Power supply help!

[Frederic Gagnon]

Hello all,

I have a little experience in electronics, but it's the first power supply I really build from scratch. I have put together a 36v power supply using the following components :

-toroid transformer 115V/36V
-a rectifier bridge 12A/100v max
-a 30,000 microFahrad capacitor

I am testing the output voltage and it gets 50V instead of 36V. What can cause that ??

thanks!

[Al_The_Man]

You are seeing the peak voltage, the AC is an RMS value of the sine wave, so the the RMSx1.414 will give you ~50vdc.
If you want to lower it the easiest way, especially with a toriod is to take a few turns off.
The secondary is wound on last and usually amounts to around 2 turns/volt.
Al.

[Frederic Gagnon]

I measured the voltage on the capacitor lugs, so the the voltage would be "flattened" at that point, no?

[Al_The_Man]

The capacitor stores the PEAK voltage, that being the 36x1.414.
Al.

[Frederic Gagnon]

Then I should have selected a 25.4V transformer, ahh well... Would a resistor be a good way to drop the voltage before entering the controller board ?

[Al_The_Man]

No, a resistor is the worst way to go, I would seriously look at taking turns off, it is not as intimidating as it sounds.
You can also put a separate small winding back on for a 5v or 12v supply if needed.
The AC you needed is a reciprocal, 36dc x 0.707 = 25.5ac.
Al.

[Frederic Gagnon]

Thanks for the help. I'll probably attempt the procedure you describe and let you know.

Meanwhile, I was reading on another thread about the TB6560 Chinese controller, which I have, and I understand that the power supply will be the least of my problems...

[Al_The_Man]

There are a few posts in the past here on removing windings, maybe a search with Toroid may find them.
Al.

[Frederic Gagnon]

Hello Al,
I have started unwinding the transformer. I have removed about 8 turns of the yellow wire in the secondary winding (see attached schematic). When I check voltage, it stays around 50V as if nothing changed. Is the capacitor "memorizing" the voltage ? (it's big)... I notice voltage stays even after I turn the switch off. Should I work with the other wires?

Thanks!

Originally Posted by Al_The_Man There are a few posts in the past here on removing windings, maybe a search with Toroid may find them. Al.

[Al_The_Man]

Firstly, only work at the AC level, do not connect the bridge, your transf. shows two primaries and two secondaries, how do you have these connected right now?
Al.

[Frederic Gagnon]

Red & Orange are tied together. So I have Red and Black going to the bridge rectifier.

[Al_The_Man]

You have a slight problem there, either the two secondaries should be in parallel or in series, assuming they are the same voltage, what is the AC voltage on each of the secondaries?.
How do you have the primaries connected?
They can be connected for 120v or 240 input.
Can you show the connection detail chart clearer?
Al.