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24V 10A 240W industrial switching power supply

(Reverse Engineered Schematics)



(click to enlarge)
Schematic of an industrial 24V 10A 240W switching power supply module S-240-24 in a metal housing. It's a halfbridge topology with two NPN transistors. The input is 110V~ or 220V~ AC mains (selected using a switch). It came from Ebay for only $13 insluding shipping. The SMPS is based on the old good TL494 control chip (on the secondary side) and a pair of bipolar (BJT) NPN transistors in a halfbridge. The switching transistors are NT2625N (400V 10A). It's very similar to an AT computer power supply (ATX predecessor), except that it has only 1 output voltage. It has a softstart, a proper fuse, NTC thermistor, Y1 and X2 safety capacitors, overcurrent protection, EMI filter, good isolation distance and a relatively safe-looking transformer (but i didn't open the transformer yet to check it from the inside). The transistors and the diode are mounted on the heatsink poorly. There's no heatsink compound and just the corner of the components is actually pressed onto the heatsink. The heatsink is very thin. One of the 4.7uF 50V capacitors (in a red circle) had both pins soldered in one hole!!! After fixing it, the power supply works well. I tested it at a full load (10A) for 1 hour. After 30 minutes the temperature of the heatsink nead one of the transistors settles at 68°C (20°C ambient). The power supply is not so bad for its price, but you have to properly mount the three components on the heatsink yourself and also make sure that the components aren't soldered into wrong holes. You can also ground/earth the negative terminal of the output (COM) for more safety. The fuse is in neutral instead of live. It may be a good idea to invert live and neutral. The NTC thermistor is connected an odd way. During 110V~ opeation, it limits the inrush current only in one halfcycle. The output inductor on an iron powder core gets very hot (especially its core).


(click to enlarge)
The schematic above, redrawn to a better shape by one of my YouTube viewers. Thanks :).


Industrial switching power supply module S-240-24 (24V 10A 240W)


24V 10A 240W switching power supply (S-240-24) - label. The input ac mains voltage is 110V or 220V +/- 15%.


24V 10A 240W SMPS S-240-24 without the top cover.


What you can find in a mass produced device :).


Heatsink temperature near one of the power transistors vs. operating time at full load (10A).



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