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.
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