POWER SUPPLY BOARD SOLDERING INSTRUCTIONS

N. Cotter

OVERVIEW:

This document explains how to solder the power supply board for building circuits in class. (Please see "Power Supply Building Instructions" for an overview of what the power supply board will be used for and where to buy the kit.) Fig. 1 shows the completed power supply board.

Fig. 1. Power supply board.

SOLDERING IRON SETUP:

Turn on the soldering iron by pressing the "1" on the rocker switch. Set the temperature knob to the 4-o'clock position. Moisten the sponge and wipe the tip of the soldering iron on it regularly to keep it clean. Tin the iron from time-to-time by putting small amounts of solder on the tip. Your solder is in a coil in your kit, and looks like a wire. It is very flexible.

Fig. 2. Soldering iron.

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SOLDERING:

To solder a component to the board, place the iron so it is in contact with the circular pad on the power supply board and the component lead you are soldering. Bring the solder into contact with the iron so it melts onto the pad and the component lead. Use a minimal amount of solder that fills the hole through which the lead passes and coats the wire to just above the hole. Fig. 3 illustrates the soldering process. Note that components are inserted on the printed side of the board and soldered on the other side.

Fig. 3. Soldering a component lead.

A good solder joint will be shiny, indicating that the solder annealed properly as it cooled. If the joint appears dull in color, it has crystalized and may not make electrical contact. This is referred to as a cold solder joint. You may be able to reheat the solder joint succesfully.

In the event that something is soldered incorrectly and has to be removed, use the braided copper solder-wick available in the lab. The solder-wick braid acts like a paper towel for soaking up liquid solder. Place the braid on the solder to be removed and press down on it with the soldering iron. Because the solder-wick is made of copper, it absorbs heat quickly. It will tend to cool the tip of the soldering iron, so you will want to have a much surface area of the soldering iron in contact with the braid as possible. Soak up the excess solder and clip off the used parts of the braid as you go.

The following sections discuss soldering issues relevant to each type of component on the board. The sections are presented in the order recommended for soldering the components on the board.

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SOCKETS:

Note the notch on one end of the 8-pin socket and on one end of the 14-pin socket. Match this notch with the curve that looks like a smile on the power supply board. After soldering the board, you will insert the integrated circuits, Fig. 5, in the sockets, Fig 4.

Fig. 4. Integrated circuits. LM555 (8-pin chip) on left and LM324 (14-pin chip) on right

Fig. 5. Sockets.

Solder two opposite corners of each socket first and make sure the socket is flush on the surface of the power supply board. If the socket is raised up on one side, reheat the solder joints on the bottom of the board while pressing down on the socket with your finger. The joints can usually be reheated a few times without harm. Just make sure the solder joints are shiny when they cool.

SCREW TERMINALS:

Fig. 6 shows the screw terminals that are placed on one edge of the power supply board. These two terminal blocks are identical and are used to connect wires coming out of into the power supply board. Fig. 7 shows a screwdriver being used to tighten the connection to a resistor lead that goes from the output of the power supply (on left) to the input of the voltage measuring circuit (on right).

Because the screw terminals provide connections for wires coming off the board, the holes into which wires are inserted should be on the outside edge of the board. Otherwise, wires will come out of the screw terminal block over the circuitry on the board. This still works but is awkward. In Fig. 7, the holes for leads are on the front side of the screw terminals, at the bottom of the picture.

Fig. 6. Screw terminals.

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Fig. 7. Using screw terminals. Tightening screw on resistor lead.

HEADERS:

The headers, Fig. 7, and jumpers, Fig. 8, act like switches. The jumpers can short out (connect) two header pins together to complete a circuit. The jumpers are pushed down on the headers on the side with the longer pins. Thus the longer pins should be on the top side of the board. The plastic base will also be on the top side of the board.

Fig. 7. Headers.

Fig. 8. Jumpers.

The two headers are identical and consist of four pins. There is a line on the header that you may match to the line on the power supply board, although the four header pins area all the same and have no real orientation.

POTENTIOMETER:

The potentiometer is a variable resistor with a screw adjustment on top. On the board, it is labeled PD1 and PD2. There are five holes for the potentiometer, but only three are used. Holes on either end of the footprint of the potentiometer remain empty. When soldering components with leads, bend the leads out so the components remain in place while soldering, as shown in Fig. 10.

Fig. 9.

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Fig. 10. Bending leads out for soldering.

After soldering leads, clip them off just above the solder bump. Be careful not to clip into the solder bump itself.

LED:

The LED lights up with an intensity proportional to a voltage being measured. Place the LED in either of the two positions provided on the board. The LED is a bicolor LED that incorporates two LED's in one.

On the LED case is a flat side, (it is on the side of the shorter lead), and this side should be placed away from the edge of the board in order for the LED to light up green when voltage is positive and red when voltage is negative. The LED leads are somewhat brittle and can be bent only a few times before breaking.

Fig. 11. LED (Light Emitting Diode)

VOLTAGE REGULATORS:

The voltage regulators, Fig. 12, look almost the same, but they are different and must be placed in the correct locations. The part numbers are printed on the front of the devices. The parts layout diagram and the lettering on the power supply board show where the voltage regulators are placed on the board. The three leads are configured in a triangle on the board, requiring you to bend the middle lead back slightly and then down. These regulators will sit 1/16" above the board when soldered in place.

Fig. 12. +5 V regulator 78L05 (left) and –5 V regulator 79L05 (right).

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DIODES:

The diodes are about the side of resistors but have black plastic bodies with stripes on one end, as shown in Fig. 13. The stripe must be placed at the correct end as indicated on the board. The diode conducts current only in one direction and must be placed in the circuit in the correct direction. Components like diodes and resistors should be place right on the surface of the board.

Fig. 13. Diode.

RESISTORS:

The resistors have colored bands to indicate their values. Fig. 14 shows the example of a 24 kΩ resistor. A guide to this color code may be found on the web.

Fig. 14. 24 kΩ resistor. Color bands are red (2), yellow (4), orange (3 for 103) and copper (% tolerance).

The values of resistors may also be verified by direct measurement with an ohmmeter. Fig. 15 shows how to measure a resistor. Note that the meter is set to the Ohm (Ω) position. The display shows kΩ to indicate "thousands of ohms" and MΩ to indicate "millions of ohms". There is no polarity for resistors. They may inserted on the board in either direction, although aligning tolerance bands makes the color codes easier to read.

Fig. 15. Measuring resistor value with an ohmmeter.

SMALL CAPACITORS:

There are three small capacitors on the power supply board, as shown in Fig. 16. The capacitors have numbers printed on them that have the same interpretation as the resistor color codes. That is, the first two digits represent a number, such as 10 for 1 and 0. The third number is a multiplier that is a power of ten as indicated by the number. Three, for example, means multiply by 1000 = 103. The values are in picofarads. Thus, 103 means

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10,000 pF = 0.01 F, 104 means 100,000 pf = 0.1 F, and 223 means 22,000 pF = 0.022 F.

Fig. 16. Small capacitors.

ELECTROLYTIC CAPACITORS:

The electrolytic capacitors look like small cola cans with leads coming out of the bottom. They have a white stripe on one side with minus signs. The board shows small plus signs, which must be oriented away from the minus signs. Values are printed on the capacitors. The values are 22 F and 100 F (three of these are used).

Fig. 17. Electrolytic capacitors.

INSERTING CHIPS IN SOCKETS:

When inserting integrated circuits (IC) into sockets, be sure to place the notch on the IC toward the notched end of the socket. The pins on the IC will be wider than the socket and must be bent inward slightly. Be careful when inserting the IC's so as not to bend the pins up under the IC.

TESTING:

Before soldering battery holders, use alligator clips to connect a 9 V battery to the power supply board. Connect the negative battery terminal to the round battery holder terminal underneath the smaller IC, and connect the positive battery terminal to the square battery holder terminal underneath the smaller IC. Only one 9 V battery is needed to power the circuit.

For testing, connect the output of the positve voltage supply (left side) to the input of the voltage measuring circuit (right side), as shown in Fig. 18. The LED will light up green

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if all is well. Note the position of the jumpers. One battery is used, and it is placed on the left side (under the smaller integrated circuit).

Fig. 18. Testing positve voltage output.

BATTERY HOLDERS:

Put the battery holders, Fig. 19, on the bottom side of the board. Solder the battery holder terminals on the top side of the board. Use a piece of wire or twist-tie to lash the battery holders to the board through the holes in the board. Use one battery, and place it in the battery holder under the smaller IC.

Fig. 19. Battery holders.

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