LM317 Variable Voltage Regulator                                                                                           Back        Home

In many cases we can manage with a suitable fixed voltage regulator such as 5v or 12v, but occasionally we need a non-standard voltage. In these circumstances the LM317 range of regulators proves to be very useful.

LM317 circuit

The output voltage is selected using two resistors. Normally R1 is chosen to be around 220R or 240R. The formula for calculating the value of R2 is V = 1.25(1+(R2/R1)) or to put it another way R2 = R1((V/1.25)-1).

For example, for an output voltage of 8.5v.   R2 = 220((8.5/1.25)-1) = 1276 ohms. The nearest preferred value to this would be 1K3.

Setting R2 to zero, ie. grounding the Adjust pin, will cause the output voltage to drop to 1.25v.

R2 may be replaced by a pot to give an adjustable output voltage range. The maximum voltage is approx. 37v although a higher voltage variant is available.

 A kit and PCB is available to make a variable voltage regulator using the LM317. Click on the picture for more details.
  LM317T variable voltage regulator kit