adequate phase margin until the loop gain drops below 1.Replacing caps with types having lower ESR pushes this 'zero' into a higher frequency band and thus lowers the phase margin, which can jeopardize the loop stability.So, be careful with what seems to be an improvement to the circuit, it may contain some traps and pitfalls for young players as D. Jones says.

author: cvanc
author_rank: Frequent Contributor
timestamp: April 12, 2017, 11:25:05 am
content: Here you go, Polybus!

author: CJay
author_rank: Super Contributor
timestamp: April 12, 2017, 01:29:10 pm
content: Quote from: cvanc on April 09, 2017, 08:21:35 pm*nods* my rule of thumb for switchers that haven't blown a fuse is to replace any capacitor that's got a 4 and a 7 in the value, it's not quite so accurate nowadays but the general rule is always check high value resistors and low value caps before you go digging into the circuit operation, it'll fix a large percentage of problems.