VERSATILE POWER SUPPLY Build this bench power supply that offers five different output voltages. The versatile power supply is one of the most basic but overlooked items of test bench equipment. Good commercial bench-type power supplies are expensive, and they might not have some feature you want. The best solution is to build your own supply that can provide the voltages, currents, and features you want. Circuits such as operational amplifiers and analog to digital. converters (A/D) converters require both a positive and negative voltage, so a supply with a dual-polarity output is essential. A + 12-volt DC source is necessary for testing car audio equipment and CB radios. A display section that will indicate the active voltages and the exact output voltage is also essential for your convenience. The prototype power supply described in this article fills those needs. It includes both positive and negative variable power sources that produce between 1.2 and 28 volts at about 1 ampere. A separate +/-5 volt source powers the A/D voltage meter, and also provides a second dual-output voltage. A third section provides 12 volts at 1.5 amperes, maximum. LEDs indicate the status of the five available outputs, and a digital display provides a digital readout of either voltage or current. The prototype has banana jacks as output connections, but binding posts or multi connectors can be substituted. The switches can be either pushbutton, toggle, or slide; it's really just a matter of your personal preference. Circuit operation Figure 1 is the main schematic. All five power supply sections conform to standard design practice. The AC voltage from transformer T1 is rectified by bridges BR1, BR2, and BR3, and filtered by capacitors CI, C2, C5, C6, and C9. Voltage regulators IC1 to IC5 reduce the voltage to the desired fixed or variable levels. The LM317 regulator, IC1, provides a positive variable output from 1.2 to 28 volts DC, while an LM337, IC2, provides a negative variable output with the same range. The LM7805 regulator, IC3, supplies a fixed + 5 volts, and the LM7905, IC4, supplies a fixed -5 volts. The LM7812 regulator, IC5, supplies a fixed + 12 volts. Capacitors C3, C4, C7, C8, and C10 improve transient response and prevent oscillation. Resistor networks Rl-R3 and R2-R4 for IC1 and IC2, respectively, provide the necessary feedback to obtain the variable output voltages. An LED and current-limiting resistor is wired across each output to indicate when each output voltage is present. The main power indicator for the entire unit consists of LED5 and Rl8. Switch S1 controls AC power to the transformer primary, and switches S2 and S3 connect the secondary voltages to the 5- and 12-volt regulator circuits. The +/-5-volt supply powers the voltmeter and display circuitry, so this section must be turned on to power the output meter. Fuse F1 protects the supply in the event of an overload, and disables it to prevent electrical shock or fire. Each voltage output is connected to one of the panel jacks and to one throw position of switch S4. Switch S4 is a two-pole, six-throw (2P6T) rotary unit. Pole A switches the positive input to the voltmeter, and pole B switches the ground input. (There are three separate grounds in the circuit.) Pole A of S4 is connected to one pole of DPDT switch S5 so that the signal can be routed directly to the meter for voltage readings, or to the LM741 current-to-voltage converter IC7 for current readings. The second pole of S5 is connected to the +5-volt supply. This pole is switched to the second decimal point of the display in the voltage mode or to pin 7 of the LM741 in the current mode. The heart of the digital meter is the Harris ICL7107 A/D converter (see Fig. 2). The 7107 contains all the active devices needed to construct the voltmeter. These include the segment decoders and LED display drivers. The optional pushbutton switch connected to pin 37 of the ICL7107 permits the display to be tested. When that pin is connected to +5 volts, the display will indicate "-1888," testing all of the segments. If a liquid-crystal display is preferred, substitute an ICL 7106CPL for the 7107. All pin connections will be the same, except that pin 21 will become the backplane instead of the digital ground. Building the circuit The circuit can be built by point-to-point wiring on perforated construction board, but a printed circuit board will make construction easier. A foil pattern is included here if you want to make your own board. All components are mounted on the PC board except items on the front and back panels. Solder all components to the board as shown in Fig. 3, starting with the discrete capacitors and resistors. Then add all the semiconductor devices. IC sockets are recommended for IC6 and IC7. Be sure to observe the proper polarity on the bridge rectifiers, diode, and electrolytic capacitors. Mount heatsinks on all of the voltage regulators to prevent them from overheating. Figure 4 shows the completed PC board. The prototype is housed in an 8- x 6- x 2.5-inch instrument case, about the smallest commercial enclosure that will contain all of the parts. You can also make one of your own design. If the case is not ventilated, cut slots in the back, top, and sides to allow air to circulate to dissipate the heat. All controls and jacks, except for the rotary mode-control switch, are mounted on the front panel. The mode switch is located on top of the case. Wire the front-panel connections with stranded No. 22 AWG hookup wire, except for the meter display and AC power lines. Wire any lines that carry the 120-volt AC voltage with 16-gauge stranded wire, and route them away from the main board. A foil pattern is also provided here for the seven-segment, four-digit meter display circuit. Mount the parts for the display as indicated in Fig. 5, and connect the display board to the main circuit board with a length of 24-conductor No. 28 AWG flat cable. The fuse holder is mounted on the rear panel, and the panel allows access for the 120-volt AC line cord. A No. 14- or No. 16- AWG, three-conductor linecord is recommended. Connect the green ground lead to the frame of the transformer. Figure 6 shows the inside of the completed power supply. Calibration With switch S4 set to the positive variable voltage position and S5 set to the voltage mode, adjust R1 for any desired voltage, and measure the output with a voltmeter. Adjust R12 until the built-in digital display shows the same voltage. Move S5 to the current position, and connect a resistive load of about 10 kilohms in series with an ammeter to the output. Adjust R15 so that the ammeter and the built-in meter both read the same current value. The power supply is now complete and ready for you to use on your test bench. DIAGRAM: FIG. 1 -- MAIN POWER-SUPPLY SCHEMATIC. The supply provides both positive and negative variable outputs, a positive 5-volt output, a negative 5-volt output, and a positive 12-volt output. DIAGRAM: IN POWER SUPPLY FOIL PATTERN DIAGRAM: FIG. 2--VOLTMETER CIRCUIT. The Harris ICL7107 A/D converter Includes segment decoders and LED display drivers. DIAGRAM: FIG. 3--PARTS-PLACEMENT DIAGRAM. Mount heatsinks on all voltage regulators to prevent them from overheating. PHOTO (BLACK & WHITE): FIG. 4--THE COMPLETED PC BOARD. Be sure to observe the proper polarity on the bridge rectifiers, diode, and electrolytic capacitors. ILLUSTRATION: FIG. 5--DISPLAY BOARD PARTS PLACEMENT. Connect the display board to the main circuit board with a 24-conductor flat cable. DIAGRAM: DISPLAY BOARD FOIL PATTERN. ~~~~~~~~ By CARL J. BERGQUIST -------------------------------------------------------------------------------- Inset Article PARTS LISTS All resistors are 1/4-watt, 5%, unless noted. R1, R2--5000 ohms, panel-mount potentiometer R3, R4--220 ohms, 1/2-watt R5, R6, R9, R16--1000 ohms R7, R8--470 ohms R10--470,000 ohms R11--1 megohm R12--100,000 ohms, PC--mount potentiometer R13--22,000 ohms R14--100,000 ohms R15--500,000, PC--mount potentiometer Capacitors C1, C2, C9--1000 muF, 50 volts, electrolytic C5, C6--470 muF, 50 volts, electrolytic C3, C4, C7, C8--0.47 muF, 50 volts, electrolytic C10--0.33 muF, 50 volts, electrolytic C11--0.22 muF, polyester C12--0.047 muF, polyester C13--0.01 muF, polyester C14--0.1 muF, polyester C15--100 pF, ceramic disc Semiconductors IC1--LM317T positive adjustable voltage regulator IC2--LM337T negative adjustable voltage regulator IC3--LM7808T positive 5-volt regulator IC4--LM7905T negative 5-volt regulator IC5--LM7812T positive 12-volt regulator IC6--ICL7107CPL A/D converter/LED display driver (Harris) IC7--LM741 operational amplifier BR1 - BR3--200 PIV, 1.5-amp bridge rectifier LED1, LED3--red light-emitting dicode LED1, LED2, LED4--green light-emitting diode LED5--orange light-emitting diode DISP1, DISP2--dual 7-segment common-anode LED display (MAN6710 or Equivalent) Other components S1-S3--SPDT switch S4--2P6T rotary switch S5--DPDT push-on/push-off switch T1--120 VAC primary, 28/18/16-volt center-tapped secondary Electronic Goldmine # G2713 or equi., 602-451-7454) F1--1/2-amp slow-blow fuse Miscellaneous: five heatsinks for TO-220 devices, panel-mount LED holders, 8- x 6- x 2.5-inch instrument case (JDR Micordevices GPB862 or equiv.), control knobs, eight panel-mount banana jacks, panel-mount banana jacks, panel-mount fuse holder, 6-foot grounded AC line cord and strain relief, 1- x 2-inch red display cover (optional), hardware, dry-transfer lettering to label the front and back panels, two 40-pin IC sockets, one 8-pin IC socket.