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Constructing the +12 Volt Supply

Introduction

While power supply requirements for digital circuits are rather simple and direct, a power supply for analog circuits is less specific, and therefore harder to precisely define. Instead of a simple, "All circuits require a 5 volt power source," we can have a wide range of power supply requirements.

A common power requirement for portable low-power equipment such as a basic transistor radio is 9 volts, so it can be powered from a compact 9-volt battery. Equipment designed for use in cars typically operates on 12 volts. Others require 3 volts, or 6 volts, or 15 volts. Or something else. Many circuits require both positive and negative power supplies. There is no end to the variations.

The reason for this is that where digital circuits are designed so that the transistors are either fully turned on (saturation) or off (cutoff), analog circuits can't work that way. Instead, they involve continuously changing voltages and currents, and it is these changing values that carry the information.

Therefore, analog circuits are designed with their operating conditions very much in mind.

A breadboard socket.

Another concern when constructing experimental circuits is that the power supplies and other auxiliary circuitry can take up a significant amount of room on the breadboard socket, which would be better used for the actual circuit being explored. Therefore, the best plan for anyone doing serious experimental work is to purchase or build a breadboarding facility, which has the necessary power supplies, logic inputs and outputs, and analog signal sources that will be needed for various experiments. You should also have the test instruments appropriate to the experiments you plan to perform. Pending your acquisition of such a breadboarding unit, however, you can construct all of the required power supplies on the left side of the breadboard socket shown above. This breadboard socket is 6½" long and 2¼" wide, and will be able to contain all power supplies while leaving plenty of room for experimental circuits on the right side of the socket.

The power supply we want will be able to provide either ±12 volts or ±15 volts, according to the requirements of the task and the devices used in a given experiment. It is possible to have any number of voltages, of course, but ±12/15 will work nicely for a wide range of projects. To obtain this capability, we will first built basic +12 and -12 volt power supplies. Then we will see how a dual op amp and a few extra components can easily turn these into highly accurate power supplies that can easily be switched between 12 and 15 volts. In addition, the negative supply will precisely mirror the positive supply in either case, so that the positive and negative supplies will always be balanced.

In this project, we will construct and test the basic +12 volt power supply. Later we will upgrade it to the precision supply we really want.



Schematic Diagram

This power supply is not designed to use the prefabricated IC voltage regulators, even though such regulators do exist. The 7812 and 7912 ICs are quite capable of providing regulated +12 volt and -12 volt outputs, respectively. However, we have two problems with doing it this way. First, each regulator provides an output voltage accurate to within 5%. For a 12 volt supply, this allows a ±0.6 volt variation. That's not bad, but we'd like something a bit more accurate for our purposes. Even more important, however, is the fact that the 7912 and the 7812 are not in any way guaranteed to mirror each other. We could easily have +12.5 volts and -11.5 volts as power supplies, and we want to avoid this disparity if we can. Besides, we'd like to easily switch between ±12 volts and ±15 volts, and still keep the output voltages accurate. That's a bit more complex with these ICs. (Plus, we'd like to use op amps for the purpose so we can demonstrate a very practical application for op amps.)

Note: The original design for this page was to construct both the positive and negative 12/15 volt supplies at the same time. Unfortunately, a flaw in some versions of IE5 as supplied with Windows 98 prevents this browser from allowing Javascript to handle that many steps. Therefore, the basic supplies will be constructed as two individual supplies on different pages. Then we will add the op amp control circuitry to complete the project. No single assembly procedure will exceed 30 steps.


Schematic diagram of a basic +12 volt power supply.

The power supply shown in the above schematic diagram uses a Zener diode as the main regulating device, to maintain a constant output voltage in spite of changes in input voltage. The type 1N4742A diode is rated at 12 volts, with a power dissipation limit of 1 watt. Since we want to be able to deal with load currents of at least 100 mA and possibly more, we cannot use the Zener diode by itself. (Under no-load conditions, the Zener diode would have to handle that 100 mA, which would result in a power dissipation of 1.2 watts. This would overheat and destroy the diode in short order.) Therefore, we add a power transistor as the main current-handling device for the power supply. The TIP31 NPN silicon transistor is rated to carry up to 3A collector current and a power dissipation of 20 watts. This is far more than we will ever need.

We also need to take into account one other concern. If the experimental circuit should attempt to draw too much current, or in the event of a miswire or short circuit, you could easily destroy the power transistor before you realized there was a problem. To prevent that, we have added some extra components that will serve to limit the current that this supply will provide to a safe level.

The 2.2Ω resistor drops only a fraction of a volt in normal use, and therefore does not cause a problem. However, as the load current rises, the voltage across that resistor increases to the point where the 2N4124 (or 2N3904) transistor starts to turn on, thus decreasing the forward bias on the main power transistor and limiting the load current that this circuit will permit to a safe value.



Parts List

To construct and test the +12 volt power supply circuit on your breadboard, you will need the following parts:

You will also need your longnose pliers, diagonal cutters, and wire stripper, as well as a voltmeter to help test the operation of the circuit.



Constructing the Circuit

You will construct the ±12 volt power supplies on the left-hand end of the breadboard socket, just to the right of the +5 volt supply, with the +12 volt supply on top.

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 or bare copper; any of these will work equally well here.



Circuit Assembly

Start assembly procedure























Starting the Assembly

Make sure that the left-hand side of your breadboard socket is clear of all components, jumpers, etc, as shown to the right. You'll need the top half of this space for the +12 volt power supply.

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.

Remove the Transformer Connections

To make it easier to move the breadboard socket around and install various components and jumpers, temporarily remove the wires connecting the breadboard socket to the transformer. You'll put these wires back when the basic +12 volt supply is complete.

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

0.3" Black Jumper

Prepare a 0.3" black jumper and install it in the location shown in the assembly diagram.

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 in the assembly diagram.

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

0.3" Black Jumper

Prepare a third 0.3" black jumper and install it in the location shown in the assembly diagram.

As before, 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 in the assembly diagram.

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

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

0.5" Orange Jumper

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

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

0.5" Orange Jumper

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

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

0.3" Orange Jumper

Prepare a 0.3" orange jumper and install it in the location shown in the assembly diagram.

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

0.2" Orange Jumper

Prepare a 0.2" orange jumper and install it in the location 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 (or use a clipped component lead of similar length), bend it in half, and install it in the location indicated in the assembly diagram.

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

10K, ¼-Watt Resistor

Locate a 10K, ¼-watt resistor (Color code brown-black-orange) and form the leads to a spacing of 0.3". It will be necessary to bend the leads around the resistor body so you can form the leads to the required spacing without risking damage to the resistor body. Clip the leads to a length of ¼" and then install this resistor in the indicated location. As you do so, you will see why we needed to keep those long black jumpers away from the contact holes.

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

1N4742A Zener Diode

Locate a 1N742A 12-volt Zener diode and form its leads to a spacing of 0.5".

If you've performed any of the Digital Logic experiments, you know that these procedures usually call for diode leads to be left longer than ¼". In this case, however, the diode will not be carrying much current so air circulation is not an issue. Therefore, when you have formed the leads of the Zener diode to a spacing of 0.5", clip the leads to a length of ¼". This will allow the diode to fit right against the breadboard socket and avoid interfering with the installation of other components. Be sure to observe the required orientation of the diode when you install it in the location indicated to the right.

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

Rectifier Diode

This is the first of two main rectifier diodes you will now install. Since these diodes carry the main current of the power supply, they need to be raised up from the surface of the breadboard socket to allow air circulation for cooling.

Accordingly, locate a rectifier diode and form its leads to a spacing of 0.3". Clip its leads to a length of ½" and then install this diode as indicated in the assembly diagram. Observe the required polarity of this diode.

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

Rectifier Diode

Locate another rectifier diode and form its leads to a spacing of 0.4". Clip its leads to a length of ½" and then install this diode as indicated in the assembly diagram. Observe the required polarity of this diode.

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

.01µf Disc Ceramic Capacitor

The capacitor you need here needs to be marked either ".01" or larger, or "103" or higher. In the latter case, the first two digits can be higher than "10," but the third digit must be at least a 3.

If necessary, form the capacitor leads to a spacing of 0.3". Then, if the leads are not already cut shorter, clip the leads to a length of 3/8" and install this capacitor in the location shown.

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, as shown in the pictorial here. Note that one lead is clearly marked (-). It is essential that this lead be connected to the more negative voltage; ground in this case. Clip the leads to a length of ¼". Then refer to the assembly diagram to the right as you install this capacitor so that the negative lead will be oriented to the right as shown.

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

2N3904 or 2N4124 NPN Silicon Transistor

Locate an NPN silicon switching transistor, type 2N3904, 2N4124, or similar. Form the leads to a spacing of 0.1" so it will fit easily on the breadboard socket. Clip the leads so that the expanded wires are ¼" long, allowing the body of the transistor to sit as close as possible to the breadboard socket. The shaped section of the leads will keep the transistor up, so it won't have any trouble sitting just above the Zener diode you installed earlier. Install the transistor as shown in the assembly diagram. Be sure to observe the correct orientation of the transistor.

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

2200µf Electrolytic Capacitor

Locate a 2200µf, 35-volt electrolytic capacitor with axial leads (one lead at each end of the package). If necessary, a unit with higher capacitance may be substituted.

Install the 2200µf capacitor as shown in the assembly diagram. If the disc capacitor you installed earlier has 0.3" lead spacing, follow the dashed line for the negative lead of this capacitor to complete the ground connection. Otherwise, use the corner contact as indicated by the solid line.

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

Power Transistor Lead Configuration

To make the proper connections to your power transistor, you must now determine its lead configuration. To do this, look at the package containing that transistor. Our example, the TIP31 sold by Radio Shack, has the pin configuration shown to the right. Note that the collector lead is in the center. If you are using this transistor or one with the same pinout, select "Collector" below. If your transistor has the base lead in the center, select "Base" below.

The center pin on my power transistor is the connection to the:

      Collector 
      Base 

Power Transistor

You specified a power transistor with the base lead in the center (pin 2 in the pictorial). If this is not the case, restart the assembly procedure and step forward again, making sure you correctly specify the pin configuration of your power transistor.

Install your power transistor in the location shown in the assembly diagram. Make sure the collector lead is to the left and the emitter lead to the right, connected to the 0.3" horizontal orange jumper you installed, just below the 10K resistor. We've assumed a TO220 package here; if your power transistor is in a TO5 round metal can, you can easily form the leads to an in-line configuration to fit in the designated location.

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

2.2Ω, ¼-Watt Resistor

Locate a 2.2Ω, ¼-watt resistor (red-red-gold) and form the leads to a spacing of 0.5". Install this resistor in the location indicated in the assembly diagram. Take time to be sure the third band is actually gold, not orange. An orange third band would indicate a 22K resistor, which would prevent your power supply from operating at all.

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

Power Transistor

You specified a power transistor with the collector lead in the center (pin 2 in the pictorial). If this is not the case, restart the assembly procedure and step forward again, making sure you correctly specify the pin configuration of your power transistor.

Install your power transistor in the location shown in the assembly diagram. Make sure the base lead is to the right and the emitter lead to the left, connected to the 0.3" horizontal orange jumper you installed, just below the 10K resistor. We've assumed a TO220 package here; if your power transistor is in a TO5 round metal can, you can easily form the leads to an in-line configuration to fit in the designated location.

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

2.2Ω, ¼-Watt Resistor

Locate a 2.2Ω, ¼-watt resistor (red-red-gold) and form the leads to a spacing of 0.5". Install this resistor in the location indicated in the assembly diagram. Take time to be sure the third band is actually gold, not orange. An orange third band would indicate a 22K resistor, which would prevent your power supply from operating at all.

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

Connect the Transformer Secondary Leads

Connect your three transformer secondary leads to their proper locations on the breadboard socket as shown to the right. However, do not turn power on until you are told to do so later on this page.

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

Assembly Complete

This completes the construction of your +12 volt regulated power supply. Check your assembly carefully against the figure to the right, and correct any errors you might find. Then, scroll down to the test procedure on the next part of this page.

Restart assembly procedure


Testing the +12 Volt Supply

Make sure your switched power strip is turned off, and plug your line cord into one of its outlets. Set your voltmeter to measure dc voltages up to 20 volts, and connect the black ground lead of the voltmeter to the negative lead of the 2200µf capacitor. Connect the red input lead of the voltmeter to the upper end of the 2.2Ω resistor you installed during this procedure, or else use an orange jumper to connect the voltmeter red lead to the upper bus strip on your breadboard socket.

Turn on power and obvserve your voltmeter. It should register a steady voltage close to +12. It may rise slowly a few fractions of a volt, and then hold steady. If you see this result, your power supply is wired correctly and you can go on to the discussion below.

If you've miswired anything, several possible things might happen. Take note of the observed symptoms, and then turn power off at once. Use the following list to help identify and correct your problem:

Output voltage rises to +12 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 only about +1 volt.

Your Zener diode is installed backwards. Refer back to the assembly diagram and correct the orientation of this diode. 

Output voltage is over 14 volts.

The Zener diode is open or the circuit is miswired. Refer back to the assembly diagram and verify correct installation. 

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

Although the power supply circuit is relatively simple and basic, it is far from trivial. Like any other electronic circuit, it must be wired accurately in order to perform correctly. However, unlike many types of circuits, miswiring the power supply can lead to damaged components and even injury. Extremely serious miswires can even overload your household wiring and cause fires under some circumstances. This is why any line-powered circuit should always include a fuse or circuit breaker: Not to protect the circuit being powered from the line, but rather to protect the line and all other circuits deriving power from it from any wiring errors or damaged components, that might otherwise overload the line and cause even larger problems.

At the same time, it may be that a poorly-designed or miswired experimental circuit may draw enough current to destroy experimental components without overloading the power line. While this cannot always be prevented, we can take steps to limit the amount of current the power supply will deliver, even if its load becomes a short circuit. The 2.2Ω resistor and 2N3904/2N4124 transistor serve this function in your +12 volt power supply.

The current provided by this supply must also flow through the 2.2Ω resistor. For normal current levels, the amount of voltage it drops is insignificant. However, if the load current starts to rise above 200 milliamperes (mA), the voltage aross this resistor begins to become significant, and comes close to allowing the current limit transistor to conduct. This in turn draws base current away from the main power transistor, and acts to reduce the forward bias on that transistor. If we assume that the load current rises to 300 mA, we find that the voltage across the 2.2Ω resistor rises to 0.66 volt. This would turn on the 2N3904 transistor enough to cut off the main power transistor altogether. Since that would mean zero load current, such a condition is not possible, and the load current cannot ever get this high. Thus, even if the power supply output is shorted to ground, the amount of current it will pass is limited to a value that will not overload the power supply components, or most experimental components.

When you have finished testing your +12 volt power supply, make sure power is turned off. You are now ready to move on to your next project.


Prev: +5 Volt Power Supply Next: Constructing the -12 Volt Supply

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