Vol. DIY Electronics Projects

Chapter 4 AC Circuit Projects

AC Lab - Using a Transformer to Build a 12 VAC Power Supply

In this hands-on AC electronics experiment, build a 12 V alternating current (AC) power supply using a step-down transformer and learn about transformer operation and the purpose of tapped windings.

Project Overview

In this first AC project, you will construct the simple circuit illustrated in Figure 1.

AC step-down transformer circuit schematic diagram.

Figure 1. AC step-down transformer circuit schematic diagram.

This circuit uses a step-down transformer to convert 120 VAC wall voltage to a 12 VAC output with a center tap that can then provide 6 VAC outputs. This is an excellent first project for the AC Lab series, as the completed circuit can be used in other projects.

Parts and Materials

Power transformer, 120 VAC step-down to 12 VAC, with the center-tapped secondary winding
Terminal strip with at least three terminals
Household wall-socket power plug and cord
Line cord switch
Box (optional)
Fuse and fuse holder (optional)

If you decide to equip your power supply with a fuse, be sure to get a slow-acting or slow-blow fuse. Transformers may draw high surge currents when initially connected to an AC source, and these transient currents will blow a fast-acting fuse. Determine the proper current rating of the fuse by dividing the transformer’s VA rating by 120 V; in other words, calculate the full allowable primary winding current and size the fuse accordingly.

Learning Objectives

To determine the transformer voltage step-down behavior
To determine the purpose of tapped windings
To illustrate safe wiring techniques for power cords

Instructions

Warning! This project involves the use of dangerous voltages. You must make sure all high-voltage (120 V household power) conductors are safely insulated from accidental contact. No bare wires should be seen anywhere on the primary (120 V) side of the transformer circuit, illustrated in Figures 1 and 2.

AC step-down transformer circuit implementation

Figure 2. AC step-down transformer circuit implementation.

Step 1: Build the circuit as shown in Figures 1 and 2. Be sure to solder all wire connections so that they’re secure, and use real electrical tape (not duct tape, scotch tape, packing tape, or any other kind!) to insulate your soldered connections.

Step 2 (Optional): If you wish to enclose the transformer inside of a box, you may use an electrical junction box obtained from a hardware store or electrical supply house. If the enclosure used is metal rather than plastic, a three-prong plug should be used, with the ground prong (the longest one on the plug) connected directly to the metal case for maximum safety.

Step 3: Before plugging the plug into a wall socket, do a safety check with an ohmmeter. With the line switch in the on position, measure resistance between either plug prong and the transformer case. There should be infinite (maximum) resistance. If the meter registers continuity (some resistance value less than infinity), then you have a short between one of the power conductors and the case, which is dangerous!

Step 4: Next, check the transformer's primary windings themselves for continuity. With the line switch in the on position, there should be a small amount of resistance between the two plug prongs.

Step 5: Now check the transformer's primary windings with the switch turned off. The resistance indication should increase to infinity (open circuit—no continuity).

Step 6: Measure the resistance between pairs of wires on the secondary side. These secondary windings should register much lower resistance than the primary. Why is this?

Step 7: Plug the cord into a wall socket and turn the switch on. You should be able to measure AC voltage at the secondary side of the transformer, between pairs of terminals. Between two of these terminals, you should measure about 12 V. Between either of these two terminals and the third terminal, you should measure half that. This third wire is the center-tap wire of the secondary winding.

Step 8: It would be advisable to keep this project assembled for use in powering other experiments shown in this online textbook. From here on, I will designate this low-voltage AC power supply using the illustration shown in Figure 3.

Low-voltage (12 V) AC power supply with a center tap.

Figure 3. Low-voltage (12 V) AC power supply with a center tap.

SPICE Simulation of the AC Step-down Transformer

You can simulate, via SPICE, the AC step-down transformer with the circuit illustrated in Figure 4 and the netlist below.

AC step-down transformer circuit with node numbers and extra devices for SPICE simulation

Figure 4. AC step-down transformer circuit with node numbers and extra devices for SPICE simulation.

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

AC Step-down Transformer with Center-tap Secondary
v1 1 0 ac 120 sin 
rbogus1 1 2 1e-3 
l1 2 0 10 
l2 5 4 0.025 
l3 4 3 0.025 
k1 l1 l2 0.999 
k2 l2 l3 0.999 
k3 l1 l3 0.999 
rbogus2 3 0 1e12 
rload1 5 4 1k 
rload2 4 3 1k 
* Sets up AC analysis at 60 Hz: 
.ac lin 1 60 60 
* Prints primary voltage between nodes 2 and 0: 
.print ac v(2,0) 
* Prints (top) secondary voltage between nodes 5 and 4: 
.print ac v(5,4) 
* Prints (bottom) secondary voltage between nodes 4 and 3: 
.print ac v(4,3) 
* Prints (total) secondary voltage between nodes 5 and 3: 
.print ac v(5,3) 
.end

Two extra resistors have been added to the circuit to assist with the simulation. R_bogus1 is a small resistance in series with the primary transformer winding, L₁. R_bogus2 is a high-value resistance to provide a pseudo-ground connection between the primary and secondary sides of the transformer.