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Constructing the +5 Volt Supply

Introduction

The +5 volt supply is useful for both analog and digital circuits. DTL, TTL, and CMOS ICs will all operate nicely from a +5 volt supply. In addition, the +5 volt supply is useful for circuits that use both analog and digital signals in various ways.

More importantly for our purposes, the +5 volt supply will be used as the primary reference for regulating all of the other power supplies that we will build. We can do this very easily if we use operational amplifiers as the controlling elements in the power supply circuits. We'll see how this works after completing the basic +5 volt supply.



Schematic Diagram

The +5 volt power supply is based on the commercial 7805 voltage regulator IC. This IC contains all the circuitry needed to accept any input voltage from 8 to 18 volts and produce a steady +5 volt output, accurate to within 5% (0.25 volt). It also contains current-limiting circuitry and thermal overload protection, so that the IC won't be damaged in case of excessive load current; it will reduce its output voltage instead.

The 1000µf capacitor serves as a "reservoir" which maintains a reasonable input voltage to the 7805 throughout the entire cycle of the ac line voltage. The two rectifier diodes keep recharging the reservoir capacitor on alternate half-cycles of the line voltage, and the capacitor is quite capable of sustaining any reasonable load in between charging pulses.

The 10µf and .01µf capacitors serve to help keep the power supply output voltage constant when load conditions change. The electrolytic capacitor smooths out any long-term or low frequency variations. However, at high frequencies this capacitor is not very efficient. Therefore, the .01µf is included to bypass high-frequency changes, such as digital IC switching effects, to ground.

The LED and its series resistor serve as a pilot light to indicate when the power supply is on. I like to use a miniature LED here, so it will serve that function without being obtrusive or distracting while I'm performing an experiment. I also use this LED to tell me when the reservoir capacitor is completely discharged after power is turned off. Then I know it's safe to remove or install components for the next experiment.



Parts List

To construct and test the +5 volt power supply on your breadboard, you will need the following parts (all available from Radio Shack):

You will also need your longnose pliers, diagonal cutter, wire stripper, and voltmeter.



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 botton 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.

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 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, 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 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.



Constructing the Circuit

The +5 volt power supply will go on the left end of your breadboard socket. There should not be any components mounted here when you begin; the analog experiments will be mounted on a separate breadboard socket from the digital experiments, until you have constructed or obtained a comprehensive breadboarding system.

As you install each part, an arrow will point to it on the assembly diagram below, and, where necessary, a pictorial will appear to show you how to form the component leads. To help avoid confusion between the colors grey and silver, all component leads will be shown in gold color, even though most of them will actually be silver colored. This merely means that the component leads are solder-coated rather than gold plated; either will work equally well here.



Circuit Assembly

Start assembly procedure




















Starting the Assembly

Make sure that the left end of your breadboard socket is clear of all components, jumpers, etc. You will build the +5 volt power supply in this space.

Click on the `Start' button below to begin. If at any time you wish to start this procedure over again from the beginning, click the `Restart' button that will replace the `Start' button.

0.3" Black Jumper

Prepare a 0.3" black jumper as described in the section on Preparing Jumpers, and install it in the location shown in the assembly diagram to the right.

Click on the image of the jumper you just installed to continue.

0.3" Black Jumper

Prepare a second 0.3" black jumper using the same method. Install this jumper in the location indicated in the assembly diagram.

Again, click on the image of the jumper you just installed to continue.

0.3" Black Jumper

Prepare another 0.3" black jumper and install it in the location indicated in the assembly diagram to the right.

Again, click on the image of the jumper you just installed to continue.

0.3" Black Jumper

Prepare another 0.3" black jumper and install it in the location shown to the right. Note that this jumper connects two columns of contacts where you have already installed jumpers in previous steps.

As before, click on the image of the jumper you just installed to continue.

0.3" Black Jumper

Prepare a 0.3" black jumper and install it on the breadboard socket as shown to the right.

Again, click on the image of the jumper you just installed to continue.

0.3" Black Jumper

Prepare one more 0.3" black jumper and install it as shown to the right.

Once more, click on the image of the jumper you just installed to continue.

0.3" Yellow Jumper

Prepare a 0.3" yellow jumper and install it on the breadboard socket as shown to the right.

Click on the image of the jumper you just installed to continue.

0.3" Yellow Jumper

Prepare a second 0.3" yellow jumper and install it as shown in the assembly diagram.

Again, click on the image of the jumper you just installed to continue.

0.3" Red Jumper

Prepare a 0.3" red jumper and install it as shown in the assembly diagram.

As before, click on the image of the jumper you just installed to continue.

0.1" Bare Jumper

Cut a 5/8" length of bare hookup wire and bend it in half as shown in the pictorial. Insert this jumper into your breadboard socket to connect adjacent contact columns as shown in the assembly diagram.

Click on the image of the jumper you just installed to continue.

Rectifier Diode

Locate a 1 Ampere rectifier diode and form its leads to a spacing of 0.3". Clip the formed leads to a length of ½" to raise the diode off the surface of the breadboard socket to allow air circulation for cooling, and install the diode in the location shown in the assembly diagram. Note that the diode will be installed over one of the bare jumpers you installed earlier, but will not touch it or make any electrical connection with it.

Click on the image of the diode you just installed to continue.

Rectifier Diode

Locate a second 1 Ampere rectifier diode and form its leads to a spacing of 0.4". Clip the formed leads to a length of ½" and install this diode in the location shown to the right.

Again, click on the image of the diode you just installed to continue.

1K, ¼-Watt Resistor

Locate a 1K, ¼-watt resistor and form its leads to a spacing of 0.4". Clip the formed leads to a length of ¼" as shown in the pictorial, and then install this resistor on your breadboard socket as shown to the right.

Note: We could require that the resistor leads be formed to a spacing of 0.3". However, there is no need to do so, and 0.4" spacing avoids the need to bend the leads around the ends of the resistor.

Click on the image of the resistor you just installed to continue.

Miniature Red LED

This LED serves as a pilot light to indicate that the +5 volt supply is active. I prefer a miniature (2mm) device here because it is unobtrusive but remains visible. The leads are already formed to a spacing of 0.1", so simply clip them so ¼" of lead length can be inserted into the breadboard socket.

As you install this LED in the location indicated, make sure that the flat side is oriented to the right, so it will be connected to the black grounding jumper.

Click on the image of the LED you just installed to continue.

.01µf Disc Capacitor

This capacitor can have a higher value than .01µf. It may be marked ".01" or "103," which indicates 10,000pf. In the latter case, your capacitor must have a third digit of 3, although the other two can be higher than 10.

In order to bridge the channel in the breadboard socket, this capacitor must have its leads formed to a spacing of 0.3". Therefore, shape these leads if necessary to achieve this spacing. Then, if the capacitor leads are longer than 3/8", clip them to 3/8" before installing this capacitor in the indicated location.

Click on the image of the capacitor you just installed to continue.

10µf Electrolytic Capacitor

Locate a 10µf, 35 volt electrolytic capacitor with radial leads (both leads emerging from the same end of the package). Note that one lead is clearly identified as the negative lead. This polarity must be observed to avoid overheating and eventual damage.

Clip both leads to a length of ¼" and install this capacitor in the location indicated, with the negative lead facing to the right as shown.

As before, click on the image of the capacitor you just installed to continue.

7805 Voltage Regulator IC

Locate a 7805 voltage regulator IC and install it in the location indicated to the right. Make sure you have oriented this IC correctly. Pin 1 should connect to the two rectifier diodes, and pin 3 should connect to the bottom end of the red jumper.

Click on the image of the IC you just installed to continue.

1000µf Electrolytic Capacitor

Locate a 1000µf, 35 volt electrolytic capacitor with axial leads (one lead at each end of the package). This capacitor also has its negative lead clearly identified.

Use your wire strippers to remove 1¼" of red insulation from your spool of hookup wire. Slip this length of insulation over the positive capacitor lead to help prevent inadvertent contact with other wires or components. Then, install this capacitor as shown to the right, with the insulated positive lead connected to the rectifier diodes and the negative lead connected to the ground bus strip.

Click on the image of the capacitor you just installed to continue.

Connect the Transformer Secondary

Connect the wires from the transformer secondary to your breadboard socket as shown so you'll be able to power up this circuit and test its operation.

Click on the image of the jumpers you just installed to continue.

Assembly Complete

This completes the construction of your +5 volt supply. Check your assembly carefully against the figure to the right, and correct any errors you might find. Then, proceed to the next part of this page to test this circuit's operation.

Restart assembly procedure


Testing the +5 Volt Supply

Set your voltmeter to measure DC voltages up to 20 volts, and connect the black (Common or Ground) lead to the negative lead of the 1000µf capacitor. Connect the red lead to the upper end of the 0.3" red jumper wire. Turn on your voltmeter, then turn on power to your transformer and power supply circuit.

You should measure a steady +5 volts (+4.75 to +5.25) here, at the power supply output, and the red pilot LED should turn on. If you get these results, move your red voltmeter lead to the positive lead of the 1000µf reservoir capacitor. You should see about +17 volts here, possibly higher.

If you get the correct results, turn off your power supply and voltmeter, and skip down to the Discussion below. If your results are different, quickly note the results you did obtain; then turn power off and look through the following troubleshooting chart.

Output voltage is steady at +5, but LED remains off.

LED is reversed. Remove it and re-insert it in the opposite direction. Then try the power supply again.

Resistor is the wrong value or connected incorrectly. Make sure it is a 1K resistor (brown-black-red) and is connected fron the lower end of the red jumper to the left (anode) end of the LED. Then try the power supply again.

Output voltage rises to +5 volts, but then declines steadily.

One or both electrolytic capacitors is reversed. The reversed one will be warm or hot to the touch. If you leave power on too long, it will explode and leave a large mess to be cleaned up. Check and correct capacitor orientation, and then try the power supply again.

Output voltage is negative.

Your main rectifier diodes are installed backwards. Refer back to the assembly diagram and install them correctly.

Output voltage is incorrect.

7805 voltage regulator is installed incorrectly or is defective. Verify correct installation and replace if necessary.

Once you are sure that your power supply is working correctly in all respects, turn off power to your circuit and your voltmeter. Then move down to the concluding discussion below.



Discussion

The +5 volt power supply is based on the commercial 7805 voltage regulator IC. This simplifies the design and layout of the circuit considerably, because all of the regulating circuitry as well as current limiters and overload protection are built into the IC. As a result, little is needed in the way of support circuitry.

We do still need the external capacitors. One thing that is very difficult to achieve in ICs is a capacitor of high capacitance value. Therefore, the electrolytic capacitors must be provided to work with the IC. The disc ceramic capacitor must also be of a higher value than is readily obtainable within an IC, so it, too, must be provided externally.

The resistor and the LED pilot light are not necessary for the correct operation of the power supply. However, they do serve to indicate when power is on, and also help to discharge the 1000µf reservoir capacitor when power is turned off.

The 7805 voltage regulator IC is capable of handling load currents up to an ampere or so. However, the IC will dissipate a fair amount of heat when the load current gets this high. Without a heat sink, the IC will get hot and shut itself down at load currents above about 150 mA. If you add a heat sink for a TO-220 case (available at Radio Shack), this power supply can easily deliver an ampere or more to its load. The placement of the components was carefully selected to allow room for such a heat sink to be installed. You may have to bend the IC over a bit to allow the heat sink to remain clear of all other components and jumpers on the breadboard. The heat sink will not be required for any of the experiments and projects on these pages.

When you have finished testing the operation of your +5 volt supply, make sure power to your circuit is turned off.


Prev: Wiring the Power Transformer Next: Constructing the +12 Volt Supply

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