# DIY an Adjustable Power Supply Source With Voltmeter Function

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In some cases, we need a DC power supply of 4V while conducting our electronic experiment. What should we do? To buy a 4V battery sounds reasonable. But if we need a 6.5V power supply next time and what should we do? We can buy an adapter of 6.5V DC output on Amazon.com. BUT that is uneconomic as when we need different voltage of power supply, we need to pay for them. The better solution is to make an adjustable DC power supply. You will go into the detail of how an adjustable DC power supply works by the DIY process and enrich yourself.

Materials:

1 x LM317 Voltage Regulator

2 x 470uF Electrolytic Capacitors

2 x 104 ceramic capacitors

1 x 10uF Electrolytic Capacitor

2 x 4148 Diodes

4 x IN4007 Diodes

1 x LED

2 x Connector

1 x 180Ω Resistor

1 x 1K Resistor

1 x 5k Variable Resistor

1 x Switch

1 x Heat Sink

1 x 10cm Cable

4 x Clips

1 x 7 Segment Digital LED Display Tube

1 x Transformer

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## Step 1: Solder the Resistors to the PCB

There are only two resistors needed in this project. R1 is 180Ω, R2 is 1kΩ. Please use a multimeter to measure out each resistor and then insert them to the corresponding position on the PCB. As shown on image 1, the 180Ω resistor belongs to R1 and the 1kΩ belongs to R2 printed on the PCB.

## Step 2: Solder the IN4007 Rectifier Diodes to the PCB

Please note that the rectifier diodes have polarity, as shown in image 2 and 3, the white band printed on the IN4007 diode should be placed at the same side of the smaller rectangle on the PCB.

## Step 3: Solder the 4148 Switching Diodes and Ceramic Capacitors to the PCB

The 4148 switching diodes have polarity, as shown in image 5, the black end of the diodes should be placed at the same side of the smaller rectangle on the PCB. The ceramic capacitors have no polarity, no need to pay extra attention to the direction.

## Step 4: Solder the Electrolytic Capacitors to the PCB

The electrolytic capacitors have polarity, the long leg is positive which should be inserted into hole near the ‘+’ symbol printed on the PCB. Please NOTE that do not insert them into the PCB in reverse or it may cause damage to the whole circuit.

## Step 5: Solder the LED and Switch to the PCB

The LED has polarity, as shown in image 12, the long leg is positive which should be inserted into the hole near the ‘+’ symbol printed on the PCB. Please pay attention to the gap between each pad while soldering the switch and do not let the melted tin cause short-circuit.

## Step 6: Solder the Wire Connector to the PCB

Please note that the ports of the connectors should be faced toward you or it may cause trouble in the few further assembly.

## Step 7: Solder the Adjustable Resistor to the PCB

Insert the adjustable resistor into the PCB and then solder each pin. The things you should keep in mind in this step is to keep the adjustable resistor vertical to the PCB. After that, then install the cap to the knob of the adjustable resistor.

## Step 8: Assemble the 7 Segment Digital LED Display Tube

Please note that you must pay more attention to this step and follow from image 22 to image 27 to complete this step. If you assemble in a wrong way, it may cause permanent damage to the circuit.

As shown in image 22, put the bundle of wires through the hole near the adjustable resistor. And then use the screw I marked by a red circle in image 23 to fix the digital LED tube. Next is as shown in image 25, to split the integrated wires into three individual pieces. The MOST important thing in this step is as shown in image 26, the red and white and black wires should be inserted into the holes in sequence of from right to left respectively. If you do not follow this guide line, the digital LED tube may be damaged permanently.

## Step 9: Screw the LM317 to the Heat Sink

Use the screw I marked by red circle in image 28 to fasten the LM317 to the heat sink and as shown in image 29, no need to put a nut to the screw. Then insert the assembly into the PCB, as shown in image 30. When solder the pins please mind the gap between each pin and DO NOT let the melted tin short-circuit the pins. And you need to check again whether the pins are short-circuited after cutting off the pins by a multimeter.

## Step 10: Solder the Transformer to the PCB

As shown in image 33, the black wires should be inserted into the holes I marked by red circles. Because the AC power supply has no direction requirement, each black wire has no its own exclusive hole, just solder them in any sequence as you like.

## Step 11: Deal With the Outer Connection Wires

As shown in image 35, cut the wire in half and split it into two individual pieces. Rip off small amount of skin from the two ends of each wire and as shown in image 37, use solder iron to add some melted tin to the bare wire.

## Step 12: Solder the Metal Clips to the Wires

Put the wire through the hole at the bottom the metal clip and as shown in image 39, solder the tin wire to the connection point until the melted tin cover it. And then follow from image 40 to 42 to complete this step.

## Step 13: Deal With the Acrylic Shell

As shown in image 43, tear down the cover from the acrylic board. From image 44 to image 47 there are the bottom board, side boards, the fore board and back board, top board respectively. Before you assemble the PCB to the acrylic board, please try to build up a box with these acrylic boards to roughly recognize the position of each board.

## Step 14: Screw the Transformer to the Bottom Board

Install the transformer to the position I marked by red circle and make sure the red wire is facing towards you. As shown in image 51 and 52, install the hollow screw to the bottom board. And then as shown in image 53 and 54, screw the PCB to the board and make sure the knob is at the left side of the transformer.

## Step 15: Install the Other Acrylic Board

Image 55: Install the right side board

Image 56: Install the fore board. The three hollow rectangles I marked by red arrows are aligned to the two connection port and switch.

Image 57: Tighten the screw to fasten the fore board to the main body

Image 58: Install the other side board and tighten the screw

Image 59 and 60: Put the two red wires through the hollow rectangle in the back board and tighten the screw to fasten the back board to the main body

Image 61 and 62: Install the top board and tighten JUST ONE screw to fasten the top board to the main body, leave the other screw holes empty. However, you can tighten screws to the other screw holes but one screw is enough.

## Step 16: Deal With the Power Supply Wire

Before soldering the power supply wire to the red wires, please add some melted tin to the black wire by solder iron, just as shown in image 63. And then use some Electrical Insulation Tape or Heat shrinkable tube to wrap around the bare wires to protect you from electrical injury.

## Step 17: Assemble the Wires Finished in Step 12 to the Connectors

Use a screwdriver to fasten the wires finished in Step 12 to the connectors. Please note that the red wires should be inserted into the right port of each connector as they represent the positive polarity while the black wires represent negative polarity.

When using as a voltmeter, you need to connect the target testing object such as a battery to the Voltmeter Input Port I marked in image 66 and PUST the switch to the left side. The red wire is connected to the positive side of the battery and the black wire is connected to the negative side of the battery.

When using as an adjustable DC power supply, you need to use an DC Power Supply Output Port I marked in image 66 and PUSH the switch to the right side. The red wire is the positive end and the black wire is the negative end. It can be used to output the DC voltage from 1V to 15V.

## Step 18: Testing

Image 67 is showing how to use it as a voltmeter. The red wire in the left connector is connected to the positive end of the battery, the black wire is connected to the negative end of the battery. We can see from the 7 segment digital LED tube that the voltage of this AAA battery is about 1.5V.

Image 68 is showing how to use it as an adjustable DC power supply. Take away the AAA battery and use the other connector to output voltage to the multimeter. Rotate the switch of the multimeter to the voltage measurement position and then use the red clip to clamp the red probe of the multimeter and use the black clip to clamp the black probe of the multimeter. Rotate the the knob of the adjustable resistor and you will get different DC output from about 1.24V to 15V.

## Step 19: Analysis

The LM317 is an adjustable 3-terminal positive voltage regulator capable of supplying in excess of 1.5 A over an output voltage range of 1.2 V to 37 V. This voltage regulator is exceptionally easy to use and requires only two external resistors to set the output voltage. Further, it employs internal current limiting, thermal shutdown and safe area compensation, making it essentially blow-out proof.

From the schematic we can see that when the 12AV voltage applied to T11 and T12, the bridge rectifier circuit composed of four IN4007 diodes trim the AC to DC, the 0.1uF ceramic capacitor, C3 is an bypass capacitor which plays a role in reducing the sensitivity to input line impedance. The electrolytic capacitor C1 and C4 is in the use of smoothing the voltage into a near-level DC voltage. The adjustment terminal may be bypassed to ground to improve ripple rejection. This capacitor C5 prevents ripple from being amplified as the output voltage is increased. For more details of the electrolytic capacitors in a rectifier circuit please right click your mouse and visit this blog in a new tab.

The IN4148 diode, D1 is used to prevent VCC from discharging through the LM317 during an input short circuit. The diode, D2 is used to protect against capacitor C5 discharging through the LM317 during an output short circuit. And the combination of D1 and D2 prevents C5 from discharging through the LM317 during an input short circuit. To adjust the the adjustable resistor RP1 you will get the output DC voltage from about 1.24V to 15V.

The DIY materials are available at mondaykids.com

The the below projects I posted at Instructables.com are all using this LM317 DIY Kits as power supply:

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DIY an Air Raid Siren With Resistors and Capacitors and Transistors

DIY a Basic Common Emitter Amplifier for School Studying

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DIY a NE555 Circuit to Generate Sine Wave

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