5 v Power Sply

8/2/2019 5 v Power Sply

1/13

Constructing the +5 VoltSupply

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 invarious ways.

More importantly for our purposes, the +5 volt supply will be used as theprimary reference for regulating all of the other power supplies the we willbuild. 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 voltageregulator IC. This IC contains all the circuitry needed to accept any inputvoltage 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 thermaloverload protection, so that the IC won't be damaged in case of excessive load

current; it will reduce its output voltage instead.


2/13

The 1000f capacitor serves as a "reservoir" which maintains a reasonableinput voltage to the 7805 throughout the entire cycle of the ac line voltage.

The two rectifier diodes keep recharging the reservoir capacitor on alternatehalf-cycles of the line voltage, and the capacitor is quite capable of sustaining

any reasonable load in between charging pulses.

The 10f and .01f capacitors serve to help keep the power supply output

voltage constant when load conditions change. The electrolytic capacitorsmooths out any long-term or low frequency variations. However, at high

frequencies this capacitor is not very efficient. Therefore, the .01f is included

to bypass high-frequency changes, such as digital IC switching effects, toground.

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 thatfunction 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 toremove 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):

(1) 1K, -watt resistor (brown-black-red). (1) 0.01 f or larger ceramic disc capacitor. (1) 10 f, 35 volt electrolytic capacitor. (1) 1000 f, 35 volt electrolytic capacitor. (2) silicon rectifier diodes. (1) 7805 +5 volt voltage regulator IC. (1) miniature red LED. Black hookup wire. Red hookup wire. Yellow hookup wire.

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


3/13

Preparing Jumpers

One thing you'll need to do constantly for any experiments on a breadboardsocket 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 " of insulation from each end. Thisworks 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 almostguaranteed to pull off all of the insulation.

One answer is to make the jumper a bit longer and bend it as shown to theright. The added length is enough to let you hold the body of the jumper firmly

while removing the " of insulation at each end. This has the added advantagethat 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 thatis 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 moreconfusing in some cases.

Since the power supply will be on your breadboard socket for some time, itmakes 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 endsbent at right angles to just fit where they need to go, wherever we can. We willuse looped jumpers only where necessary.

To easily make jumpers this way, start by removing about 4 to 5 inches ofinsulation from the end of a spool of hookup wire with the appropriate color

insulation. Throw this away. Then, bend " 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


4/13

nearest 1/16" that is shorter than the desired length of the jumper. For a 0.3"jumper, for example, you can cut " (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 " from the second bend. This will leave you with a jumper thatwill 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; theanalog 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 formthe component leads. To help avoid confusion between the colors grey andsilver, 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 wellhere.

Circuit Assembly


5/13


6/13


7/13


8/13


9/13


10/13


11/13

Starting the Assembly

Make sure that the left end of your breadboard socket is clear of all components, jumpers, etc. You willbuild 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 fromthe beginning, click the `Restart' button that will replace the `Start' button.

Testing the +5 Volt Supply

Set your voltmeter to measure voltages up to 20 volts, and connect the

black (Common or Ground) lead to the negative lead of the 1000f 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 supplycircuit.

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 1000f reservoir

capacitor. You should see about +17 volts here, possibly higher.

If you get the correct results, turn off your power supply and voltmeter, andskip 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 followingtroubleshooting chart.

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


12/13

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 ofthe 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 willbe warm or hot to the touch. If you leave power on too long, it willexplode 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 theassembly diagram and install them correctly.

Output voltage is incorrect.

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

Once you are sure that your power supply is working correctly in allrespects, turn off power to your circuit and your voltmeter. Then move downto 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 overloadprotection 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 toachieve 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 anIC, so it, too, must be provided externally.


13/13

The resistor and the LED pilot light are not necessary for the correctoperation of the power supply. However, they do serve to indicate when power

is on, and also help to discharge the 1000f reservoir capacitor when power isturned 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 shutitself 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 carefullyselected 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.

5 v Power Sply

Documents

Transcript of 5 v Power Sply