Cheapy Chinese SMPS Wall Wart

[dragon-architect]

Hello there! I'm not quite a beginner in electronics, but my knowledge of switch mode power supplies is rudimentary at best, even after watching lots of Big Clive take lots of cheapy Chinese power supplies to bits to figure out how they work. Earlier today whilst attempting to breathe life into a 60mm case fan (which turned out to be fried anyways), I was using a 9V wall wart to provide power for the fan. When I went to unplug the supply, you can guess what happened. Yep! It fell apart.

So I took the opportunity to extract the circuit board and try to reverse engineer it. I got as far as figuring it out to be a flyback topology SMPS with a bootstrap feedback coil on the transformer and... I think it's a 2-transistor oscillator... but that's as far as I got. After that I'm just completely dead lost as to how this circuit actually works.

There anybody that can help me out with a play by play run-down of how this thing works? Specifically, how do the two NPN BJTs work as an oscillator? What component(s) is(are) used in the feedback loop to set the output voltage of this circuit?

If need be I can post more detailed images of the circuit board itself on both sides. It's a single-sided PCB so all the traces are on the bottom.

[Zero999]

The schematic doesn't look quite right. Try tracing it out again. One thing which may help is to take pictures of either side of the PCB, flip one picture over and overlay it on the other side with it set to semi transparent, using image editing software.

[eblc1388]

I agree to comment by Hero999. There is no obvious current path through the main switching transistor, perhaps you've got the pinout wrong.

I'll provide you with a similar circuit which I believe will be 95% similar to yours so you can verify your schematic.




 

[dragon-architect]

I tried doing the layered transparency trick, but it didn't really work out very well for me. Everything just looked all muddied up and I couldn't tell traces apart from components. (Keep in mind I'm working with an awful camera for this sort of thing.)

But I gave the PCB another really thorough go-over, and I more closely looked at the datasheet for the optocoupler in particular and quadruple-checked how the optocoupler was connected into the circuit, and this is what I came up with a second time around. Now I'm even more confused by how the heck this thing is supposed to work, but somehow it works and puts out a steady 9V DC under no load and about 8.2V under load.

That 10uF/50V electrolytic off the negative end of the primary coil also kinda bugs me a bit, because it's not in an AC signal. It's in a full wave rectified DC signal. So I don't think it would be acting on the circuit with capacitive reactance, would it? Especially with that 2u2F/400V electrolytic across the terminals of the diode bridge smoothing out the rectified AC?

Here's links to the optocoupler datasheets that I found:

• http://www.datasheetcafe.com/fl817c-datasheet/
• http://www.datasheetspdf.com/datasheet/FL817C.html

(I couldn't find a direct link to any OEM datasheets. Even the NJE3003B is a defunct part)

Another friend seems to think the feedback coil of the transformer bootstraps the 2 transistor oscillator and gets it kickstarted, and the zener diode somehow plays a part in all this. I'm guessing when the output voltage rises above 8.2V, the zener clamps it down and shuts off the oscillator.

Anyways, here's more detailed pics of the PCB, top and bottom, and a new sketch of the circuit. Maybe that can help a bit? ^^;

EDIT
Iiiii made a mistake in both circuits. The little signal diode ST4148 is not connected to the base of the transistor NJE3003B. It's connected to the node between the 4u7F film capacitor and the other end of the feedback coil. NJE3003B is connected to nothing else but one end of the feedback coil. I think that's part of what's been messing me up this whole time.
TIDE

[eblc1388]

This is what I got from the images that you've posted.

[dragon-architect]

That corrected circuit definitely looks a whole lot more sensible, especially for the oscillator.

I'm curious to know how you found that pinout for the MJE3003B, because the only source I could find was this page which linked to this datasheet about the MJE2955T/MJE3055T, both pages of which talk about the transistor in the TO-220 package instead of the TO-126 package of the transistor on the circuit I have in my hands.

[eblc1388]

Try MJE13003. But the pinout is obvious just from inspecting its PCB connection.

[dragon-architect]

........ Son of a biscuit. I completely overlooked the 1 in that part number because it's so worn off. O.o I just checked again and sure enough, MJE13003B.

I know next to nothing about flyback power supplies, and barely anything about transistor oscillators, so the PCB connections aren't quite so obvious to me.

[amyk]

The keywords to search for are "ringing choke converter".

http://mmcircuit.com/understand-rcc-smps/
http://www.deltartp.com/dpel/dpelconferencepapers/S19P6.pdf

[dragon-architect]

Okay, next question: I'm looking at the PCB again, but now I'm not sure how it's possible to determine the connections for the transformer coils. I'm guessing that each coil's pair of pins are spaced close together, and each pair of pins are separated by a bit more space between them? I was going on the assumption that the two outermost pins were the primary winding (to correspond with the two pins of the secondary on the opposite side) an the two innermost pins were the feedback winding.

... I'm a derp. In writing this I remembered the multimeter I've got on hand has a continuity test mode. Just tested the pins on the transformer and YEP. Each pair of pins is for a separate winding, and the pairs of pins are separated.

Trap for young players like me! 

[dragon-architect]

@amyk: Thanks for the links and the keywords!
@blueskull: Yeah the 9004 was a misprint on my behalf. It's an SS9014 (with just one S printed).