Questions about modifications to AliExpress SMPS units

[Johnny B Good]

 Looks like you'd placed the positive rail capacitor backwards! The positive terminals of both caps are commoned to the zero volt rail.

 Oh, and btw do you happen to know why your YT video comes up as "Your browser can't play this video.". I'm seeing this "error" message more and more frequently in Opera these days.

 I used to fire up Firefox and paste in the offending page's url. This works but since such videos are so badly infested with adverts (and often "Click Bait" to boot), I no longer bother any more, electing to close the tab instead and view the rest of YT's selection on the original page.

 I always open any videos of interest in a separate tab - that stops YT from "rearranging the video list" after watching one of a few possible videos of interest. I assume this is simply a ploy by YT to randomly shuffle the list forcing me to scroll around in order to find the next video or two that had also piqued my interest on the original page.

[intabits]

Sorry, I thought this thread had gone dormant, so haven't checked in very often...

Looks like you'd placed the positive rail capacitor backwards! The positive terminals of both caps are commoned to the zero volt rail.

I assume you mean my little DIY PCB?
No, that is only for the negative supply, the two caps are in parallel.

I've had a look at it now, and I'm more confused than ever.
There was no wiring error, the dual diode (which seems to have survived) was connected across the outside transformer outputs, just like the original one for the positive supply. Two wires bring it's common anode to the DIY board and the caps -ve, and two more bring the 0v rail to the caps +ve.
The cap was about to blow not because of reversed polarity, but because of over voltage! I measured 57V coming out of the diodes, so tried a 63V cap, which promptly exploded.
Looking on the scope I saw there were peaks of 120V. So I tried a 450v cap which survives - but I'm getting a -120V rail.

I'm currently trying to learn more about the nuances of these things, and while I've only scratched the surface so far, I believe these are, or are related to, "asymmetric half bridge converters". And I'm wondering if the "asymmetric" part has something to do with why my symmetric mirror-imaged addition is not providing symmetric outputs...

Oh, and btw do you happen to know why your YT video comes up as "Your browser can't play this video.". I'm seeing this "error" message more and more frequently in Opera these days.

No!?
I had just assumed that the reason I was seeing no views at all, was because the video is unlisted, so that "Anyone with the video link can see this video".
It's working for me in both firefox and chrome (with different google accounts).
And recently had a similar issue with an unlisted video linked from a short that also getting no views, even though the short got hundreds...

I've just made it public, maybe that will help.

[Johnny B Good]

Thanks for clarifying the purpose of that circuit board.

 Your experience with converting the output from a bi-phase full wave rectified single pole output into a centre tapped full wave bridge rectifier by adding the diodes 'missing' from the full wave bridge topology of such a bi-polar output echoes a similar experience when I had the bright idea that I could improve the +/- 12 rails on the cheap 10W class II smpsu board used in the FY6600 arbitrary wave function generator.

 I too saw the output voltages go north (and south ) of their nominal +/- 12v to the tune of some 20 odd volts or so. Luckily, I'd thought to test it before connecting it to the main circuit board whilst monitoring the output rails so was able to switch it off within just a few seconds to save the 16v rated smoothing caps from blowing up.

 I reverted this ill advised mod and had a good think about what had happened before I realised that this type of smpsu cannot benefit by such a conversion on account of it being of the flyback type. The centre tapped 24v winding had the diodes connected effectively in series, cathode  to one end and anode to the other, such that they simultaneously conducted on the same 'half cycle as the 5v winding rectifier diode.

 My attempt to "improve" this smpsu board had been doomed all along from the start. However, the topology of your smpsu seems to be based on a  design using PWM pulses that drive the transformer primary with equal width pulses to each end of this centre tapped winding, effectively using it purely as an ac transformer rather than as a magnetic energy store with a separate primary winding, so on the face of it, your modification, unlike mine, should have produced the desired result.