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.
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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.
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