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Full wave bridge rectifier but doing half wave

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sassyboy

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Hi. I have been tormented by a simple full wave bridge rectifier. Simulation shows full wave from 50hz to 100hz, but on breadboard is just a half wave.

Diode used is 1N4148. I attach here a YouTube link if anyone could spare a moment to point me in the right direction.

https://youtu.be/5LBmmnxPy2I
 

You have one rectifier branch shorted through ground connection of generator and oscilloscope. Either rectifier source or load must be floating, but you have both grounded.

Possible solution: Remove the ground connection from rectifier negative output, use oscilloscope in differential (A-B) mode to measure the rectifier output.
 
Yes, it's a common mistake for newbies to connect the input and output of a bridge circuit to a common ground, which will not work.
It you look at the current path, you will see why.
 
Thanks for the reply. I see what you mean. The two grounds must be separate but since the function generator and oscilloscope are both power from mains which have the same earth, how can one avoid this. If I plug the oscilloscope to a different earth from another room so the two instruments are isolated during measurement. Would this work?

The term floating, does it mean not connecting the ground clip from the oscilloscope probe?

Not sure if my oscilloscope has differential mode, does it involve using inv to invert the negative output? Would it be better to swap the function generator with an AC power source at 50hz.
 

Not sure if my oscilloscope has differential mode, does it involve using inv to invert the negative output?
Yes.
And adjust both inputs to have identical gain.
You can check that by connecting both inputs to the same signal (such as the AC voltage) and adjusting for the minimum displayed voltage.
 

I followed advice from you guys. I got hold of a AC power supply with voltage selection from 2V to 10V rms, i.e., a transformer. However it didn't work as the transformer is plugged in via mains which means it shares the same earth as the oscilloscope. I realised I needed to plug them in with isolated earth. However I tried all of my main sockets in the room and they are all shared the same earth.

A light bulb moment when I thought of the danger of blowing up measuring equipments if classroom full of novice students wrongly ground the circuits. Therefore, science classrooms must comply to the safety standard by isolating the earth between sockets. So, I took my equipments to the science classroom and connected the wiring including the grounds hoping they are isolated. And then there it was the 100hz full wave rectified curve on the oscilloscope. I was so relieved.

Question now remains is how I could isolate earth in the original classroom? Is there a device I could buy to plug into the mains?
 

Unless you got an autotransformer, the primary and secondary of a transformer are isolated.

And I don't see how AC main's grounds can be "isolated".
By definition all AC grounds go to earth.
 

And I don't see how AC main's grounds can be "isolated".
By definition all AC grounds go to earth.

Yes, the respective statement sounds confused. I guess that the lab has some sockets supplied by a safety transformer. Such sockets have no ground terminal at all rather than an "isolated ground".
 

use two scope probes and subtract them ( Ch1-Ch2 ) to see what is going on without shorting the 0v of the ckt ( cos the sig gen has an earthed o/p and so does the scope )

no need to connect the scope grounds to anything ( as they already are connected to the sig Gen 0v via the earth wires in the mains wiring )
 

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