Rectifier Circuit with Higher Voltage

Hi all!

Sorry if I'm in the wrong subforum. But this one includes Power. :)

We have created a very very simple rectifier circuit, from AC mains to 15v DC. We have a transformer with center tap on its secondary winding of 18v. We measured the output voltage AC of this transformer, without connecting anything, and from the center tap to one 18v end measures 18.6V, from center tap to the other 18v end still measures 18.6v. We want to regulate the DC using 7815. This regulator has 2V drop-out, so it is fine we use full bridge rectifier. But initially, we used Full Wave Rectifier which uses the center-tap.

The output of the regulator is indeed 15v, but when we measured the voltage before the regulator: whopping 25V ! I don't know why this happened. So we modified the circuit and used Full Wave Rectifier which does not utilize the other 18v terminal.

Still, the output of regulator measures 15v, but the voltage before it now measures: 22V !

The regulator is functioning properly, but we don't want the regulator to drop large voltage that would contribute much to heating it.

Why is this happening? Anyone had the same experience? What should we do then?

Thanks in advance!

If your using a typical multimeter then AC mode measures either RMS voltage or the peak voltage times 1/sqrt(2) (which is exactly the same as the RMS if the wave form is a sin wave). In other words I would expect to see about 26v (=18.6*sqrt(2)) peak at the center tap. Once rectified and smoothed with a capacitor I would expect to see something a little under 26v.

The only way to "fix" this would be to build a switching power supply instead of a regulator or get a different transformer. If you're not using a lot of power then I'd just keep it as is. (if the regulator isn't getting too hot then you're okay.)

Wow! You explained well! It's very silly of me, I forgot about the multimeter reading only the RMS. Since the smoothed dc is the peak, then that is the very reason.

Thank you mspguy!

Consider this topic solved. ^^