Cheap Chinese 24V 15A PSU from ebay

So i got this PSU here from ebay for an insanely cheap price of 21.91 € (that includes shipping). Usually this PSU would've cost me 10-15€ more (even from china). I was hesitating for a while, cause it seemed too good to be true, and the bad reviews of the buyer sounded a bit nightmareish. In the end i decided to go for it and see what i end up with. The picture of the listing depicted a dented, but otherwise functional, PSU, so i figured these might be units with shipping damage, that cant be sold for a higher price. And indeed when my PSU arrived, the acrylic cover of the terminals fell off. The little pins that held it in place were broken off. But the cover didnt had any information printed on it, so i didnt count that as a loss.

When getting a cheap item, i always inspect it before using it. so lets take the thing apart.

We get a 1 sided circuit board with through hole components. Key elements here are the switching transistors at the back(1), the switching controller(2), the switching transformers(3), the huge inductor(4) and the diode rectifier on the left(5). Now how do you test for overheating components? Some people try a laser thermometer, but i find that very tedious and not satisfying at all. So i got myself a thermal imaging camera in january 2016 for exactly that purpose. Lets take a first look at the device seconds after plugging it in.

Uhoh... that doesnt look good. We have a ton of hot components here. Lets investigate further:

i luckily found a blog that already reverse engineered such a supply. The schematic is pretty close to what my PSU has, so i will tell you about the differences further down.
blog: http://imajeenyus.com/electronics/20151028_smps_variable_voltage/index.shtml
schematic: http://imajeenyus.com/electronics/20151028_smps_variable_voltage/s-400-12_supply.pdf


R41 (3K9): This is a smaller resistor. probably rated .25W.  It's directly connected to the power LED and is supposed to run the LED off from 24V. i let it be, as there are no components next to it that could fail due to its heat, and if it fails, the LED will die, and the led isnt vital to the project.


R16 (330R): A big 1W resistor wedged in between 2 caps. This is a bleeder resistor that should discharge the 24V filter caps. Now lets calculate the powerdrop going through it using this formula: P= U^2/R. so P = 24^2 / 330 = 1,745W. This is way over the resistors Rating. Now i dont have any 1W resistors, but i could put some higher value .25W resistors across it. Or even just one, as the PSU is supposed to be part of an enclosed appliance, and thus it doesnt matter if the 24V stay on there for a bit longer. Lets calculate a good value for a .25W resistor.

0.25 = 576 / R
R = 576/0.25 = 2304

So using a 3k or higher resistor should suffice.


R37 (22R): (Update: the fans speed is changing with the load that is applied to the PSU, so i dont have to change anything here) Again a 1W resistor. This one is in series with the fan. Looking at the silkscreen it seems like its in the wrong hole and a component next to it is missing. Looking at the schematic and the full image on the blog we can see that this component is a NTC thermistor, which should reduce its resistance with increasing temperature. Its a crude way to make a fan react to higher temperatures. With the current configuration the fan is idling at a low speed, and only revs up when the PSU is unplugged (probably a voltage spike in the fans supply). I am unsure if i should just bridge the resistor and let the fan run full speed, or change its value so i get a bit better cooling. Im going to measure the voltage on the fan pins with my oscilloscope to see if the resistor is actually also there for dropping voltage. Im going to use a voltage divider for this, as the scope can only display up to 20V and will cut of anything above that. Also the computer disconnects the usb oscilloscope when that happens, and it takes a few seconds to recover. Now i tested the voltage, and i forgot i had a x10 probe :P so no need for a voltage divider after all. But the voltage is at 20V, so i dont just want to bridge the resistor. Instead im going to add a buck converter, and with that i can properly adjust the fan's speed.


R2&3 (150k on the right): Here we have 2 small probably .25W resistors that are there for bleeding the 2 big 250V caps that are filtering the positive and negative rail (each ~170V when feeding 240VAC in the device). Lets calculate the power dissipated in them. P=170*170/150000 = 0.193W. this is still under the .25W that the resistors are rated, so i will leave them, especially as i need a fast discharge for these big caps when unplugging the PSU.

In the same shot we see R11 on the left and C5 on the bottom. C5 is a small ceramic cap, so i just hope it can tolerate the heat


R1 (680K): This resistor is meant to bleed out the capacitor C4 that is across the AC line, so you dont get zapped by several 100 Volts. It gets hot, but again not close enough to other components to bother changing it. Also the bleeder resistors of the main caps will take care of C4 if this resistor fails.


R11 (1K5 on the left): This one is a mystery to me. Even with the schematic i have no idea what its purpose is. But it also gets searing hot and seems to be an important part of the circuit. The thing im going to build is intended to run ~14 hours a day for years. cooking the PCB and its components around it is not acceptable. I cant afford to let this resistor fail eventually, i will have to replace it. Probably with several higher value resistors.

now that we have sorted through these hot resistors lets look at another problem. Before i disassembled it, i plugged it in and moved my finger across the case. i felt a vibration. this means there is a voltage on the casing. when measuring it using a voltage divider made from 2 10k resistors it stopped, so grounding it with 20K is enough to eliminate that voltage. The reason why this voltage is there, was, that i only used phase and neutral to power the PSU. The PSU has a terminal for the ground wire, and the case is connected to that, so i guess when i put a 3 wire plug in there it should be fixed. The voltage itself is probably induced in the casing by the numerous magnetic fields that are generated inside the PSU.

Now for the missing parts: Im missing DB1. Here the part of the schematic with DB1:

i redrew this part to show how my DB2 is wired.

The difference this makes is, that i only get a half wave rectifier for my 24V output. Now this PSU is supposed to give me 15A, and i wonder if a half wave rectifier is the job for such a high current demand. It might cause the switching controller to switch the transistors harder. Luckily DB1 can be replaced by a bunch of rectifier diodes, But i dont have a 15A one. Also i would have to place some jumpers on the board. As you see C20 and R25 are also missing in my sketch, but they are still present on the board, they just go nowhere. Also yes C17 and R26 are wired the other way round. I know this doesnt make a difference, but i wanted to show that here too.

The next missing part is the NTC thermistor that should regulate the fan. In his PSU its put inside the inductor. I would personally put the thermistor somewhere over the switching controller and all that, cause those components are much more sensitive than the inductor.

I Also want to mention my PCB is labeled with a V2.0, which probably is the revision number. Maybe also a reason why his schematic is wired a bit differently

And thats it, a look at an extremely cheap psu. Lets hope it doesnt burn my house down. I probably didnt save money with this thing. I had to invest a lot of time, and time is money. If you only count the components i mightve saved something. As this is my hobby i dont count the time, so all in all: win? i will post a follow up when i replaced the components mentioned above