Electronics Activity

The Adjustable Voltage Regulator

Purpose: To construct an adjustable DC to DC converter.

Resources

bullet LM217T adjustable voltage regulator TO220 package
bullet R1 - 240Ω 1/4 watt resistor
bullet R2 - 5kΩ multi-turn trimmer potentiometer
bullet Aluminum heat sink
bullet Pre-etched circuit board
bullet Heat sink compound
bulletStranded hook-up wire
bulletMounting hardware

Background

The LM217 family of positive voltage regulator integrated circuits have a voltage output range of 1.2v to 37v (depending on the input). They require very few additional components and the basic circuit configuration is simple to implement. They are especially useful in a robotic application where a single voltage main battery makes the use of components with different voltage needs difficult. For example, the circuit when connected to a 12 volt battery can be adjusted to output a voltage in the range 1.25 to 10.75 volts. This useful IC can also be configured as a constant current source which will be the focus of the next activity.

Circuit Configuration

The LM217 is a three terminal device which can pass a current of 1.5a when mounted on a suitable heat sink. Please see the datasheet for terminal identification.

The basic circuit configuration is shown next. The components will be mounted on a pre-etched circuit board.

 

Circuit Fabrication

The components for the circuit are shown next. The pc board is a section from a larger board. This board can be cut to provide for 4 circuits plus additional pieces for the next activity.

Step1

Start by soldering the heat sink to the circuit board. Two holes have been enlarged with a 5/64" dill bit to accommodate the heat sink legs. Make sure it is seated as far as possible.

Step2

Mount the LM217 voltage regulator. DO NOT SOLDER YET! First put a dab of heat sink compound on the back tab and bolt it to the heat sink with a short machine screw and nut. Now solder the three terminals to the circuit board as shown. Trim the leads.

Step3

Mount the fixed resistor vertically by bending one of the leads, Make sure the resistor is flush with the board.

Step 4

Mount the potentiometer flush with the circuit board for stability. As shown, the voltage will increase with a clockwise turn of the adjusting screw.

 

Step 5

Finally, solder 4 wires to the board, 2 input and 2 output. Use the traditional red for the positive input, black for the two grounds. The pairs can be twisted and should be attached to the board with a cable tie or hot clue for strain relief.

Testing

Connect the output leads to a voltmeter. Connect the input to a 12v source.  Rotate the adjusting screw on the potentiometer. The voltage should change from roughly 1.25 to 10.75 volts.