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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 |

Constructing the -12 Volt Supply

Introduction

The -12 volt power supply is not required for all possible analog circuits; many require only the +12 volt supply you have already built and tested. However, a number of analog circuits do require power supplies of both polarities so that their inputs and outputs can handle and deal with both positive and negative voltages. The most generally familiar device that requires dual power supplies is the Operational Amplifier, or op-amp. Since a number of our experimental circuits will involve op-amps, we will need the -12 volt supply. In addition, we want to improve the precision and versatility of this power supply as we demonstrate one of the many applications of op amps. Hence, a dual power supply is essential.

A breadboard socket.

As we mentioned in connection with the +12 volt supply, 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 system, however, you can construct all of the required power supplies on the left end of your 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..



Schematic Diagram

The -12 volt supply, like the +12 volt supply, uses discrete components rather than a 7912 voltage regulator IC. Here, we use a 1N4742A Zener diode, rated at 12 volts, 1 watt, as the main regulating element.

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 dual 12/15 volt supply will be constructed as two individual supplies on different pages. Then we will add the op amp control circuitry. No single assembly procedure will exceed 30 steps.


Schematic diagram of a -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 TIP42 PNP silicon transistor (available from Radio Shack) is rated to carry up to 10A collector current and a power dissipation of 65 watts.



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 ±12/15 volt power supplies on the left-hand side of the breadboard socket, just to the right of the +5 volt supply. The +15 volt supply will be on top and the -15 volt supply below. The right side of the breadboard socket will be allocated to experimental circuits that will use these power supplies.

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 end of your breadboard socket is clear of all components, jumpers, etc, other than those shown to the right. You'll build the basic -12 volt power supply here.

We have shown the +5 and +12 volt supplies, which should already be present. The -12 volt supply will in many ways be a mirror image of the +12 volt supply, but with some subtle differences. Therefore, be careful to follow the step-by-step instructions on this page exactly as given.

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 -12 volt supply is complete.

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

0.5" Blue Jumper

Prepare a 0.5" blue jumper 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.

0.5" Blue Jumper

Prepare a second 0.5" blue 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" Blue Jumper

Prepare a 0.3" blue 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.

0.2" Blue Jumper

Prepare a 0.2" blue 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.

10K, ¼-Watt Resistor

Locate a 10K, ¼-watt resistor (brown-black-orange) and form the leads to a spacing of 0.3". Install this resistor in the location indicated in the assembly diagram.

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

1N4742A Zener Diode

Locate a 1N4742A 12-volt Zener diode and form its leads to a spacing of 0.4". The body of this diode is likely to be glass, but you will have enough space that you need not bend the leads right at the diode body. Clip the leads of this Zener diode to ¼" to allow the diode to sit firmly on the surface of the breadboard socket. Observe the indicated diode polarity as you install this diode in the location shown 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.

Again, 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.

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

.01µf Disc Ceramic Capacitor

Look through the package of disc capacitors you obtained. There are various markings on each capacitor, but the one 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 the leads are not already formed to a spacing of 0.3", adjust them to this spacing now. Be careful not to damage the body of the capacitor when doing so. Then, if the resulting leads are longer than 3/8", clip them to this length and install the capacitor in the location indicated to the right.

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; -12 volts in this case. Clip the leads to a length of 3/8" to allow some flexibility in positioning. Then, refer to the assembly diagram to the right as you install this capacitor so that the negative lead is oriented to the left as shown.

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

2N3906 or 2N4126 PNP Silicon Transistor

Locate a PNP silicon switching transistor, type 2N3906, 2N4126, 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. Be sure to observe the required polarity; the negative lead must be oriented to the right as shown, since this is the more negative voltage.

As before, 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 TIP42 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 emitter lead is to the right and the collector lead to the left. We've assumed a TO-220 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.

2.2&Omewga;, 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 emitter lead is to the left and the base lead to the right. We've assumed a TO-220 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.

Restore the Transformer Connections

Reconnect your three transformer 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 jumpers 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

Set your voltmeter to measure dc voltages up to 20 volts, and connect the black ground lead of the voltmeter to a ground connection. Connect the red input lead of the voltmeter to the -12 volt output bus.

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 these results, 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 positive.

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

Output voltage is only about -1 to -1.5 volts.

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

Output voltage exceeds -16 volts.

The Zener diode is open or 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 2N3906/2N4126 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 2N3906 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.


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