Full-wave rectifier circuit showing half-wave rectification on oscilloscope?

Thread Starter

AverageMoss

Joined Apr 24, 2021
8
I'm new to electronics and decided to learn something new by trying to put together a small rectifier circuit that converts 24VAC to DC. So I put together a little full-wave rectifier circuit that consists of four diodes and an LED and ran into an issue when I tried to measure the DC output.

IMG04.jpg

This is a diagram of the circuit I put together on a breadboard and how I connected the probe of my oscilloscope to test it (btw I know the LED should have a resistor in the circuit). The next image is a screenshot of the half-wave rectification reading I am getting on the oscilloscope without the LED as a load, so DC positive connected to probe and DC negative connected to probe GND.

PNG02.png

But when I add the LED to the circuit as shown in the diagram, this is the reading I get.

PNG03.png

So after that, I decided to test the transformer alone connecting it to the oscilloscope like this.

IMG05.jpg
PNG01.png

The transformer seems to have no problem. It's just there to step-down 120VAC mains to 24VAC for testing.
If anyone possibly knows why I'm getting a half-wave rectification out of a full-wave rectifier circuit, I would really appreciate the help.
 

WBahn

Joined Mar 31, 2012
27,942
I would also recommend verifying that all of your diodes are good and that they are installed correct.

(btw I know the LED should have a resistor in the circuit).
So why don't you have a resistor in the circuit?

You are going to let the magic smoke out of something.
 

Thread Starter

AverageMoss

Joined Apr 24, 2021
8
Why did you not connect the scope-probe ground to the other transformer output?
It should be for a proper measurement.
When I connect the probe ground to the other transformer output I get a flat line. The peak voltage reading drops from around 26v to 800mv
 

KeithWalker

Joined Jul 10, 2017
2,623
If you are doing everything exactly as you describe, the results displayed are not what is expected. Do you have a ground connection to one side of the transformer secondary that is not shown on your diagram? If not, I suspect that there is leakage between the primary and secondary windings or one of your bridge rectifier diodes is open circuit.
 

crutschow

Joined Mar 14, 2008
31,151
When I connect the probe ground to the other transformer output I get a flat line.
Then there is something seriously wrong with your setup, as the transformer outputs should be totally isolated from your scope ground and they apparently are not.
If one of the transformer outputs is grounded, that would explain the half-wave from the bridge.
 

tautech

Joined Oct 8, 2019
307
Then there is something seriously wrong with your setup, as the transformer outputs should be totally isolated from your scope ground and they apparently are not.
If one of the transformer outputs is grounded, that would explain the half-wave from the bridge.
This ^^^
Check transformer output isolation from mains ground with a DMM.
 

Thread Starter

AverageMoss

Joined Apr 24, 2021
8
Then there is something seriously wrong with your setup, as the transformer outputs should be totally isolated from your scope ground and they apparently are not.
If one of the transformer outputs is grounded, that would explain the half-wave from the bridge.
So I completely disconnected the transformer from ground and tried testing it again. I connected the probe to the first transformer output and the probe ground to the second transformer output and still got no voltage. I then disconnected the probe ground and now I'm getting around 8VAC.

Now I know my transformer is f*cked but what would cause this kind of behavior?
 

WBahn

Joined Mar 31, 2012
27,942
So I completely disconnected the transformer from ground and tried testing it again. I connected the probe to the first transformer output and the probe ground to the second transformer output and still got no voltage. I then disconnected the probe ground and now I'm getting around 8VAC.

Now I know my transformer is f*cked but what would cause this kind of behavior?
Use a DMM to check the resistance between the transformer secondary winding and the primary winding.
 

WBahn

Joined Mar 31, 2012
27,942
There is something else seriously wrong: There is no resistor in series with the LED, thus the LED clips off half the waveform.
Only the middle scope trace has the LED in the circuit (with the resulting clipping). The first is with just the diode bridge and the third is with only the transformer.
 
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