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| Wiring the Power Transformer | +5 Volt Power Supply | +12 Volt Power Supply | -12 Volt Power Supply | Op Amp Control of the Power Supplies |

Wiring the Power Transformer


Electronic circuits, whether analog or digital, all require power in order to operate. There are many possible sources of electrical power, but for long term use in the home or office, the most consistent source available is the ac power wired throughout the building. If we can use this power source, we can take our experimental circuitry into any room, pack it away to bring it back out later, or even move to another building, and still expect the power to be available when we need it.

There are, however, a few factors that we must take into account. The ac power line, as its name implies, provides power as an alternating current at a fixed frequency, of either 50Hz (in most of Europe) or 60Hz (in North America). The ac line voltage available is nominally 120 volts (North America) or 240 volts (Europe). We need to take our line voltage and somehow convert it to a steady dc (direct current) value. We also need to reduce the voltage level to something suitable for our experimental circuits. This would be +5 volts dc for most logic circuits, -5 volts for some circuits (such as standard ECL ICs, which are actually designed to use -5.2 volts), and ±12 to 15 volts for many analog circuits. Clearly, we need to somehow convert the available line voltage to something more practical to meet our needs.

The first step is to reduce the voltage level to something much closer to our requirements. We could use resistors, perhaps as a voltage divider, to reduce the line voltage to a more suitable level. However, this method would be very wasteful of power, and would require that the resistors be able to dissipate all of the wasted power as heat. There must be a better way!

The better way is to use a power transformer. A transformer is an electrical component that is capable of converting an ac voltage and current to a different voltage and current, at very high efficiency. With modern transformers, approximately 98% of the power applied to the input, or primary side of the transformer is available at the output, or secondary side. The voltage level can be stepped up or down, thus allowing us to obtain a reduced ac voltage without wasting a lot of energy in the process. On this page, we'll obtain and wire our power transformer for use with later pages. Once this is done, the next set of pages will cover the construction of the electronic circuitry required to convert the ac output from the transformer into a smooth, regulated dc voltage suitable for use in the various experiments.

Parts List

To prepare the power transformer for your power supplies, you will need the following components:

You will also need your longnose pliers, wire stripper, and diagonal cutter to prepare various lengths of hookup wire.

Preparing Jumpers

One thing you'll need to do constantly for any experiments on a breadboard socket is to construct and install jumper wires. You'll need a number of different lengths, of course, but two common lengths will be 0.3" and 0.5". These lengths match the spacing between the main component area in the middle and the bus strips along the top and bottom of the large breadboard socket.

The traditional way to create a jumper is to cut a piece of insulated wire from the roll or bundle, and then remove ¼" of insulation from each end. This works fine for longer jumpers (2" or more), but is a problem when you try to remove insulation from the end of a 1" length of wire. You're almost guaranteed to pull off all of the insulation.

A looped jumper wire.

One answer is to make the jumper a bit longer and bend it as shown to the right. The added length is enough to let you hold the body of the jumper firmly while removing the ¼" of insulation at each end. This has the added advantage that it is easy to insert and remove the jumper from the breadboard socket, or to move it from place to place during an experiment. Another advantage is that is can be positioned to avoid interference with other components on the breadboard socket. It also has the downside that the breadboard socket can get filled up with loops of jumper wire all over the place, making it more confusing in some cases.

A minimum-size, squared-off jumper wire.

Since the power supplies will be on your breadboard socket for some time, it makes sense to build them as neatly and compactly as possible. Therefore, we'll make the jumpers as shown to the right: as short as possible and with ends bent at right angles to just fit where they need to go, wherever we can. We will use looped jumpers only where necessary.

To easily make jumpers this way, start by removing about 4 to 5 inches of insulation from the end of a spool of hookup wire with the appropriate color insulation. Throw the removed insulation away. Then, bend ¼" of wire at the end into a right angle.

Now, use your wire stripper to separate the required length of insulation from the main body still on the wire. As a rule of thumb, make this length to the nearest 1/16" that is shorter than the desired length of the jumper. For a 0.3" jumper, for example, you can cut ¼" (0.25") of insulation from the current end and slide this short length of insulation up to the bend. For a 0.5" jumper, make the insulation 7/16" long. This makes it easy to use a standard ruler, marked in sixteenths of an inch, to measure the required length of insulation.

Next, bend the wire again at the end of the cut length of insulation. Finally, cut the wire ¼" from the second bend. This will leave you with a jumper that will fit precisely into place, and will sit snugly on the surface of the breadboard socket.

Constructing the Circuit

Note: Due to a limitation in some versions of Internet Explorer 5, we cannot put too many assembly steps on a single page. To ensure that no problems occur as a result, no experimental page will have more than 30 steps in the assembly procedure. This may require breaking an experiment up into smaller parts, and some experiments will build on circuitry left over from prior experiments.

Most of the assembly you will do here will involve the transformer and its connecting wires. However, you will install a pair of jumpers on your breadboard socket to allow additional connections to the transformer secondary winding.

Circuit Assembly

Start assembly procedure

Starting the Assembly

Preparing your power transformer for use is direct and straight-forward, but must still be undertaken with care. For your power supplies to operate correctly, and for your own safety, it is essential that you follow each step accurately.

Click on the `Start' button below to begin.

Examine the Transformer

Before doing anything else, look over your power transformer. It consists basically of a few coils of wire wrapped around the center of an iron core. The core itself is built up of layers of iron plates held together with lacquer and an outer shell. Some wires emerge from one side of the outer shell, as shown to the right.

One side has three wires emerging from it; most likely two are of one color and the third is another color. In our Radio Shack example, two wires are yellow and the third is black. These three wires are the connections to the secondary winding, with the black wire denoting the centertap connection.

The two wires on the other side are the connection to the primary winding. This is the side that will be connected to your ac power.

Click on the image of the transformer to the right to continue.

Yellow Secondary Wire

Cut a 6" length of yellow hookup wire and remove ¼" of insulation from each end. Also trim the exposed wire of one of the yellow secondary leads to ¼" to match. Insert the transformer lead and one end of the yellow wire you just prepared into a wire nut, and twist the wire nut as if you were tightening a screw. Tighten up the wire nut as far as it will go.

Click on the image of the transformer to the right to continue.

Yellow Secondary Wire

Cut a second 6" length of yellow hookup wire and remove ¼" of insulation from each end. Trim the other yellow secondary transformer lead as before, and use a second wire nut to connect the wire you prepared to the transformer lead.

Click on the image of the transformer to the right to continue.

Black Secondary Wire

In exactly the same way as before, cut and prepare a 6" length of black hookup wire, and use a wire nut to connect it to the remaining transformer secondary lead.

Click on the image of the transformer to the right to continue.

Wire the Transformer Primary

Locate your line cord and, if necessary, remove ¼" of insulation from the free end of each wire. If more then ¼" of bare wire is exposed already, trim the exposed ends to ¼". Also trim the exposed primary wires from the transformer to ¼".

Now, use two wire nuts to connect the wires of the line cord to the transformer primary leads as shown to the right. It doesn't matter which wire of the line cord goes to which transformer lead.

Click on the image of the transformer to the right to continue.

Assembly Complete

This completes the wiring of your power transformer. Check your work carefully, and then scroll down to the next part of this page.

Restart assembly procedure


The power transformer is the start of a set of four power supplies that you can build to provide ±15 volts for experiments with analog circuits and ±5 volts for experiments with both digital and analog circuits. You will find that you can build and explore a wide range of experimental circuits powered by these supplies.

Your wiring of the transformer primary winding does not include either a power switch or any means of current overload protection. It is for this reason that we specified the use of a power distribution strip that would provide both of these functions. If at any time you should wish to build or obtain a more permanent experimental setup, you should make sure that your system includes both a power switch and an appropriate fuse or circuit breaker to protect your house circuitry against accidental overloads and wiring errors.

In addition, we did not specify a third, grounding wire for the line cord. This is acceptable almost everywhere because this circuit is "double-insulated." That is, the primary side of the transformer is electrically insulated and isolated from all of the secondary circuitry. As a result, there is no direct electrical connection between your experimental circuitry and your house wiring. However, you should check with your local electrical codes. In those few areas that may require grounding in spite of the double-insulated arrangement, connect the grounding wire to the transformer shell. This will ensure that even if the transformer primary should somehow become short-circuited to the transformer shell, it will merely pop the circuit breaker or blow a fuse; it will not cause any major problems or disasters.

Prev: Getting Started Next: Constructing the +5 Volt Supply

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