Switched mode power supplies

[bertiedog]

I, like a lot of others use the modern 110v/240volt switched mode power supplies from China. These are used for lighting supplies at 12 volts 5amps in my case. I also use one to power the controller, and had no problems.

 

But to test a new one in from Ebay, I connected up an LED cob strip at 12volt 0.5 amp,(6 watts) which came on for a fraction of a second and then blew a 2 amp mains fuse in the test bench connector....... Further checks show the unit works without blowing a 5 amp mains fuse. All the components are in working order as far as can be tested in situ.

 

I suspected an initial surge must have blown it, and put the unit on my variable mains transformer at 110 Volts, all works fine, but when tried on 240volts there is an initial surge. To blow the 2amp mains fuse uses 480 watts. I know some early types of switched mode liked a load connected, but the latest seem not to.

 

I do not like to up rate fuses in a situation like this really, and will get in another one and test it out, as I was planning on using the PS for supplying the controller circuits, where a surge might cause problems to the regulator, although it could be smoothed by adding capacitors before the regulator.

 

Any ideas?

[royaloak]

It sounds as if it a cheap and nasty copy, ebay is full of them and I wouldnt use them in my house.

 

Why spend £hundreds on controllers etc and then power them with a cheap transformer of unknown quality?

[bertiedog]

They are mainly feeding LED strips, equivalent transformers are very expensive these days  with the controller it is an older branded unit, (Panasonic), and the controller is 317 based, about £1.80's worth. I opened up the one that chews fuses, and all is the same design and parts as others I have doing duty with household LED strips.

Edited August 19, 2017 by bertiedog

[Crosland]

I, like a lot of others use the modern 110v/240volt switched mode power supplies from China.

Direct from China? That's your problem. Buy a UK brand like PowerPax. Even though they may be made in China they will be well specified and tested.

 

See http://www.edn.com/design/pc-board/4441830/Teardown--12V-AC-adapters---The-Horror

[bertiedog]

I have used suppliers like Farnell, not direct.  The fault has been traced to the input capacitor, too high a value, but it will be confined to use on the bench for testing etc. Also the fuses were suspect, although marked 2 amp, they blow at under 1 amp (British made BS marked)

[Suzie]

With multi-input-voltage PSUs you will find that the input smoothing capacitor has to be rated for 90V mains input so is twice as big as is required for 220/240V making for a bigger surge at power on. A good quality power supply should mitigate the surge with an inductor, but this is an expensive component and might be missing/inadequate on a cheap PSU thereby requiring a bigger fuse.

 

It is possible to source PSUs in the UK for a reasonable price, this one from Rapid might be suitable:-

 

https://www.rapidonline.com/stontronics-t6366st-12vdc-5a-60w-installation-psu-85-3794

 

I think you are pushing it trying to use a 2A fuse - even this PSU quotes an inrush current of 75A and 1.5A max input current so 5A in the plug will be more suitable to avoid straining the fuse too much. Overheating the fuse and melting the plug (or even the socket) can often be a bigger hazard than using a higher current fuse - the fuse in the plug is there to protect the mains cable.

[bertiedog]

The PSU has got a ferrite inductor on the input part of the board, I compared two units and the faulty one had double the value of capacitor, so altered it to be the same. It works but I do not now trust it. Previous PSU of the type all ran on 2amp fuses, but I find on test they blow at about an amp. They are an old batch of British made fuses. I can change the 2 amp to 5 amp.though. The older PSU still in use have other brands of fuses fitted in the plugs.

Typical of the Chinese confusion is the rating of 12 volts at 3 amps equating to 48 watts  and some 12 volt LED strips claim 12 watt output from 12 volts at .5amps.

[DCMarvel]

I have used suppliers like Farnell, not direct.  The fault has been traced to the input capacitor, too high a value, but it will be confined to use on the bench for testing etc. Also the fuses were suspect, although marked 2 amp, they blow at under 1 amp (British made BS marked)

I work for such a company you purchased from in product compliance and can confirm power supplies from China are a pain in the neck. We thoroughly test all PSU's sold and included in products so they comply with RoHS, CE, WEEE, LVD, BS etc but the Chinese do have a nasty habit of changing the spec or modifying components on a re-order but not telling us. They also slip in the odd counterfeit item, sometimes deliberately, sometimes they get caught out too by sub-suppliers of parts. We do get odd ones that, until someone lets us know, sell many. Another problem we have is when users are buying a PSU for a purpose other than what it is intended for, and I guess this would be such a usage, as it is difficult then to determine whether the problem is purely in this instance, especially if no one else using them for the intended use has reported any issues.

 

[bertiedog]

The power supply in question will be junked, I had a closer look at under the capacitors, and one has corroded leads, and in odd corners there is a white deposit, ( not heat compound) . There are also some iffy dry joints, looks like hand assembly not flow soldering. Frankly the unit looks well made, but you never can tell.

The type is the near universal aluminium box with perforated covers that are sold to power LEDS, which this one was intended for and at a rating that was no strain on it. I was going to use three units.

I think in future the most economic way to power the layout overhead lights will be on much higher rated one from a known supplier, or a computer supply unit, which are usually more reliable.

The lighting requires less than 100 watts, about 36 watts in theory ( six strips), plus a bit for LEDS on the layout etc. A ten amp unit should cover it with ease.

I had thought of using one in a power supply for the trains, but I have gone off switched mode at present, back to an H&M transformer to provide the power.

[DCMarvel]

Most of this kind of stuff made in China is hand soldered, I have seen photos usually by women sat at well lit work benches quite cottage industry looking, and quality is generally quite good. It's actually amazing how they can turn out large quantities in short time with a pretty good consistency. At the price they sell to UK resellers for they would never be competitive if they invested in flow solder technology. Occasionally a dud or lesser quality one slips through and if a buyer returns one as faulty, it would be tested and opened up and if anything unusual found they would be made to correct it. You would get a replacement from another batch or your money back. If you have already opened it up then you probably wouldn't but the product development team where you bought it from would still have liked to see it.

Edited August 21, 2017 by DCMarvel

[bertiedog]

It's all repaired now, new caps, cleaned the board, and tests OK. The caps in it were unbranded, which is odd.

I wonder if the work from one makers is farmed out to cottage industry, like the Japanese used to do. One design, many makers who source components themselves perhaps.

Just got is some replacements , cheap types, slim line version, and they work fine,(and are flow soldered). It's just a nuisance it is such a lottery as to quality, especially as safety is involved.

 

Stephen

[AndyID]

It's all repaired now, new caps, cleaned the board, and tests OK. The caps in it were unbranded, which is odd.

I wonder if the work from one makers is farmed out to cottage industry, like the Japanese used to do. One design, many makers who source components themselves perhaps.

Just got is some replacements , cheap types, slim line version, and they work fine,(and are flow soldered). It's just a nuisance it is such a lottery as to quality, especially as safety is involved.

 

Stephen

 

Dump it!

[bertiedog]

It is not being used except as a test power supply, and is repaired. The ones for the layout are from China, but UK supplier, with paperwork, and opening up one are a different class of construction, and are flow soldered, generous isolation on the boards,and have the power in at one end and DC the other end, not in one bunch of connectors. All much better made, with higher rating, and should handle the 12 volt Cob strip lights.

The colour balance is excellent, warm white, for illuminating the whole layout with shadowless lighting, and one strip only for photography, to allow shadows. Pleasant to get stable lighting balance for working on painting scenery and buildings.

The bits that are junked are the fuses!

Stephen.

[AndyID]

It is not being used except as a test power supply, and is repaired.

 

Obviously it was a piece of junk. How good do you think the isolation in the transformer is? If that breaks down the output might approach 350 volts DC. It's up to you of course, but I wouldn't take the chance.