Cheap bench power supply from China 30V 10A 2022.09.21 at 14:50

The names of those power supplies are different, but inside they are all the same, with veeery slight differences. The brand names are Wanptek, Hanmatek, Naweisz, Sky TopPower, Circuitspecialists, Eventek and bunch of other brands and even without any brand, they all look pretty much the same and are priced at about 70-80 €.

The internals of the power supply are the same and looks like this:

Pictures takes from the power supply I got for repair, exact model is KPS1505D, one of other models, sold by this „manufacturer”:

The power supply had burned main fuse (3A), so it was not long to find out that power transistors and diodes gave their souls to the god of electronics in form of smoke. And we all know – all electronics is made of smoke, when it goes out, electronics stops working. Well, before repairing, lets study this board for a while. By the way – as this power supply is so typical, I am not explaining about its functionality, controls and basic things like that.

The main PCB is marked POWER-ZB-02 2022.01.10:

The white connector on the bottom right is for 230AC input and all this bottom part along the PCB is for input filtering, rectifying and stabilizing. So right to left – yellow input capacitor, common mode (CM) filter, which, with its ‘current compensated’ windings, provides high impedance to CM noise without saturation from line current, two small blue caps and one more yellow capacitor. Then a small diode bridge, yes, yes, the fuuuuuuull bridge rectifier (DB107S). This rectifier, together with the nearby electrolytic capacitor, that strange IC with 7 pins (TNY280GN) and JKL-10-12 transformer (skipping some minor SMDs) forms low power supply for logic and control ICs, LED indicators, etc.

The bigger rectifier (KBL610) and two big electrolytes are for the power section of the device.

And on the left side of the PCB a place for missing connector, it wasn’t there, but on the other boards the main power selection switch (115VAC/230VAC) is connected. This version of device has no connector on the PCB and switch on the back panel, so, if you want to use it with low voltage power system just connect those two pins on PCB or put a switch – when pins shorted use 115VAC, when open use 230VAC. The rest of the board is, basically used to hold the main controller (TL494C). The TL494 or its clones or variants are used in all sorts and kings of PWM power supplies and, probably, in 95% of their population.

As this IC is so popular, there are plenty of schematics with it on the Internet, which would help you to make the repairs. I was surprised, but this version of board has all electrolytic capacitors marked as low ESR, which is good (hoping, that its not just marking 😋). Even those big input filtering caps (470 µF 250V) are low ESR, which is not really needed for their job.

One of three output caps is also Changxin 2200 µF 35V Low ESR, other two Sanyo 1000 µF 50V, together with a big and small choke, they form the output filter (on the top of PCB picture). And thick copper wires on the negative trace are shunts for current measuring.

In the middle part of the PCB the main component is TL494, couple of op-amps, one of them turns on the fan when aluminum board with transistors and diodes get hot. The temperature is sensed by 1N4148 diode, drowned into thermal paste. The last things are power transistors and diodes:

The big thing is a dual Schottky diode with common cathode – MBR30200PT. But the two transistors are blank, not marked 🤨, guess what are they. Ok ok, I tell you, its 2SK3569.

And yes, it’s not recommended for new designs, most probably you wouldn’t find them for purchase eather. Unless fake AliExpress clones. Luckily, we have me and I already selected proper substitute – AOTF15S60L. Why particularly this one ? Because at the date and time of repair it suits all requirements:

  • Analog or better than 2SK3569
  • Cheap
  • Available for purchase

This transistor a slightly better that original, is almost cheapest (~2,5€/piece) from the selection and is available for order – perfect :).

Parts received and soldered in, surprisingly nothing exploded, and power supply shows the set voltage. But the test under load was not successful – connected 0,62 Ω resistor and raised voltage to raise the current. At some point heard a clap and everything turned off. The current was not high, I think it was about 6-7 A, but can’t say for sure – I did not expected it to explode, so did not looked at the display all the time and also display refresh rate is not instant. Nevertheless, fuse blown, most probably transistors and diodes also. Let’s disassemble it again and see what happened. But it worked with some small current, liek couple of amps, so it might be, that TL494 is counterfeit and cannot work normally under bigger loads, while the idea of this power supply is to be fearless when speaking about currents up to 10 A or short circuit. Luckily, I still have some transistors and diodes left 😅.

To be continued after parts received and soldered in.

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