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basic electronic question - rectifier

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confounded

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hi i'm reading these notes and i come across something i dont understand can someone help please?
I've attached the drawing of a full wave rectifier, the notes say:

''one rectifier diode conducts during the positive half cycle, and the other diode conducts during the negative half cycle''

I do not understand this as both diodes face the same direction.
Is it that the middle wire of the secondary transformer (centre tap?) acts as the return path for both the positive and negative cycle?
 

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Roff

Well-Known Member
hi i'm reading these notes and i come across something i dont understand can someone help please?
I've attached the drawing of a full wave rectifier, the notes say:

''one rectifier diode conducts during the positive half cycle, and the other diode conducts during the negative half cycle''

I do not understand this as both diodes face the same direction.
Is it that the middle wire of the secondary transformer (centre tap?) acts as the return path for both the positive and negative cycle?
Yes. The ends of the transformer are 180 degrees out of phase with each other.
 

colin55

Well-Known Member
Get a long piece of thick wire and wrap 6 turns around a tin of fruit.
Now remove it and assume a magnetic field is passing through the middle of the coil (where the fruit was located).
If you hold one end of the wire, the other end will produce a voltage. The end you are holding is called "earth" or 0v and the other end is positive.
You can "see" the far end is positive because the wire is coming off the coil in a clockwise direction.
Now, if you pinch your fingers on the third turn, you have created an earth or 0v in the middle of the coil and if you look at the top turn you can see it will still produce a positive voltage but at only half the original voltage as only three turns are producing the voltage.
Now look at the lower output and you will see the wire is coming off the coil in an anti-clockwise direction and this means the voltage will be negative or lower than "earth" or 0v.
This means that for a part of the cycle, the voltage appearing on the winding in your diagram, will have a positive at the top - zero from the "centre tap" - and a negative at the bottom.
Now look at the two diodes.
The top diode will have a positive voltage on the left side of it and this voltage will pass through the diode.
The lower diode will have a negative voltage on its left side and the diode will not conduct. The only two wires from the transformer that will conduct (during this time) are the top and middle wires.
Now we take the instant when the magnetic flux is moving in the opposite direction.
The top wire will be negative and the lower will be positive. This time the lower diode will conduct and the centre tap will be the only other wire that will conduct current.
Do you want more detail?
 
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