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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 |

Op Amp Control of the Power Supplies

Introduction

The ±12 volt power supplies you constructed in the previous two projects will work for our experiments, but will introduce some inaccuracies in some cases because they aren't really precise. Therefore, if we were to use a voltage divider to obtain a 10 volt reference, we would have a hard time getting an acurate reference voltage.

In addition, we want to be able to have either ±12 or ±15 volt power supplies, and we want the positive and negative supplies to accurately mirror each other. This isn't easy with Zener diodes or IC voltage regulators because, as good as they are, they still contain internal tolerances which will allow a certain amount of error.

In this project, we will modify our power supply circuit so that it will produce highly stable and accurate output voltages of either ±12 or ±15 volts, as we choose. Then we will be able to use these supply voltages as references for our experiments as well as power for our experimental circuits, and to provide other reference voltages if we wish.



Using an op amp to regulate the +15 volt supply.

Schematic Diagram

The circuit shown to the right is basically a variation on the non-inverting op amp amplifier circuit. The key difference is that we have incorporated the power supply regulating circuitry into the op amp output, so that now the op amp will assume whatever output voltage it must to maintain the power supply output at a preset voltage. The Zener diode is required to provide a voltage offset in this case, since the op amp cannot develop a 15 volt output directly. (We have left out the rectifier diodes and external capacitors for clarity; they are still part of the final circuit.)

Because we want an output voltage of either +12 or +15 from the positive supply, we want a reference voltage that is a submultiple of both possible outputs. We could use a +1 volt reference and amplify that by either 12 or 15, but it's easier and a bit more reasonable in this case to use a +3 volt reference, and multiply that by a factor of either 4 or 5 to obtain the desired output voltage.

The gain of the amplifier is set by the ratio of the feedback resistors, R1 and R2, and is equal to (R1/R2) + 1. Thus, we can get a +15 volt output by setting R1 = 12K and R2 = 3K. These are both standard 5% resistance values and are readily available. If we add a 1K resistor in series with R2, we will have a total of 4K for this resistance, which will reduce the gain of the amplifier to 4, and hence reduce the output voltage to +12 volts. This is exactly what we wanted, and we can simply use a small DIP switch or a jumper to short out that 1K resistor any time we want the power supply to output +15 volts again.


Using an op amp to regulate the -15 volt supply.

For the -12/15 volt supply, we use a second op amp connected as a summing amplifier. The control of the power supply transistors is the same although of the opposite polarity. This time, however, we are summing the outputs of the two power supplies and forcing the sum to be zero volts. As a result, this op amp will constantly cause the -12/15 volt power supply to mirror the +12/15 volt supply. No other reference is required here.

One bonus of setting up the power supplies this way is that their outputs can even be used to power the op amps that are controlling them. This helps to ensure clean and accurate output voltages under varying conditions. Another is that even the small voltage losses caused by the 2.2Ω resistors are eliminated until the power supplies become overloaded. Placing such components inside the feedback loop helps to maintain the accuracy of the output voltage.



A 15-turn trimmer potentiometer.

The Trimmer Potentiometer

A new component that you will need for this project is a trimmer potentiometer. A potentiometer consists of a resistive element with a movable electrical contact touching it. This permits the potentiometer to serve as a continuously-variable voltage divider.

The figure to the right shows one of the many kinds of potentiometers available for a wide range of applications. This one uses a screw to slowly advance the moving contact along the resistance element. This allows accurate placement of the contact and reduces the likelihood that an accident may move the contact away from the desired position. The particular potentiometer that we will use requires 15 turns of the screw to cover the entire resistance range.

This particular type of potentiometer is typically known as a trimmer potentiometer (or trimpot for short), because it is intended to be adjusted or "trimmed" to a particular setting, and then left there to retain its setting. It will seldom need to be readjusted in normal use.



Parts List

To construct and test the op amp control circuit on your breadboard, you will need the following experimental parts:



Constructing the Circuit

You will build the op amp control circuitry just to the right of your existing ±12 volt supplies. In this case, you'll build the circuit in two stages, with a pause in the middle to make preliminary adjustments to your trimpot after you install it. Refer to the image and text below and install the parts as shown.



Circuit Assembly

Start assembly procedure




























Starting the Assembly

Make sure that the space to the right of your power supplies, as shown in the assembly diagram to the right, is clear of all components, jumpers, etc. You will install the components for this project in the open 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" Orange Jumper

Prepare a 0.3" orange jumper in the same manner as in previous projects, and install it in the location indicated to the right. This jumper will connect one of the right-hand bus strips to the +12/15 volt power supply.

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 in the location indicated to the right. This jumper will connect one of the right-hand bus strips to the +5 volt power supply.

Again, 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 in the location indicated to the right. This jumper will connect one of the right-hand bus strips to ground.

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

0.3" Blue Jumper

Prepare a 0.3" blue jumper and install it in the location indicated to the right. This jumper will connect one of the right-hand bus strips to the -12/15 volt power supply.

Once more, 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 in the location indicated to the right.

Again, 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 in the location indicated to the right.

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

0.3" Bare Jumper

Prepare a 0.3" jumper with no insulation at all, and install it in the location indicated in the assembly diagram.

The use of a bare jumper this long is unusual, but is necessary in this case to fit underneath the trimpot you will install in the next step.

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

10K, 15-Turn Trimmer Potentiometer

Locate a 10K, 15-turn trimmer potentiometer and install it so that the three connecting pins are inserted into the breadboard contact holes indicated by gold squares in the assembly diagram. This will connect the full resistance across the +5 volt supply, and connect the slider to the bare jumper you installed in the previous step. The trimpot can rest on the bare jumper if necessary; it will do no harm.

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

Adjust the Trimmer Potentiometer

Before installing any further components, turn on your power supply and set your voltmeter to measure voltages up to 10 or 20 volts. Connect the common voltmeter lead to a ground connection, and use the red lead to monitor the voltage on the bare jumper you installed earlier.

Use a thin-bladed screwdriver to adjust the trimpot output voltage to 3.00 volts. When you have made this adjustment as accurately as possible, turn off power and your voltmeter, and click the `Continue' button below to go on to the next step.

Remove Black Jumper

Remove the indicated grounding jumper and put it aside for later use.

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

Remove Black Jumper

Remove the indicated grounding jumper and put it aside for use later in this project.

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

Remove Bare jumper

Remove the bare jumper indicated to the right. Set it aside for future use.

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

0.3" Green Jumper

Prepare a 0.3" green jumper and install it in the location indicated in the assembly diagram.

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

2.7K, ¼-Watt Resistor

Locate a 2.7K, ¼ watt resistor (red-violet-red) and form its leads to a spacing of 0.4". Install this resistor in the location indicated to the right.

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

1458 Dual Op Amp IC

Locate a type 1458 (may be labelled 5558) IC. This device actually contains two independent 741-type op amps in a single 8-pin mini-DIP package. Note that there is a small notch or indentation at one end of the package, and possibly an indented dimple in one corner. These markers must be oriented to the left as you carefully insert this IC into the indicated location on your breadboard socket. Be sure you don't bend any of the pins under the body of the IC; all eight connections are required.

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

0.5" Orange Jumper

Prepare a 0.5" orange jumper and insert it in the location shown in the assembly diagram.

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

0.5" Blue Jumper

Prepare a 0.5" blue jumper and insert it in the location shown in the assembly diagram.

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

12K, ¼-Watt Resistor

Locate a 12K, ¼ watt resistor (brown-red-orange) and form its leads to a spacing of 0.5". Install this resistor in the location indicated to the right.

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

4.7K, ¼-Watt Resistor

Locate a 4.7K, ¼ watt resistor (yellow-violet-red) and form its leads to a spacing of 0.4". Install this resistor in the location indicated to the right.

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

3K, ¼-Watt Resistor

Locate a 3K, ¼ watt resistor (orange-black-red) and form its leads to a spacing of 0.4". Install this resistor in the location indicated in the assembly diagram.

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

1K, ¼-Watt Resistor

Locate a 1K, ¼ watt resistor (brown-black-red) and form its leads to a spacing of 0.4". Install this resistor in the location indicated to the right.

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

0.4" White Jumper

Prepare a 0.4" white jumper in the usual fashion and install it as shown to the right.

As usual, 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 in the location indicated in the assembly diagram.

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

10K, ¼-Watt Resistor

Locate a 10K, ¼ watt resistor (brown-black-orange) and form its leads to a spacing of 0.5". Install this resistor in the location indicated to the right. Note that this resistor must be installed on a diagonal, because there is no contact point on the -12/15 volt bus directly in line with this pin of the IC.

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

10K, ¼-Watt Resistor

Locate a 10K, ¼ watt resistor (brown-black-orange) and form its leads to a spacing of 0.6". However, unlike the resistors you have installed previously, you'll need to keep the leads longer so the resistor can be installed over the top of the IC already in place. Therefore, clip these resistor leads to a length of ½" after forming them. Then, install this resistor in the location indicated to the right. Be sure that the upper resistor lead does not make contact with the Zener diode lead beneath it.

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

0.2" Removable Black Jumper

Cut a 1¼" length of black hookup wire and remove ¼" of insulation from each end. Form this jumper as shown in the pictorial below, with a spacing of 0.2" between the exposed ends. Insert this jumper in the location indicated in the assembly diagram.

This specific jumper is Jumper A, and is made to be easily identifiable and removable. You will find references to it when you test your completed power supplies. For now, leave it installed in its place.

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

Assembly Complete

This completes the construction of your experimental circuit. Check your assembly carefully against the figure to the right, and correct any errors you might find. Then, go on to the test procedure on the next part of this page.

Restart assembly procedure
Resume assembly procedure


Testing the Power Supply

When you have completed the installation of the 1458 dual op amp and its associated circuitry, check your assembly one last time. Then turn on your voltmeter and set it to measure dc voltages up to ±20 volts. Connect the ground or common test lead to a convenient ground point, such as the grounded end of any of the three main reservoir capacitors.

Turn on your power supply and measure the voltage present on the +15 volt bus (along the top edge of the breadboard socket). This should be almost precisely 15.00 volts (the test model for this circuit showed a voltage of +15.02 at this point). If necessary, adjust the 15-turn trimpot to set the +15 volt bus to precisely 15.00 volts.

Now move your voltmeter test lead to check the voltage on the -15 volt bus. This voltage should ideally read precisely -15.00 volts, and should in reality be very close to this value. If it is not, make no further adjustments at this time, but consider making the changes described in the Discussion below.

Move the voltmeter test probe back to the +15 volt bus. Locate Jumper A, immediately to the right of the 10K trimpot you installed as part of this project. Remove Jumper A from your breadboard socket and set it aside. Now check the voltage on the positive bus. It should now read +12.00 volts, or very nearly. The negative bus should simlarly read -12.00 volts. If these voltages are incorrect, make no adjustments at this time, but do refer to the appropriate sections of the Discussion below.

Restore Jumper A to its on-board location and verify that the positive and negative supplies have reverted to ±15.00 volts. Then turn off your power supply and voltmeter and go on the the Discussion below.



Discussion

The output voltage of the positive power supply is set by the +3 volt reference from the trimpot and by the 12K and 3K resistors which provide the negative feedback to one of the op amps in the 1458. The 1K resistor is initially shorted out by Jumper A, so the gain of the op amp circuit is initially (12K/3K) + 1 = 4+1 = 5. With a +3 volt reference input, the output will be +3 × 5 = +15 volts.

Any error at this point is due to either an incorrect setting of the trimpot or the tolerances of the 12K and 3K resistors. Ideally, these resistors should be high-precision components: either 1% or even 0.1% tolerance if at all possible. They don't actually have to have values of 12K and 3K, but their ratio must be 4:1. However, the manufacturing process for even the 5% resistors you have been using is generally precise enough that these resistors will serve adequately.

The negative power supply is simply a mirror of the positive supply. Its accuracy depends on the two 10K resistors you installed just before installing Jumper A. As with the 12K and 3K resistors, they need not be exactly 10K, but they do need to have the same value. So long as this is the case, the negative supply will faithfully mirror the positive supply.

When you removed Jumper A, you removed the short circuit across the 1K resistor. This left the 1K resistor in series with the 3K resistor, forming a total series resistance of 4K. The resulting ratio of 12K/4K is now 3:1, and the op amp gain has been reduced from 5 to 4. Therefore, the output voltage dropped from +15 volts to +12 volts. Again, the accuracy of this output voltage is determined by the precise ratio of resistances; ideally the 1K resistor would be just as precise as the 3K and 12K resistors.

The negative supply simply mirrored the change in the positive supply with the same degree of accuracy as before.

The +15 volt supply will be the base reference for other voltages which we will generate as part of future projects. Therefore, this is the voltage that you must set as accurately as possible to +15.00 volts with the trimpot.

When you have completed this project, make sure power to your experimental circuit and voltmeter is turned off.


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