Microelectronics | 12 V / 4 A Switching Mode Power Supply

Synopsis

I bought this power supply from a general hardware store around eight years ago for a LED lighting project. It was cheap, and it worked well with a five meter 12 V warm LED strip around our bed. We used it for a couple of years, then I retired it. Why? Because of fear of blowing up. When I bought this 4 A power supply unit, I also bought two smaller ones, each rated 2 A. At the time, I used those for LED lighting around my desk. Unfortunately, one of them blew up, releasing a fair amount of smoke from the main storage capacitor. Needless to say that a couple of other parts went out bursting in flames. Thankfully, nothing caught fire.

For many years, all three power supplies were left forgotten in a drawer. Then came the moment to decide what to do with them. My options were to either throw them away in the bin, or repair the broken one and improve the reliability of them all. Because I dislike waste, I chose to rebuild all three supplies. In this article, I show how I rebuilt the 12 V / 4 A unit.

Disclaimer
This article is not to be treated as a do-it-yourself tutorial on how to fix, restore, rebuild, or improve this unit. This was not my initial intention. But you can consider this whole content as a general guideline, should you decide to attempt any service procedure on an identical or similar unit.

The entire documentation is just a reflection of my work and I cannot be held responsible if you damage your unit, or even harm yourself in the process.

Maintenance

First, I stripped away all the parts. I took some measures, and then I classified each part so that I knew what to use as replacements.

I replaced almost every cheap failure-prone part with high quality alternatives. I chose metal film resistors from Vishay, Dale, and KOA Speer. Only four resistors are carbon film and I used Ohmite parts. As for capacitors, I went with Wima and Siemens film capacitors. I replaced all semiconductors with quality parts made by Texas Instruments and Vishay. All electrolytic capacitors are long life Nichicon parts rated for 2000 to 20000 hours at +105 °C. The variable resistor is made by Bourns and is of Cermet type.

I kept the inductors though.

I cleaned the PCB with acetone and isopropyl alcohol and then soldered every component with quality eutectic solder.

Pictures

Here are some pictures that I took during this operation.

This is before the rebuild.

The overall construction is cheap. Even the silkscreen is of questionable quality.

Some parts are not seated correctly and the brand of the electrolytic capacitors is obscure.

And after the rebuild.

Components that are bound to get warm during normal operation are now elevated from the PCB with glass beads.

I used metal film resistors and good capacitors. Semiconductors are from a reputable manufacturer.

There was nothing wrong with the green LED. But I absolutely love deep ruby red LEDs with a wavelength of around 700 nm. I have a sufficiently large stock of these LEDs, enough for all my projects. I particularly like them as power supply indicators.

The solder side generally looks OK. There were some scratches but overall no lifted pads and no soldermask issues.

Old parts. Dongbaohe!? Go figure.

Even if the Dongbaohe electrical ratings are identical to those of the Nichicon main storage capacitor, their physical dimensions are not similar. The Chinese part is smaller and weights considerably less.

I'm pretty sure most of the resistors were probably fine, albeit in some places, a bigger derating would've been better. But why take the risk anyway? I replaced all of them with better parts and ensured consistent derating where needed.

After the successful smoketest, I connected a load and then adjusted the variable resistor for a 12 V reading. Next, I connected a dynamic load consisting of a NAS device. Then, I repeatedly took measurements of the output voltage, which proved to be a steady 12.00 V, practically stable in time.

The power supply dissipates very little heat at around 60 % load.

Now I'm happy with this power supply and I would dare to leave it on, without any surveillance.