Switch mode power supply

[Barlidge]

I know its not welding but of all the forums I visit the variety of knowledge on here always amazes me.

I have three 30v SMPSs used for powering an audio/video distribution system, they are about 10 years old and the system has been in use 24/7 since day 1.

A year or so ago they started to fail, the symptoms would be that the supply would emit a low pitched cyclic whine and fail to boot properly while the system was plugged in, if I unplugged the supply from the system the supply would then boot up allowing the system to be plugged back in and would function normally until the next power outage. In this failed state the output was only 12v.

One has failed completely and will no longer boot using the above procedure.
One is well on the way and will still boot as above but takes approx 5 minutes.
One is in earlier failure mode and will boot as above within 20 seconds.

The whole system although good was very expensive and replacement power supply's are approx £180 each and I am struggling to find a 30v 4A output generic supply from anywhere. I have also rang around a couple of local electronic engineers who have been reluctant to take a look. I have looked for the obvious such as dry joints, bulging caps or any other visual sign of failure but none apparent.

Is there anyone one here who could help in any way, either by looking themselves (beer tokens of course) or by advising components to change. I have attached a pic and also have an audio file of the cyclic whining increasing in tempo until the supply finally boots but I cant upload an MP3.

Many thanks.

 

[Flapdisk]

It's usually the capacitors that fail with time, they work hard in a smpsu, make sure the replacements have a 105 degree temperature rating and they will be more robust. That's the usual, but over the years I have also had various resistors increase in value due to age and temperature stress, but that would be a permanent fault and there would be no sometimes working. The fault you are describing sounds like the caps to me.

 

[500e]

Switch modes are nasty little things not a lot of people repair
for new try sunpower
S-240-30, 30V 8A

www.sunpower-uk.com
Chris Gane
01189811001.
OR

http://www.lhvpower.com/precisionscientific.htm

Tel: +44 (0)1903 712400

 

[ratherbewindsurfing]

Not cheap but a little better the £180

http://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/7541211

 

[MattF]

Like Flapdisk says, most likely caps. Anything electrolytic wants replacing, including the large reservoir cap. Going over the board with a soldering iron wouldn't go amiss either. The solder and soldering on those is usually somewhat lacking, plus you'll get dry joints and oxidised legs on some stuff after a while. If you order any caps, check the dimensions of the replacements before ordering. You tend to find with a lot of the switch-modes that they're so cramped on space in the design that some caps of correct rating won't physically fit.

That board looks fairly clean and not discoloured in the photo, so at least it looks like it's not running at a stupid heat in a poorly ventilated enclosure.

Switch modes are nasty little things not a lot of people repair

PSU's used to be one of my favourite things for working on. Few people seem to want to go down to component level repair in general these days. They can be a cow to do, and a few would end up hitting the bin, but at least they're generally easy to cover with stock once you know which parts are prone to fail. The're aren't enough components to throw up many surprises on that front.

 

[wheresmejumper]

Welderfix used to be on here, the company specialise in swps

 

[Flapdisk]

They are really not that complicated, the whole idea was to get dc power at high current for small physical size and without having huge transformers or very hot running wire wound resistors.
They take the mains and rectify it to dc around 400 volts, then they usually have a chip made for the job which is mainly an oscillator with various frequency control feedback and over current and voltage sensing, then the output is boosted by a high frequency power transistor which drives the primary of a small high frequency transformer. The oscillator is running somewhere around 60 to 80 thousand hertz or kilohertz the idea being that at this high frequency it's almost DC and will require very little smoothing so the caps can be small and low values. The secondary of this transformer will have several windings on it . the amount will depend on what DC outputs it was designed for, then, say in the case of having a 12 volt output and a five volt there would be two secondaries each would have fast schotky diodes rectifying the high frequency to dc and then the caps mentioned above smoothing the outputs.

A bit of feedback from the outputs to the overvolt and current sensing part of the first chip I mentioned and your good to go.
Faults are nearly always caps, main oscillator transistor short circuit, diodes short circuit, sometimes when the main transistor goes down it takes the chip with it.
All modern televisions and audio equipment are powered with them and I have worked on hundreds too. The monitor you are reading this on, if you are on a home pc has two in, one for the power for the electronics and another hich voltage one for the backlight tubes, unless like me you have led backlights now

 

[DanielW]

In the pic you will see a variable resistor on the board this will reduce the voltage from 36v to 30v and at 10amp which will draw as much your system needs,more than enough even if your own supplies are only 4amps.

http://www.ebay.co.uk/itm/CNC-Servo...ial_Automation_Control_ET&hash=item3a993e357a

 

[MattF]

Faults are nearly always caps, main oscillator transistor short circuit, diodes short circuit, sometimes when the main transistor goes down it takes the chip with it.

That just reminded me of one time, (as obviously owt barring the caps would likely have trashed the fuse, and also a lesson in why it's not smart to wind another tech up), when I spent a good while scratching my head over why a freshly repaired PSU on test suddenly had a black fuse when I came back in from making myself a cuppa. Spent ages checking everything over on the board again, and not a thing wrong that I could find. The sod had only gone and shoved a ******** fuse in there whilst I was out making the cuppa, to get his own back for me winding him up earlier on in the day.

 

[DanielW]

Just what others on the bay,I have bought three from China sometime ago for the build of a CNC and had no problems importing.

http://www.ebay.co.uk/sch/i.html?_o...wer+supplies&_nkw=cnc+power+supplies&_sacat=0

 

[Barlidge]

Apologies for not replying back sooner, been out with the kids this afternoon and saw some fantastic fireworks at a local village pub.

Thank you all for the replies, every one has given me an option to look at. As I said in the beginning I am a member of a number of forums but the wide spectrum of knowledge on here is unequalled. I will update when I have some further answers.

 

[normspanners]

my vote is replace all caps on your psu's with quality 105's look on the site called bad caps http://www.badcaps.net/

 

[ukracer]

my vote is replace all caps on your psu's with quality 105's look on the site called bad caps http://www.badcaps.net/

Question is where does he get good caps at that guy is in the states...

 

[DanielW]

Reminds me of a guy in an electronics course I did,came in one day with his defunct getto blaster,tutor told him its probably a bad capacitor(tutor didn't like the guy and was being funny with him),guy spent all day swapping capacitors out.

Oh what a laugh that was.

 

[normspanners]

Question is where does he get good caps at that guy is in the states...

no I meant for him to read that site, as to the explanation of how caps can effect electronics, and why we have this problem now, they also have tutorials. etc

 

[DanielW]

Few questions that hasn't been asked....

1.is these three seperate supplies for three seperate pieces of equipment? or this is the third supply to go down in the same piece of equipment?

2.how do you know if the supplies are at fault?

3.have you tested the supplies under load outside the equipment they're intended for? ie load them with a 12v motor or something you have that requires a good amperage pull.

 

[Barlidge]

In the few times I've changed caps out in the past I've used RS Components. There's some interesting reading on that site though ukracer.

Few questions that hasn't been asked....

1.is these three seperate supplies for three seperate pieces of equipment? or this is the third supply to go down in the same piece of equipment?

2.how do you know if the supplies are at fault?

3.have you tested the supplies under load outside the equipment they're intended for? ie load them with a 12v motor or something you have that requires a good amperage pull.

There's some good point there I missed Danielw,

1. The system is an 8 zone audio/video distribution hub running 1 pair of speakers per zone. The hub has two PSU input ports, PSU 1 powers zones 1-6 and the optional PSU 2 powers zones 7-8, I'm not sure if there is any load balancing with both PSU's plugged in but the system powers zones 1-6 happily from the first PSU (as per manufacturers spec) and only powers zones 7-8 with the second PSU plugged in, although both PSU ports are marked as 4.33A. The third PSU is for zones that have more than one pair of speakers which is only one in my case, the PSU is connected to a distribution block (PIM3) and distributed directly to the additional speakers.

2. I cant be sure an underlying hub fault hasn't caused the PSU's to fail but the hub does operate normally one the PSU's have booted and . . .

a) The faulty symptoms of each PSU stays with it regardless of where it is in the system.
b) With the PSU's sat on the bench one will not boot up at all, one takes approx 5 minutes and the third about 20 seconds, this is without going anywhere near the hub.

3. No I haven't and for sure that would be a good test but I cant think of anything I have around that's 30v.View attachment 47993 View attachment 47994