# Trying to build a power supply unit

#### greendean

Joined Dec 20, 2008
19
ok, i'm pretty new to this stuff (you've heard it all before).

I am trying to design a simple yet efficient PSU out of a 16volt 3.5amp center tap transformer i have.

Can someone point me in the right direction for the size of the capacitors, diodes, and inductors.

I am also looking to knock the voltage down to 15v.

So when all is said and done I would have +15, 0, -15.

THANKS!
-Dean

#### Wendy

Joined Mar 24, 2008
22,702
Basically the capacitors are the thing, and diodes rated for the current. Since we are talking peak voltages that sets the PIV (Peak Inverse Voltage) you'll need for the diodes. 50V PIV ought to do it well. You can get diode bridges as a component, which is what I recommend. I seem to remember ThingMaker (one of our mods, also a techie) posting some rules for capacitor sizes vs. load (ie, current) around here, if/when I find it I'll add a link.

#### greendean

Joined Dec 20, 2008
19
Thanks Bill.

I figured out the inductors. Pretty simple.
And i figured as much for the diodes.

The caps are whats driving me stupid.

EDIT:

So i found this equation for the size of the caps

uf = (10xcurrent) over (maximum ripple).

which is great, but now how do i figure out my maximum ripple?

Last edited:

#### Wendy

Joined Mar 24, 2008
22,702
That depends on what you're going to use it for. For RF and audio, it may need to be quite small, whereas digital isn't nearly so picky.

The thing is, get it down to millivolts, and a regulator will smooth the rest.

#### beenthere

Joined Apr 20, 2004
15,819
If the transformer is 16 VAC with a center tap, the maximum voltage on the filters will be about 11.2 volts. From that, you could use 9 volt regulators for +/- 9 volts, but you can't get +/- 15. For that, you need to have about 14 volts on the filters capacitors, or a 20 volt CT transformer.

#### Audioguru

Joined Dec 20, 2007
11,249
If the transformer is 16 VAC with a center tap, the maximum voltage on the filters will be about 11.2 volts. From that, you could use 9 volt regulators for +/- 9 volts, but you can't get +/- 15. For that, you need to have about 14 volts on the filters capacitors, or a 20 volt CT transformer.
No.
The rectifier diodes will reduce the 11.3V peak down to about 10.3V and the ripple will reduce it more which is too low for ordinary 9V regulators.

To get plus and minus 15VDC you need to have a peak voltage of 18V to 20V so the transformer must be 26V center-tap or more.

#### greendean

Joined Dec 20, 2008
19
It's a 30 volt transformer.
16v to center tap. 3.5 amps.

so i have enough power.

and this is for an audio application.

#### Audioguru

Joined Dec 20, 2007
11,249
Terminology??
You said a "16volt 3.5amp center tap transformer". Then it is 8V-0V-8V.
A 30V center-tapped transformer is 15V-0V-15V and has a peak voltage of 21.2V which is reduced to 19VDC by the rectifiers and ripple from the filter capacitors.
Then you have plenty of voltage for 15V regulators.