# Variable Power Supply question

Hello everyone,

I want to make this power supply because buying one is more expensive.

Now, i got a few questions:

1. Is the circuit correct?

2. When i measure the outputs + and - is DC for continuity, should it be possitive?

3. Would a 220v 12-0-12 transformer work same way? (using 12 and 12 to get 24  one draws other gives)

4. Finally, should the 1000uf capacitor get hot or I have short circuit?

I hope someone knows how to help me...

GT

## Comment viewing options

Hmmm.

1. The circuit looks fine.
2. When you measure what? The output measured at +V should be positive relative to Gnd. I don't know what you mean about continuity.
3. Yes. As long as you are getting 24V AC measured from the secondary winding of the transformer, you should be able to substitute a 220V transformer for a 115V transformer. Also be sure that the transformer is rated for enough power to supply your circuit. If the original transformer was rated for 12VA (or 500mA), then your substitute transformer should be rated at least as high.
4. Your capacitors should NOT get hot. Your first sentence says that you want to make this power supply, but it sounds like you already made it. Check all your connections carefully with the power off.

Here is a very similar circuit shown with a 230V transformer.

Thank you ignoblegnome and OddBot for your help, it seems like the circuit is working fine now.

Im not really sure what was wrong... I started heating all the connections to correct bad soldering moved some cables, checked connections and like magic, it worked.

Now finally, I dont know if you will be able to answer this but, do you have an idea why when i plug the circuit in, my mouse stops working?

When is plug it off the mouse works again...

What is going on?

Is the circuit too noisy?

Thanks, GT

Also keep in mind that the LM317 has sufficient cooling (heat sink) especially if you have low output voltage.

With a 24VAC transformer you will get approx. 34VDC (24V * 1.41) over the 1000uF cap.

If you have the output at 5VDC and as an example using 100mA the LM317 must be able to handle 3W. (34V-5V = 29V * 0.1A = 2.9W) Without heat sink it get hot very fast.

If you dont need to get over approx. 10V you can just use one 12VAC from the transformer.

Yes I noticed the LM317 can get hot in a past project in which i didnt have a proper heatsink but some tin foil folded and wrapped around it.

Now I`m using One of these and it works just fine :)

Thanks.

Good to see you are using a heat sink. You should use a very think layer heat sink paste between the device and the heat sink. This fills in the tiny spaces between the two surfaces, and ensures good heat transfer.