About electronics: Power Supply for Logic Circuits

Thursday, September 13, 2007

Power Supply for Logic Circuits

Introduction

In order to continuously power modern experimental logic circuits, you'll need a power supply capable of delivering a regulated +5 volt output with a current-handling capacity sufficient to power any experimental circuit you may wish to build and test. Fortunately, such a power supply is easy to build with inexpensive parts, which in turn may be readily purchased at an electronics parts store such as Radio Shack.

We will not go into experimental demonstrations in constructing this power supply, since you probably want to just build it and get on with your experiments. Such experiments will be reserved for a set of pages on analog electronic circuits, which will be designed for the purpose.

If you happen to have a spare power supply from an a typical PC, you might be tempted to try to use it instead of building one. Unfortunately, that's not a practical option. These power supplies are rated for 20 amps or more from the +5 volt source, and require a significant current drain (3 amps or more) to maintain proper regulation. Your breadboarding system won't draw enough current to allow the PC power supply to work reliably.

You can build a perfectly good, serviceable logic power supply on approximately the last half inch at one end of a large breadboard socket. Or, if you prefer, you can build the same circuit on a separate board and then run the power connections over to your breadboard socket for use with your experimental circuits. The layout on this page is for building the power supply on the left end of the socket.

Schematic Diagram

Schematic of the +5 volt logic power supply.

Since every experiment involves a particular circuit, the schematic diagram of that circuit will be included and explained. In the case of the logic power supply, most of the important circuitry is contained within the 7805 IC, which is a standard three-terminal voltage regulator.

For the most part, this schematic is here for the sake of interest and completeness. However, one point needs to be addressed at once: the transformer primary is shown connected only to the line cord and plug. This is because this circuit is to be constructed on your breadboard socket, for experimental use only. We call for the use of a power strip having both a switch and a circuit breaker, to provide the appropriate protection against miswires and short circuits.

If at any time you should decide to build this power supply or any other as a separate, stand-alone circuit, be sure to include a power switch and an appropriate fuse or circuit breaker in the hot side of the primary circuit. This power supply is double-insulated — there is no electrical connection between the primary and secondary sides of the transformer — so most jurisdictions do not require a grounding plug. However, you should check your local electrical codes in such matters.

Parts List

In order to construct the logic power supply you will need the following parts and components. Most components listed are available from Radio Shack in the US; you folks in other parts of the world will have to locate your own parts, I'm afraid:

1 Power strip. Commercial power strip with on/off switch and circuit breaker for current overload protection.
1 line cord. Appropriate for your household power.
1 power transformer. Secondary rating 25.2 volts AC, center-tapped, at 1.5 Amps or more. May also be designated 12.6-0-12.6 volts AC. Primary rating must match your household line voltage: 120 volts in the US, 240 volts in some other parts of the world. This transformer will be used for analog power supplies as well as the digital supplies.
5 small wire nuts. If you can't find these at Radio Shack, check a local hardware store such as Home Depot or Lowes.
1 470 resistor (color code yellow-violet-brown).
1 .01µf disk ceramic capacitor.
1 10µf, 35 volt electrolytic capacitor. Select the one with radial leads, having both leads projecting from the same end of the component case.
1 1000µf, 35 volt electrolytic capacitor. Select the one with axial leads, having one lead projecting from each end of the component case.
2 silicon rectifier diodes. Can be from a pack of 25 rectifier diodes available from Radio Shack. Minimum rating 1 Amp forward current, 50 volts.
1 7805 +5 volt voltage regulator IC.
1 green LED.
Hookup wire in assorted colors. 22- or 24-gauge solid copper wire. Can be leftover telephone wire, although more colors would still be a good idea. For this circuit, you'll use red, black, and yellow.

Tools Required

To construct the power supply on your breadboard socket, you'll need a pair of diagonal cutters, longnose pliers, and a basic wire stripper. If you're practiced at it, you can use the diagonal cutters in place of the wire stripper.

You'll also need your voltmeter to verify the correct operation of the completed power supply.

Preparing Jumper Wires

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 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.

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. 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.

Since the power supply will be on your breadboard socket for some time, it makes sense to build it 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.

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 this away. Then, bend ¼" of wire at the end into a right angle.

Now, use the 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.

Other Components

Other components are handled similarly to short jumpers. However, for components such as resistors, capacitors, and diodes, it is important not to bend the leads of the component directly at the body of the component itself. That could break the casing of the component or even tear one of the leads loose, thus destroying that component's usefulness. Diodes require slightly different treatment. Rectifier diodes may dissipate a significant amount of heat, while signal diodes often have glass bodies that require gentle handling. Therefore, diode leads should be cut to ½" rather than ¼", to space them above the surface of the breadboard socket. Transistor leads are generally short enough that they need not be clipped after forming.

Use the following figures as a guide to forming small jumpers and component leads to fit readily on your breadboard socket. For each component, first bend the component leads to the correct spacing without bending them too close to the component body; then clip the leads to the length indicated.