Bestec BPA201S-12V 12V@1.6A wall wart repair

[eccerr0r]

110VAC-220VAC

I had one of these croak on me, was wondering why it died.
I had been running it on 120VDC (first half of my transistor magic smoke generator, err, inverter) and that was a suspect, turns out it wasn't.

Cracked it open with a vise, it split up very cleanly. It's a cheap 3-transistor optoisolator feedback design, trimmed by sticking in resistors in a parallel slot.

noted the output was shorted. Pulled secondary rectifier - short still there. Pulled out zener crowbar... short gone and zener was shorted! Checked switch transistor... okay. Filter caps...okay.

Replaced zener... working PSU once more.

What was interesting: it's a 12V PSU that measured 12.2V OCV. It had a 13V 1N4743A crowbar. Not much room for error...

Now comes the task of gluing the plastic back together. Ugh...

probably tape + zip tie...

[PeteS in CA]

Re: Bestec BPA201S-12V 12V@1.6A wall wart repair

Adding a diode in series with that 1N4743A, cathode to cathode, would add a diode drop to that nominal 13V (you could even select the magnitude of the diode drop by choosing a Schottky, ordinary, or ultra-fast rectifier).

[momaka]

Re: Bestec BPA201S-12V 12V@1.6A wall wart repair

It's a cheap 3-transistor optoisolator feedback design, trimmed by sticking in resistors in a parallel slot.

What's a 3-transistor design?

I only know of the 2-transistor self-oscillating (with opto feedback) and single transistor self-oscillating design (with or without opto feedback.)

Either way, if it's a 2-transistor, check if there is a "critical" cap and replace it. If you've heard about the infamous Bestec ATX-250-12E "motherboard killer" PC PSU, then you should definitely inspect this design, as it's from Bestec as well. In the case of the ATX-250-12E, the 5VSB circuit uses a 2-transistor self-oscillating design with opto feedback and a "critical" startup cap. The reason why that startup cap is called "critical" is because when it failed, it would greatly overshoot the voltage on the output, often killing the attached motherboard on its output. The startup cap is on the primary side - typically rated for 10-47 uF and 25-50V. If it's a crap brand, better replace it now than have a completely dead adapter down the road with possibly burned out attached devices.

probably tape + zip tie...

If you're just going to use the adapter yourself in a dry location and not intend to give it to anyone else, you can probably just go with the zip tie method.

When fixing stuff for myself, I'm too lazy to do even that and usually just wire-tie the case together (i.e. those wire ties you get for bags/bread at the grocery store.) But if I will be giving it to someone else, I glue it together with the proper glue (PVC or super glue, depending on the type of case plastic used.)

[eccerr0r]

Re: Bestec BPA201S-12V 12V@1.6A wall wart repair

Not sure, I counted two transistors plus mosfet switching, so three transistors. The optoisolator has one more.

Well, the zener diode probably would protect against the "critical" cap failure as it would clamp the voltage to 13V.

Oddly enough I don't recall seeing any small electrolytics on the board. Only the main filter cap and a two cap pi filter on the output were electrolytic. Rest of the caps were ceramic or polyester I thought...ugh do I want to reopen this thing...

[momaka]

Re: Bestec BPA201S-12V 12V@1.6A wall wart repair

Not sure, I counted two transistors plus mosfet switching, so three transistors. The optoisolator has one more.

So 3 discrete transistors on the primary side?

I don't count the transistor in the opto, since the opto is its own device. Same with the 431 current shunt on the secondary - it does have more than "a few" transistors inside, but I count it as a separate device.

Well, the zener diode probably would protect against the "critical" cap failure as it would clamp the voltage to 13V.

That's true.
But then you may be inconvenienced to a blown primary side... unless the PSU somehow has SC protection latching (maybe that 3rd transistor??)

Oddly enough I don't recall seeing any small electrolytics on the board. Only the main filter cap and a two cap pi filter on the output were electrolytic. Rest of the caps were ceramic or polyester I thought...ugh do I want to reopen this thing...

Yeah, it's possible. I've seen 2-transistor designs done with a larger metal film cap instead of an electrolytic for the startup / pri. side aux. filtering function.