Hi, I make a dual polarity full-wave rectifier with a bridge diode, but it doesn't seems to work as the theory says it should work.

The circuit I made in a breadboard is this one:


Here is the breadboard:

The red and green cables are the end-points of the secondary coil, and the brown cable is the center tap of my transformer. I'm using R=10k (2R=20k), so I have unbalanced loads.

I'm using a 12-0-12 center tapped transformer, here it is:


I wanted to see the rectified waveforms (both positive and negative, "Vo+" and "Vo-") on the output, so I used an oscilloscope and placed the probes here:


I've been reading this post that says that, in theory, the waves on the outputs should look like this:


But when I see my scope, it looks like this:

Channels are on DC coupling, not inverted. Oscilloscope is a Tektronix TBS1102.

What am I doing wrong? I've been trying to figure out this one for more than 20 hours, any help would be very much apreciated, thanks !

 

hi
Add these caps, 25v

E

 

You're seeing AC rectified to raw DC, if you add the two smoothing capacitors you'll get a better output.

 

What am I doing wrong?

Show us the two Vs waveforms from the transformer.

You're seeing AC rectified to raw DC

But he's only seeing a half-wave, not a full-wave as he should.

 

Hi, I make a dual polarity full-wave rectifier with a bridge diode, but it doesn't seems to work as the theory says it should work.

The circuit I made in a breadboard is this one:
View attachment 285306

Here is the breadboard:
View attachment 285307
The red and green cables are the end-points of the secondary coil, and the brown cable is the center tap of my transformer. I'm using R=10k (2R=20k), so I have unbalanced loads.

I'm using a 12-0-12 center tapped transformer, here it is:
View attachment 285311

I wanted to see the rectified waveforms (both positive and negative, "Vo+" and "Vo-") on the output, so I used an oscilloscope and placed the probes here:
View attachment 285308

I've been reading this post that says that, in theory, the waves on the outputs should look like this:
View attachment 285309

But when I see my scope, it looks like this:
View attachment 285312
Channels are on DC coupling, not inverted. Oscilloscope is a Tektronix TBS1102.

What am I doing wrong? I've been trying to figure out this one for more than 20 hours, any help would be very much apreciated, thanks !

Nothing wrong at all! You are seeing the two wave forms. Adjust the position control to separate them and you will see. One is blue and the other is yellow and they are exactly what I would expect. One going positive from zero, the other going negative from zero. perfectly synchronized, as they should be.
Use the menu to select the average value of each.

 

Disconnect the transformer center tap from ground

 

One going positive from zero, the other going negative from zero. perfectly synchronized, as they should be.

No.
Each output should be a full-wave rectified signal, not half-wave.
See simulation below:

Disconnect the transformer center tap from ground

No.
You need the center tap to ground as the bridge rectifier is actually acting as two 2-diode full wave rectifiers.

 

hi
Add these caps, 25v

E
View attachment 285313

Hi! ... no, I mean, I want to see the wave fully rectified, but instead it is getting half-rectified on each output ... which makes no sense, or does it?

I don't wanna filter it ... Do you know what could the problem be?

 

You're seeing AC rectified to raw DC, if you add the two smoothing capacitors you'll get a better output.

Hi!... yes I know but I don't want to filter the output, I just want to know why am I getting a half rectified wave on each output instead of a fully rectified one as the theory says ... Do you know what the problem could be?

 

Nothing wrong at all! You are seeing the two wave forms. Adjust the position control to separate them and you will see. One is blue and the other is yellow and they are exactly what I would expect. One going positive from zero, the other going negative from zero. perfectly synchronized, as they should be.
Use the menu to select the average value of each.

No, it's not OK. It should be fully rectified but instead I'm getting half rectification.

 

Show us the two Vs waveforms from the transformer.
But he's only seeing a half-wave, not a full-wave as he should.

Thanks A LOT !!! ... BEST ADVICE!!... I checked the output from the transformer and indeed, it was wrong, it was giving me this:

This is the voltage meassured straight out of the transformer without connecting it to anything.

I touched and wiggled the cables from the transformer and that solved that problem. The transformer is now giving me this:

And the output is now this:


Which is exactly what I was expecting for.

THANKS A LOT !!!... problem solved, need a new transformer, this one is too old I guess :')

 

Disconnect the transformer center tap from ground

But I need that connection in order to have the dual polarity ... thanks anyway, I already solved it

 

, I already solved it

So you're going to leave us hanging as to what the problem was?

 

What it looks like tome is that only half of the bridge is working. So either half of the transformer is not connected, or half of it has failed, or the connections are not as shown in the drawing. any one of those will cause what we see.
My guess is a really dumb goof that the TS does not want to talk about. I already made one of those in this thread. It is only HALF of the circuit working perfectly.
And if wiggling the wires solved the problem then there needs to be a repair done wherever that poor connection was. That may be a broken wire or a poor solder connection, either one is repairable with a bit of effort.

 

The TS described the problem and fix in post #11

 

The TS described the problem and fix in post #11

Yes, I missed that post.

 

"Touch and wiggle" is not a repair of the problem, it is an approved diagnostic action, though. But the waveform displayed in an earlier post pointed at an actual problem. So at the very least a closer examination is in order.
It might have been a poor connection in a temporary setup, or it might be a damaged connection at the transformer.
Magic fixes done by trapping the hardware are not actual repairs.

 

No, but at the end of post #11 the TS stated he needed a new transformer.
And just for the record my post about removing the center tap of the xformer from ground was for diagnostics.

 

Are we sure the conclusion the OP state is 100% sure? Old by itself is not a reason for failure.

 

Are we sure the conclusion the OP state is 100% sure? Old by itself is not a reason for failure.

Certainly correct about "old" I have one useful power supply that I built using a transformer that was part of an old TV that I lbought for fifty cents back in 1956. The transformer has outlived a couple of 5U4 rectifier tubes as well as a few filter capacitors. Also another rectifier type low voltage power supply 0 to 13 volts, built in 1969 using a heater power transformer from a scrapped "hot chassis" TV set.
what damages transformers is overloading, moisture, and physical abuse. also mice and rats. Otherwise, except for the junk imported door bell transformers, they last a very long lifetime.