What it shows
Every student lab needs a variable DC supply, and shop-bought ones are costly. An LM317 regulator with two resistors gives an adjustable output. The project builds one from a 15 V adapter output and tests how much the output falls when the load current rises, which is the load regulation.
The science
The LM317 keeps 1.25 V between its output and adjust pins. A 240 Ω resistor from output to adjust therefore carries about 5.2 mA, and that same current flows through the second resistor to ground. Output voltage is 1.25 x (1 + R2/R1) plus a small adjust-pin current term. The regulator burns the difference between input and output as heat, which is why a heat sink and a minimum input margin of about 3 V are needed.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An LM317 regulator with a small heat sink
- A ready-made 15 V DC, 1 A adapter output or a 12 V battery pack (not a mains-wired circuit built by you)
- A 5 kΩ preset or potentiometer, 240 Ω resistor, 1N4007 diodes (2)
- Capacitors 1000 µF 25 V, 10 µF 25 V and 0.1 µF
- Power resistors or a 10 Ω 5 W resistor and a rheostat as load
- A digital multimeter (two if possible), a DC jack and binding posts, a plastic box
Safety
For this project: Take power only from a sealed ready-made adapter or battery. Never open the adapter or build any mains-side circuit. The regulator and load resistors get hot: an adult must supervise, and let them cool before touching.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Read the LM317 datasheet and note the pin order, the 1.5 A limit, the dropout voltage and the thermal limits.
Calculate R2 for outputs of 3, 5, 9 and 12 V with R1 = 240 Ω and compare with the 5 kΩ preset range.
Wire the input side: DC jack, 1000 µF capacitor across the input and a 0.1 µF capacitor close to the regulator.
Wire R1 from output to adjust, R2 preset from adjust to ground, and a 10 µF capacitor on the output with the protection diodes as shown in the datasheet.
Bolt the regulator to a heat sink and fix everything in a plastic box with binding posts for the output.
With no load, set the dial and measure the output at 3, 5, 9 and 12 V and compare with your calculated values.
Connect the 10 Ω load and the rheostat in turn, and record output voltage at load currents of 0, 100, 200, 400 and 600 mA at 5 V and at 9 V settings.
Compute load regulation as (V no-load minus V full-load) divided by V no-load, in percent.
Measure the temperature of the heat sink with a thermometer taped to it after 10 minutes at 500 mA and 5 V output.
Write the report with the circuit, the table, regulation graph and a note on efficiency, which is output power divided by input power.
What you should see
The supply gives a smooth variable output from about 1.3 V to 12 V. Load regulation is usually under 1 percent up to 500 mA, and the table shows efficiency dropping when the output is far below the input.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is the LM317 called a linear regulator?
It drops the excess voltage across itself as heat in series with the load, instead of switching like a buck converter.
2What is the purpose of the protection diodes?
They give the output capacitor a discharge path if the input is shorted, which would otherwise damage the regulator.
3What limits the maximum current?
The regulator has an internal limit near 1.5 A, but heat and the adapter rating usually limit it first.
4How would you improve the efficiency?
Use a buck converter such as the LM2596 or lower the input voltage to just above the output plus dropout.
Take it further
Add a second LM317 as a current limiter with a sensing resistor so that the supply works as a constant-current LED tester.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely