Adolfo Mondragon
EDN
The LM317 voltage regulator is well know for its versatility and ability to perform many different and varied functions.
The standard and classic is the adjustable positive voltage regulator with a three terminal configuration. Another important and simple configuration of the LM317 is that of a two terminal current regulator.
But when the adjust terminal and output terminal are tied together, the LM317 is converted to a two-terminal component which behaves equivalently to a 1.5 V, high-current Zener diode. This concept can provide a versatile and very useful circuit configuration that will allow it to source or sink a high current in either a positive or a negative voltage supply.
Tying the Adjust terminal to the Output terminal forces the internal regulator to compensate, pushing the internal NPN transistor into saturation. The NPN transistor in saturation has a VCE(sat) voltage drop of about 0.20 V-0.25 V which, when added to the 1.25 V drop of the internal regulator, will yield a total voltage drop of 1.45 V-1.50 V.
Using the LM317 in this configuration causes it to behave similarly to a Zener diode with a dynamic internal resistance of approximately 1 Ω or less. This dynamic internal resistance will provide a stiff reference, holding the voltage drop of the modified LM317 constant to within about 0.1 V for every100 mA change in current through the device.
