# noob question on centre tap transformers

#### noviceamateur

Joined Sep 27, 2015
4
what happens if you use four diodes to rectify voltage from a centre tap transformer

#### tracecom

Joined Apr 16, 2010
3,944
It depends on how you wire them. If you arrange them in a full wave bridge and don't use the center tap, you will rectify the total voltage. If you arrange them in a full wave bridge and use only the center tap and one other, you will rectify half the voltage, but get only half the current. And there is another way to use diodes to get half the voltage and all the current.

#### noviceamateur

Joined Sep 27, 2015
4
It depends on how you wire them. If you arrange them in a full wave bridge and don't use the center tap, you will rectify the total voltage. If you arrange them in a full wave bridge and use only the center tap and one other, you will rectify half the voltage, but get only half the current. And there is another way to use diodes to get half the voltage and all the current.
thank you!

#### crutschow

Joined Mar 14, 2008
32,848
There is an another interesting option.
If you connect the outer connections to a bridge rectifier with one side of the bridge grounded then, if you place a capacitor filter at the center-tap, you will have roughly half the DC voltage available from the center-tap output to ground as compared to the bridge output.
That can be handy for example, if you want to build a power supply with two different output voltages, such as 5v, along with a higher voltage, such as 10-12V. This minimizes dissipation in the 5V and 10-12V linear regulators and reduces the transformer requirements.

This works because the outer windings are acting as a full-wave bridge while the center-tap is acting as a full-wave rectifier (using the two diodes to common).
The two bridge rectifiers to common are, in effect, doing double duty.
Note that the total transformer and diode ratings must be observed for the sum of the currents from the two outputs.

An LTspice simulation of that is shown below.
Note the increased current in the ground diode of the bridge (ID4) due to it having to carry the sum of the two output currents.

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#### noviceamateur

Joined Sep 27, 2015
4
There is an another interesting option.
If you connect the outer connections to a bridge rectifier with one side of the bridge grounded then, if you place a capacitor filter at the center-tap, you will have roughly half the DC voltage available from the center-tap output to ground as compared to the bridge output.
That can be handy for example, if you want to build a power supply with two different output voltages, such as 5v, along with a higher voltage, such as 10-12V. This minimizes dissipation in the 5V and 10-12V linear regulators and reduces the transformer requirements.

This works because the outer windings are acting as a full-wave bridge but the center-tap is acting as a full-wave rectifier (using the two diodes to common).
The two bridge rectifiers to common are, in effect, doing double duty.
Note that the total transformer and diode ratings must be observed for the sum of the currents from the two outputs.

An LTspice simulation of that is shown below.
Note the increased current in the ground diode of the bridge (ID4) due to it having to carry the sum of the two output currents.

View attachment 109787

Thank you!