power supply problem

[prgdeltablues]

I'm struggling with troubleshooting a MFOS 12V wall-wart power supply. I've built one before, which works fine, but this one is causing me grief. After swapping out various components, I've resorted to breadboarding the basic LM317 regulator circuit. That works fine with a 9v battery. As you'd expect - but it confirmed I had the pinouts right. When I build the half-wave rectifier and smoothing caps, using a 12v AC linear AC/AC wall-wart from Farnells, and use that instead of the battery, the trouble starts. The correct voltage appears at first at the output of the regulator, but after a few seconds it starts to drop quite quickly away down to 0. Even more disconcerting, measuring the AC out, at the 2.1mm plus on the adapter, with nothing else connected, it's about 15v AC, as I'd expect from an unloaded wall-wart Plug it in to the 2.1mm socket, -still nothing connected - and I get 15V for a second or two, then it drops to about 3V or less. Which makes me suspect a partial short - resistive or capacitive - of some sort in the socket itself. I don't have another one to hand to try. anyone else ever experienced anything like this?

Peter

[John Noble]

Hi Peter.

Ray's power supply boards are right out of the National Semi data sheets, and I don't see what could go wrong except for bad components.

Now, you're building a MFOS +/- 12VDC supply, right? Does this mean you're using LM7812/LM7912 regulators? If so, 15VAC RMS is too low; you need a bare minimum of around 19-20VAC RMS (under load, not open circuit) to have enough headroom for the regulators to not drop out.

Also, I infer from the fact that the transformer has a circular connector that there is no center tap available. This will also cause problems with the MFOS board, won't it?

Lastly, I'd say the transformer is faulty if the output drops way down with no load. Transformers should do the opposite: no load voltage will be considerably higher than the rated voltage most of the time.

I hope none of the above was too elementary or too obvious.

[prgdeltablues]

Problem solved! Yeay!

Thanks for the comments John, always good to have a second view. I'm using the LM317/337 adjustable regulators, which seem happy with 15VAC. And it's the half-wave rectified version of the MFOS circuit, so no centre tap.

After a night's sleep, the problem turned out to be that I was using the wrong pins on the 2.1mm socket - must be a normalising version, and I suspect when I turned the power on, I was shortening out the transformer, but not before a brief surge of current had charged the smoothing capacitors - so the output from the regulators was briefly OK, then fell off as the capacitors discharged. Used the correct pair of pins on the socket and problem solved.

On now to calibrate my Klee!

Peter