Using The LM317T To Regulate Voltage

Use an LM317T to regulate voltage in renewable energy projects

LM317T Voltage Regulator Chip used to regulate Voltage.

The LM317T Voltage Regulator (1) is an integrated circuit which is very useful in many renewable energy applications. It can be used to regulate current - for example to regulate the current in a string of LEDs (2) - or it can be used to provide a stable fixed voltage output.

* We have LM317T integrated circuits available in the REUK Shop (3).

Regulating Voltage with an LM317T

R1 is typically a 240 Ohm resisitor and should be in the range of 100-1000 Ohms. The voltage across R1 is 1.25 volts - the reference voltage from the ADJ leg of the LM317T - and so there is a 1.25/240 = 5 milliamp load. Using a 120 Ohm resistor for R1 would give the regulator a load of 10 milliamps which helps to give slightly better accuracy.

The value of R2 is used to set the output voltage of the circuit according to the following equation:

V_OUT = 1.25 * ( 1 + R2/R1 )

which can be rearranged to give:

R2 = R1 * ( (V_OUT/1.25) -1 )

NEW We now have a handy LM317 Voltage Calculator (4) which calculates R2 for any given value of R1 and desired output voltage. On the same page are two comprehensive tables (one grid, and one sorted list) presenting the output voltages given by different combinations of the most commonly found stock resistors.

Example - Recharging a Mobile Phone Battery from a 12V Battery Bank with an LM317T

Therefore, if for example you decide to recharge your mobile phone battery directly from a 12 Volt (nominally) battery bank (5) you would typically need a voltage output of around 5 volts, and an output current of around 500ma. The LM317T can handle average currents of up to 1.5 Amps so it will work well in this situation.

Assuming that a standard 240 Ohm resistor is used as R1 all we have left to do is calculate the value of R2 with a target output voltage of 5V:

R2 = 240 * ( (5/1.25) -1 ) = 720

Therefore since no-one manufactures 720 Ohm resistors, use the next larger available size - a 750 Ohm resistor.

LM317T Voltage Regulation Accuracy

In all these calculations it is important to remember that typical resistors have error ranges of up to 5%, and that the reference voltage of the LM317T can vary from 1.2 to 1.3 Volts - another 5% of error. Therefore if you cannot afford for the output voltage to be as much as ±10% away from the required voltage - an adjustable trimming potentiometer should be used for all or part of R2 enabling you to adjust the value of R2 until the output voltage is the chosen value.

Web Link References

(1) http://www.reuk.co.uk/LM317T-Voltage-Regulator-Chip.htm
(2) http://www.reuk.co.uk/Using-The-LM317T-With-LED-Lighting.htm
(3) http://www.reuk.co.uk/REUK-Products-For-Sale.htm
(4) http://www.reuk.co.uk/LM317-Voltage-Calculator.htm
(5) http://www.reuk.co.uk/Interconnecting-Batteries-for-Battery-Bank.htm

Article from REUK.co.uk:
http://www.reuk.co.uk/Using-The-LM317T-To-Regulate-Voltage.htm
Published: 7th March 2008
© REUK 2009