How to build a variable amp and voltage power supply

Written by j.t. barett
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How to build a variable amp and voltage power supply
A current-limiting power supply prevents circuit damage. (Jupiterimages/Photos.com/Getty Images)

When you get serious about your electronics hobby, you'll need a decent power supply to run your projects. The supply should have decent regulation and a variable voltage as a minimum, but you can go a step further by adding a variable current-limiting feature. This lets you set the maximum current the supply will deliver, preventing overloads and burnouts in the circuits to which it provides power. This circuit uses an L200 voltage regulator integrated circuit (IC) to provide voltages up to 26 volts and current up to 1.5 amps.

Skill level:
Moderate

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Things you need

  • 35 to 40-volt step-down transformer, 2-amp secondary
  • Line cord with standard 3-prong plug on one end, bare wires on the other
  • 30-watt soldering iron
  • Electrical solder
  • Electrical tape
  • Hobby knife
  • Perfboard
  • Wire strippers
  • 2-amp bridge rectifier
  • Diagonal cutters
  • 4700-microfarad 100-volt capacitor
  • 220-nanofarad 50-volt capacitor
  • L200 voltage regulator integrated circuit (IC)
  • LM741 op amp IC
  • 22-gauge solid wire
  • Wire strippers
  • 2 resistors, 1K-ohm 1/4-watt
  • 470-ohm 1/4-watt resistor
  • 10K-ohm variable resistor
  • 100K-ohm variable resistor
  • .1-ohm 5-watt resistor
  • Labelling tape
  • Marker pen

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Instructions

  1. 1

    Solder the bare wire ends of the line cord to the transformer's primary wires. Give the soldered connections a minute to cool. Wrap them with a few turns of electrical tape.

  2. 2

    Locate the bridge rectifier near the perfboard's left edge and slip its leads through the perfboard, so they protrude through to the copper-foil back side. Note that two leads are marked with the tilde (~) symbol signifying alternating current (AC). Two leads have a plus (+) and minus (-) sign. Solder the transformer's two secondary wires to the "~" leads. Trim the lead excess with the diagonal cutters.

  3. 3

    Insert the 4700-microfarad capacitor's positive lead near the bridge rectifier's positive lead and its negative lead near the rectifier's negative. Solder this connection and trim as before. Place the 220-nanofarad capacitor next to the larger capacitor and solder its leads to the larger capacitors.

  4. 4

    Seat the L200 IC to the right of the capacitors. Insert its leads through the perfboard. Solder pin 1 to the board to hold the IC in. Place the LM741 IC to the right of the L200 and insert its leads through the board. Solder pin 1 to the board.

  5. 5

    Cut about five short pieces of 22-gauge wire, from 2.5 to 5 cm (1 to 2 inches) long. Remove about 6 mm (1/4 inch) of insulation from each end. Use a short piece to connect pin 2 of the 741 to pin 6 of the L200. Solder the connection. In a similar way connect and solder the negative lead of the 4700-microfarad capacitor to pin 3 of the L200 and pin 4 of the 741.

  6. 6

    Cut six 12.5 cm (5 inch) pieces of the wire and strip the insulation from each end. Solder one end of a wire to each solder lug on the two variable resistors. Cut two 30 cm (12 inch) wires, strip the ends and set them aside.

  7. 7

    Use a short piece of wire to connect the L200's pin 1 to pin 7 of the 741. Solder the connection. Insert one lead of a 1K-ohm resistor near pin 5 of the L200. Put the other lead near pin 2. Insert the end of the wire connected to the 100K-ohm variable resistor's wiper, or centre lug, to pin 2. Connect a wire from one of the other lugs also to pin 2. Connect the remaining wire to pin 2 of the 741. Connect one lead of the 470-ohm resistor to the 741's pin 2. Insert the other lead near the L200's pin 5. Solder these connections and trim the excess.

  8. 8

    Insert one lead of the .1-ohm resistor near the L200's pin 5 and the other near the 741's pin 3. Place the wiper-lug wire and one other wire from the 10K-ohm variable resistor near the 741's pin 3. Place the remaining variable-resistor wire near the L200's pin 4. Connect the remaining 1K-ohm resistor between pin 4 on both ICs. Solder these connections.

  9. 9

    Insert the end of one 30 cm (12 inch) wire near the 741's pin 3. Label this wire "V+." Insert the other wire near the 741's pin 4. Label this wire "Ground." Solder these connections.

Tips and warnings

  • The smaller capacitor isn't polarised, so it doesn't have positive or negative leads.
  • The 100K-ohm variable resistor sets the supply's current limit. The 10K-ohm variable resistor sets its voltage.
  • Use the "Ground" wire to ground the circuit you're running with this power supply, and use "V+" for the circuit's positive voltage.

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