12V adapter modded to 5V, with USB socket! :D

[ben7]

Here is my most recent PSU mod.

I changed the caps with better ones. (Used, but still, better than the original 'Elite' capacitors!)

I added a 5.1V zener diode to the output. If the two transistor circuit goes crazy, the zener will clamp the voltage and/or short. Also the PSU output voltage has a small overshoot, about two volts, when unplugged. This is handled by the zener diode just fine.

I removed the original feedback resistors, and put in two of the same value (1% also). I found the output voltage to be a little low, so a 15K resistor was placed in parallel with one of the divider resistors, and the output is perfect now.

Then I put in a better schottky diode for the secondary side rectifier and also added a heatsink to it. The original was an SB360, and it just would get too hot for my liking. The new diode runs only just warm.

I soldered a wide angle LED I got from a scrap board onto two wires, and wired a resistor in series. I then drilled out a hole in the case, and glued the LED in there. The wide angle is nice because the LED looks about the same brightness all around, not like those cheap PC speakers with the narrow viewing angle LED, and super blindingly bright!

I got the USB-A socket from a scrap board too, and I soldered metal tabs to the sides of it. Then screws were placed through the tabs and the plastic shell so that the socket is held in nicely, no fussing with trying to get glue to stick to the plastic! It has virtually no noticeable wiggling when I have a USB cable plugged into it

So, that is all I did. Quite a lot actually. And of course, I have included pictures!

P.S. Sorry for the somewhat dark pictures, the sky is cloudy outside, and those CFL light bulbs are terrible for taking pictures in!

[mariushm]

Re: 12V adapter modded to 5V, with USB socket!

Not sure 5.1v zener is a good choice... you may hit that easily during normal operation, especially if you have something that pulls some current and then suddenly goes to sleep or something like that.
I would have chosen a 5.6v one.

Other than that, the mod looks nice... you started with a good power supply which certainly helped.

[tom66]

Re: 12V adapter modded to 5V, with USB socket!

I would have chosen a 6V TVS or SCR circuit. A zener's clamp is too soft for any real clamping. The "dynamic impedance" of a typical zener may be 10 ohms. This means past the breakdown point (e.g. 5.1V) it basically acts as a ~5.1V drop with 10 ohms in series. i.e. 10V is only ~500mA extra load for the power supply. It'll just get hot and melt open. A TVS may have a dynamic impedance of around 0.01~0.1 ohms, easily clamping the output voltage.

The main disadvantage is a typical TVS is around +/-20% tolerance, so it's hard to set a particular clamp voltage. In which case, you might have to use an SCR and zener circuit. Just tie the zener to the gate and supply voltage, maybe add a resistor if you want to limit the current through the zener. The SCR will latch the current until the power supply cycles off or current drops below a certain point.

Anyway, nice mod!

[ben7]

Re: 12V adapter modded to 5V, with USB socket!

Thanks guys!

Yeah, I did have a thought about the zener, that the 5.1v is too close. But I have used it for probably about two to three weeks or so, and nothing has happened. Yes yes yes, I know, that doesn't mean that something won't happen in the future!

I should trace out the circuit to see where or how I can make it not overshoot at all.

Oh, and, I thought zeners always failed short circuit? (Of course, unless there is a very large power source - then I can see how it might melt O/C)

[tom66]

Re: 12V adapter modded to 5V, with USB socket!

Small zeners can crack open if the current is significant, and a power supply like this could probably provide around 3 amp short circuit which would be enough. I wouldn't say it's reliable that they would always short -- probably around 80% of the time but not always.