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Please help. Universal adjustable power supply, 3-12V.

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gazownik

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Hi there,

what I have here is a cheap 3-12V power supply, probably something resembling the one below:

dual-supply.jpg

more detailed pictures here:

66_1283261028.jpg
20_1283261028.jpg

The problem with it is that it's completely dead. I disassembled it and checked electrical connections with a voltmeter set to "beep" (diode test). The rectifier diodes seem to be fine (at least the multimeter is indicating "something", same goes for 230V leads and primary and secondary windings of the transformer (beeps on their respective sides). Input zener diode seems to be dead but I suppose it's parallel so it shouldn't be that much of a problem. Fuse is also ok, i.e. beeps. I also checked the transistor (marked with red ellipse) attached to the radiator using beeper (although I'm not sure if it's a viable method to do that) - putting one probe to the middle lead and checking junctions by putting the other to the remaining two. It also indicates semiconductor junction inside. I'm out of ideas - what should I check next? Any ideas?

Thanks in advance.
 

Zero signs of life everywhere at the output stages suggests nothing is getting through the transformer.

Do you read expected volts across the transformer? Does it get warm?

Components can fail open or they can fail shorted. Sometimes you have to clip leads, except one lead, in order to test a component out of circuit.

The diodes are not likely to all fail at the same time. So maybe you don't need to start clipping one lead so you can test them out of circuit.

Is there a fuse on the mains side? Is voltage getting to the transformer on the mains side?
 

Thise diode is not in parallel, they are in rectification bridge (Gertz) connection, check each. Check transformer.

**broken link removed**

You can use voltmeter to check output AC voltage from trafo, and DC voltage after Gretz, then go further.
 

Forget that 1st schematic you posted, this is a switch-mode power supply (SMPS). First thing I'd check is the voltage on the primary side buffer capacitor. That greenish round thing on top - probably rated around 2x the AC mains voltage. For example for a 230V input supply, might be rated around 100 uF / 400V.

DC voltage on that should be around 1.4x AC mains voltage, if it's much lower either a fuse is blown, one or more of the 4 diodes (upper left) has failed, or the capacitor itself has died (you'd probably smell that :wink: ). If that capacitor voltage is okay (please post exact reading you get!), chances are either that power transistor, or the circuitry controlling it has failed. Note that diode test or similar on that power transistor isn't reliable as long as it's soldered in - you'd need to get 2 of its wires unsoldered to measure. But first check buffer capacitor voltage?

Btw BE CAREFUL - everything on left side of that 'trench' in the circuit board is connected to AC mains voltage, with power behind it. One mistake here could be deadly! After unplugging, make sure that buffer capacitor is discharged before touching the circuit anywhere. Either re-arrange test circuit / test components without power, or set up measuring circuit & don't touch / move / adjust anything while powered on.

- - - Updated - - -

By the way, product page here: **broken link removed**

No schematic that I can find. Saw a list price of $2.7 somewhere while Googling... :lol:
 

Hello again,

Thanks for your help guys, I'm doing it just for fun (so no worries about the price - and I'm aware that this junk is of Chinese origin :D ). I did some tests in the meantime and seems that both the driver and power transistors are dead (the latter has cracks on its casing). I'm going to replace them and post the results :)
 

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