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AC rectification

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kal.a

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I recently realized that I don't really understand rectification of AC voltage, or is it current?

So an AC sine wave is basically alternating in amplitude between positive and negative as referenced to Zer0 volts. It's either positive or negative but never both at the same time. But in full bridge rectification it is explained that one side is positive while the other is negative at the same time then they switch polarity with positive being negative and negative being positive and we end up having one side positive at every half cycle. That doesn't make sense to me because a sine wave has only on "hump" at a time, either positive or negative.

Thanks
Kal
 

MaxHeadRoom78

Well-Known Member
Think of a bridge as steering diodes that direct each negative going and positive going pulse to a particular output path so the polarity result is constant for each.
Max.
 

kal.a

Member
Thanks Max.

I may be confusing rectifying AC from transformer VS. AC from a power line. Is there a difference?
Let's say I take 120VAC from a wall outlet and put it through a one-to-one transformer. The incoming 120VAC is live and neutral meaning the negative and positive portions of the wave are on one wire only and the other is a return path. Does the 120VAC coming out of the transformer alternates positive and negative between the two leads?

So hypothetically speaking, to rectify the 120VAC from the wall outlet a full wave rectifier would not work and I would have to use half wave rectifier. Is that correct?
 

Tony Stewart

Well-Known Member
Most Helpful Member
When an AC is connect to a full diode bridge and the bridge outputs to +/- to a load, the V/Z volts/load impedance determines the current. The bridge will alternate like a 2P2T switch with only 2 diodes on at a time out of 4, If the load is grounded and the AC swings +/- around neutral (Neutral) gets grounded at distribution transformer or sometimes at service entrance underground plumbing, then a rectified DC will result.
Diodes only rectify voltage difference between source and load, so if load is parallel RC , current only conducts before the peak voltage for a very short duration depending on % ripple voltage.
 

MikeMl

Well-Known Member
Most Helpful Member
Look at this sim. Your confusion comes from where the ground symbol is placed. To get a "full-wave rectified" voltage with a DC component across the Load, the ground (0V reference) is placed at the bottom end of the load resistance. That requires that the AC voltage source (transformer winding) be "floating", i.e. must be isolated from Gnd. This allows the diodes to steer the current from the source so that it always flows only in one direction (top to bottom) in the load resistance.

Note that the voltages at the V(1) and V(2) go negative by one diode drop. This is because neither node 1 or node 2 are connected to Gnd.

This is why connecting rectifiers directly to the AC line should never be done by newbies...

rectr.png
 
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