Vol. DIY Electronics Projects
Chapter 5 Discrete Semiconductor Circuit Projects

# Si Lab - Full-wave Center-tap Rectifier

## In this hands-on semiconductor electronics experiment, build a full-wave rectifier for AC to DC conversion and learn about center-tapped transformers.

### Project Overview

The rectifier circuit in Figure 1 is called full-wave because it makes use of both the positive and negative half-cycles of the sinusoidal AC source voltage wave in powering a DC load.

##### Figure 1. Schematic diagram of a full-wave center-tap rectifier circuit driving a DC motor.

As a result, there is less ripple voltage seen at the load than in a half-wave rectifier. It relies on the center-tapped transformer and two diodes to always provide a positive voltage to the load.

### Parts and Materials

• Low-voltage AC power supply (6 V output) with a center tap
• Two 1N4001 rectifying diodes or any of the 1N400X series of rectifying diodes
• Small hobby motor, permanent-magnet type
• Optional: Audio detector with headphones
• Optional: 0.1 µF capacitor
• One toggle switch, SPST (single-pole, single-throw)

It is essential for this experiment that the low-voltage AC power supply is equipped with a center tap. A transformer with a non-tapped secondary winding simply will not work for this circuit.

### Learning Objectives

• Design of a full-wave center-tap rectifier circuit
• How to measure ripple voltage with a voltmeter

### Instructions

Step 1: First, build the circuit illustrated in Figure 2.

##### Figure 2. Full-wave center-tap rectifier circuit driving a DC motor.

Note that our low-voltage AC power supply contains inside it the center-tapped step-down transformer that was shown in the schematic diagram of Figure 1. This transformer converts the 120 VAC wall voltage down to the 12 VAC (± 6 VAC) output by our supply.

Step 2: Use a voltmeter to measure both the DC and AC voltage delivered to the motor. The RMS (root-mean-square) value of this full-wave rectifier’s output is also greater for this circuit than for the half-wave rectifier.

You should notice the advantages of the full-wave rectifier immediately by the greater DC output voltage and lower, undesirable AC voltage fluctuations, as compared to the half-wave rectifier experiment. In addition, this full-wave rectifier design has only a single-diode voltage drop in the conduction path. This is better than the two-diode voltage drops of the full-wave bridge rectifier.

Step 3: An experimental advantage of this circuit is the ease with which it may be converted to a half-wave rectifier. Simply disconnect the short jumper wire connecting the two diodes’ cathode ends together on the terminal strip (Figure 2). Better yet, for a quick comparison between half and full-wave rectification, you can add a switch in the circuit to open and close this connection at will using the circuit of Figures 3 and 4.

##### Figure 4. Illustration of the circuit with a switch to select between full-wave and half-wave rectifier operation.

With the ability to quickly switch between half- and full-wave rectification, you may easily perform qualitative comparisons between the two different operating modes.

Step 4 (Optional): You can use the audio signal detector to listen to the ripple voltage present between the motor terminals for half-wave and full-wave rectification modes, noting both the intensity and the quality of the tone. Remember to use a coupling capacitor in series with the detector, as illustrated in Figure 5, so that it only receives the AC ripple voltage and not the DC voltage.

### SPICE Simulation of the Full-wave Center-tap Rectifier Circuit

We can simulate the full-wave center-tap rectifier, as illustrated in Figure 6, using SPICE.

##### Figure 6. SPICE circuit schematic for a full-wave center-tap rectifier circuit.

Netlist (make a text file containing the following text, verbatim):

Fullwave center-tap rectifier
v1 1 0 sin(0 8.485 60 0 0)
v2 0 3 sin(0 8.485 60 0 0)
d1 1 2 mod1
d2 3 2 mod1
.model mod1 d
.tran .5m 25m
.plot tran v(1,0) v(2,0)
.end