Programmable Current Source by LM317

Accurate and stable current supplies are generally needed in electronic labs especially for creating V-I curves, charging and discharging batteries, preloading power supplies and for many other purposes and applications.

This circuit provides a current supply that can be programmed by using three digit BCD switches that allows us to set the current from 1 mA to 999 mA. It is really a low cost easy to build and easy to use device. The components are a three terminal adjustable voltage regulator – LM317 , three BCD switches (shown in the figure 1) and a handful metal film (1% tolerance) resistors.

The circuit operation is very easy. First connect a DC power supply’s (3V to 40V) positive terminal to the “IN” and ground terminal to the “ADJ” pin of the LM317. For instance when you set the MSD (Most-Significant-Digit) to two and the other ones to zero, the BCD switch connects the 6.2 Ohm resistor from the LM317’s output to adjust pin. The LM317 forces 1.25 V across the 6.2 Ohm resistor, causing 200 mA to flow from the output pin through the resistor, and to the ground of the power supply.

The circuit maintains this regulation as long as the input voltage remains between 3 to 40V.

The voltage regulator LM317 must be cooled by connecting a heatsink. You must isolate the heatsink from the LM317 by using thermally conductive insulators.

While determining the resistors first take the base resistor of the digit and then parallelly connect it to obtain the other resistors’ values. For instance for the second digit to obtain 31 Ohm parallelly connect four 124 Ohm (base resistor of the second digit) resistors. Using this approach with ¼W resistors ensures that the highest current resistors don’t overheat.

The circuit performance is 2% accurate. You can increase the accuracy by choosing the resistors by measuring each one and finding the most suitable ones.

John Guy, National Semiconductor, Santa Clara, CA; Edited by Charles H Small and Fran Granville — EDN, 6/12/2008