In our previous discussion about rectifiers, we discussed full-wave rectifiers and how
they are different from half-wave rectifiers.
We also discussed one type of full-wave rectifier, the bridge full-wave rectifier and learned
how it operates to rectify both positive and negative half-cycles of the AC input.
In this tutorial, we will discuss the second type of full-wave rectifier, the center-tapped
full-wave rectifier, and review how it operates.
The reason why this type of full-wave rectifier is called a center-tapped rectifier is because
it uses a center-tapped transformer.
If you noticed in the schematic diagrams that we showed in the half-wave and bridge full-wave
rectifier tutorials, you can see that the transformer has only a single winding on the
secondary side.
So if we want to create a power supply that uses a half-wave or bridge full-wave rectifier,
we just use transformers that only have a single winding on the secondary side.
Transformers may have different numbers of windings on their primary and secondary sides.
Some transformers only have a single winding on their primary and secondary sides but most
of the transformers I’ve encountered have multiple windings on both sides.
For example, the Triad Magnetics VPS24-5400 that we used in other tutorials, it uses dual
winding on both sides and can be configured to have a series or parallel configuration.
If you’re interested to learn how to wire transformers, we have done a tutorial on that,
you can check it out on our website CircuitBread.com
For a center-tapped transformer, this type of transformer is designed to produce two
secondary voltages with a common connection at the center of the secondary winding.
So the two voltages are the same and they can also provide the same power.
From one end to the center tap, the voltage is half of the total secondary voltage, which
you can see by inspection.
In this image, you can see how a center-tapped transformer is connected to a center-tapped
full-wave rectifier.
One of the differences between center-tapped and bridge rectifier is the numbers of diodes
used to rectify both positive and negative half-cycles of the AC input.
A bridge rectifier uses 4 diodes while a center-tapped rectifier uses only 2 diodes.
So let’s discuss how a center-tapped full-wave rectifier operates.
You can see the polarities on the secondary winding of a center-tapped transformer during
the positive half-cycle of the AC input.
In this case, D1 is forward-biased while D2 is reverse-biased.
So the current flows through D1 and enters the load, then returns to the transformer
through the center-tap connection.
When the AC input switches to the negative half-cycle, the polarities on the secondary
winding of the center-tapped transformer are reversed.
So this time D1 is reverse-biased and D2 is forward-biased.
The current flows through D2 and enters into the load in the same direction as it entered
during the positive half-cycle of the AC input, then returns to the transformer through the
center-tap connection.
Since the current entered the load in the same direction in both positive and negative
half cycles, the voltage across the load is a full-wave rectified DC voltage.
But it’s still a pulsating DC which needs to be filtered before using as a DC power
supply.
The same with the bridge full-wave rectifier, the average value of a full-wave rectified
voltage can be determined by the equation V average equals two times the peak voltage
divided by pi.
The peak output voltage of a center-tapped full-wave rectifier is only equal to the half
of the transformer secondary voltage minus the forward voltage of the diode.
If you’re going to check the current flow during the positive and negative half-cycle
of the AC input, you will notice the load only sees half of the transformer secondary
voltage and not the total secondary voltage.
So to determine the peak output voltage across the load, check what is half of the transformer
secondary voltage and use the equation V peak out is equal to V peak secondary over two,
minus .7 volts.
Now for the peak inverse voltage (PIV) of the diodes in a center-tapped full-wave rectifier,
let’s assume that the AC input is in the positive half-cycle.
So D1 conducts with a 0.7V diode drop while D2 is reverse-biased.
By using Kirchhoff’s voltage law we get that the PIV is equal to the Voltage peak
on the secondary, minus the .7V diode voltage drop.
So the PIV that D2 (or D1 during negative half-cycle) must withstand while in reverse-bias
condition is equal to the secondary voltage of the center-tapped transformer minus one
diode drop.
In this tutorial, we’ve discussed center-tapped full-wave rectifiers and their operation.
We’ve also discussed briefly how a center-tapped transformer is designed.
I hope you’ve found this tutorial interesting or helpful.
If you have any questions or comments, leave it in the comments below or go to CircuitBread.com.
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