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10-11-2017

Whipping a cheap LED PSU into usable shape

Michael Dunn

EDN

As part of the LEDification of my house, I’m designing some wall-lighting sconces. The plan is to place an array of LED strips on the walls, covered with translucent (or in some cases, opaque!) Plexiglas plates mounted a few centimetres away. That’s the plan anyway (yes…future blog).

As before (see My kingdom for a 12V adapter), providing 12 VDC power has been a stumbling block, but in this instance, I think, not insurmountable.

Thus we return to my continuing love-hate relationship with Cheap Chinese Chotchkes – in this case, 18 W power supplies small enough to fit into an electrical box (Figure 1).

Whipping a cheap LED PSU into usable shape
Figure 1. The 12 V 18 W PSU. Notice the 100% efficiency (IN & OUT are both 18 W), and the dire
warning not to touch the surface of the plastic case because of high temperature!

I say “18W” (well, the website says 18 W), but those are Chinese specs, not to be confused with actual specs. The first batch of PSUs came a year ago, and believe it or not, could provide a full 1.5 A without collapsing. However, upon opening the cases, the component temperatures after running at full-throttle for a few minutes suggested otherwise. The switching transistor, output rectifier, and output capacitor (!) were all far too hot for comfort. I imagine the 18 W spec is only valid for one day of use.

I recently purchased a second batch of these PSUs, this time from AliExpress. They behave differently than the first batch. From an external vantage point, they’ll only supply 1.3 A as opposed to the former 1.5 A. The output voltage collapses to 8 V at 1.5 A.

Take it apart, and further differences appear. Most noticeably, the 12 V output capacitor does not egregiously overheat at, say, 1.3 A. Sure enough, the part has “Low ESR” printed on the case. The previous caps don’t.

The design appears to be a simple self-oscillating circuit. I measured the switching frequency to be about 100 kHz. The 12 V output caps are at the upper-right of each board (Figure 2). Though the legend says “1000µF 25V”, the installed caps are 470 µF.

Whipping a cheap LED PSU into usable shape
Figure 2. PSUs from AliExpress (top) and another, forgotten source (bottom). Note the
convenient dates at the bottom of the boards. Manufacture has transitioned
from phenolic to fiberglass.

After discovering the output cap heat problem (but before getting the second PSU batch), I sourced a bagful of quality capacitors – 270 µF @ 35 V, still more than enough capacitance for this circuit, but with a high ripple-current rating and low ESR. Both of the boards in Figure 2 have these new parts installed. They run cool as a cucumber, versus the slight temperature rise of the second PSU caps, and of course, the extreme rise of the first ones.

Whipping a cheap LED PSU into usable shape
Figure 3. The PCB bottoms reveal other minor design changes in the newer boards –
mainly exposed copper to pick up current-fortifying solder.

I’m constantly struck by the strange state of affairs at this level of Chinese manufacture. Clearly, there is some thought and skill put into design and production, yet we still end up with stupidities, like unsuitable parts, or wishful-thinking specs.

As I mentioned, other parts get hot too. The switching transistor can get toasty at higher loads, but it was the output rectifier I focused my measurements on. At 1.33 A, free-air TC registered 90 °C. At 1.2 A, 86 °C.

Whipping a cheap LED PSU into usable shape
Figure 4. My test setup here at EDN Labs. Note the many safety protocols employed on the bench.

Enclosed in its case, in an electrical box, I don’t think I’ll want to pull more than 1.1 A from these PSUs. Hopefully, that will be enough for my LED lighting. Other options: Squeeze two PSUs into a box (possibly swapping the case for some shrink wrap or electrical tape), or, cut a hole in the case so I can bend the rectifier out and heat-sink it to the electrical box! Hmm. We’ll see.

You may have also noticed the absence of an AC line filter on the board. Well, what did you expect for US $3.15? I would much prefer to have used this 2 A bare-board PSU, or this enclosed one. The former, at least, has a line filter, and even if they also don’t meet their output-current spec, they will certainly be better than the PSUs I’m using. But…they don’t fit into an electrical box.

EDN reader's comment

Having worked in China at a factory that manufactured electronics for their own products, I can tell you that there are a number of 'forces' at work on products from China.

First, especially the younger engineers, are very keen to design the 'best possible' circuit to do the job. I found many of them to be conscientious in this regard. What constitutes "the best" morphs over time form technically the best solution to what works with the cheapest, least number of components as they become more jaded by their environment.

The next factor is their environment - I'm not talking about air & water here, but the commercial one. The' bean counters' want the absolute lowest cost to manufacture possible - the reason is that many manufactures are on 5% margin on the products they produce, there is no 'fat' there! With no knowledge of electronics, they seek out the lowest price components that meet the majority of the top rated specs... "whats ESR?" They wouldn't have a clue - nor do they care - their job is to source the lowest cost suitable parts, but nobody fully educates them on exactly what that is and they would lose their jobs very quickly if they spend time talking to the Engineers to find out - their job is to source parts and they are measured on this alone.

Then there is quality control. If you pay for it, they will do QC to the highest possible standards, if you do not specify, they won't do it at all. It is up to the customer to specify EXACTLY what it is they want. If you don't specify, they will very rarely ask you, they will take the lowest cost approach.

As you say, for US$3.15 in the US, what do you expect? In many ways, it is the users who create this issue, everyone wants cheap, but they expect quality and immediate delivery. If you have a triangle with one of these qualities on each point, you can only hold two: which two do you want? Cheap, Quality, Fast… The one you are not holding will always be the compromise!

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