I have a 15 0 15 center tap transformer. I am trying to build a normal bridge rectifier and when I try and connect the diodes in forward or reverse bias I am getting an AC voltage reading on my voltmeter when measuring the voltage across the diode output and the center tap. It is quite counter-intuitive! I have made two figures to simplify things. Can somebody help with this basic problem??!! Thanks!

 

Each rectifier diode conducts one half of the AC waveform.
The result is a waveform of half cycles, 60 times every second (or 50Hz depending on your country). This is a waveform that is considered to be DC + AC. A voltmeter will still give a reading depending on how the meter has been designed.

If you want to measure DC only, you can filter out the AC component, i.e. smooth the signal to a constant DC voltage by putting a capacitor across the output.

In your case, a 100μF electrolytic capacitor will work. Pay attention to the voltage rating and polarity of the capacitor.

Get it wrong and the capacitor will explode!

 

Basic misunderstanding. Somebody said, "Diodes convert AC to DC." Well, they don't. They block one polarity of input and pass the other polarity. It's up to you to catch the results in a capacitor or let it continue to be a lumpy row of pulses.

 

Basic misunderstanding. Somebody said, "Diodes convert AC to DC." Well, they don't. They block one polarity of input and pass the other polarity. It's up to you to catch the results in a capacitor or let it continue to be a lumpy row of pulses.

Looking at it - I'm wondering if the OP is measuring with a high impedance DMM and getting the readings upset by the very low leakage in the rectifier, connecting a load resistor should cure that, or better still put on the 2nd diode on the other end of the winding.

 

Basic misunderstanding. Somebody said, "Diodes convert AC to DC." Well, they don't. They block one polarity of input and pass the other polarity. It's up to you to catch the results in a capacitor or let it continue to be a lumpy row of pulses.

Wonderful explanation, love it..

 

Wonderful explanation, love it..

The reason the Diode is represented as a typical check valve symbol, albeit backwards as far as electron flow!
Max.