1. Purpose of Voltage Regulators
A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate AC or DC voltages.
Electronic voltage regulators are found in devices such as computer power supplies where they stabilize the DC voltages used by the processor and other elements. In automobile, alternator and central power station generator plants, voltage regulators control the output of the plant. In an electric power distribution system, voltage regulators may be installed at a substation or along distribution lines so that all customers receive steady voltage independent of how much power is drawn from the line.

2. Electromechanical regulator

3. Voltage sources
Voltage sources provide constant voltage as long as the amount of current drawn from the source is within the source's capabilities, its output voltage stays constant. An ideal voltage source provides no energy when it is loaded by an open circuit (i.e. an infinite impedance), but approaches infinite power and current when the load resistance approaches zero (a short circuit).

4. CONSTANT CURRENT SOURCE
a current source provides a constant current, as long as the load connected to the source terminals has sufficiently low impedance. An ideal current source would provide no energy to a short circuit and approach infinite energy and voltage as the load resistance approaches infinity (an open circuit).

5. LM 317-CONSTANT CURRENT SOURCE

6. The regulator is set up as a constant-current source
The regulator is set up as a constant-current source. (As opposed to a constant-voltage source, which is how linear regulators are used most often.) An LM317 style regulator strives to always keep the voltage drop from OUT to ADJ equal to 1.25V. If you put a resistor between the two (R1), the regulator puts out whatever current is required to keep the voltage 1.25V while satisfying Ohm's Law: I = 1.25 / R2

7. LM317-PARAMETERS Parameters / Values LM317
Output Current: 500, 1000, 1500 mA
Input Min Voltage: 4.2 Volt
Input Max Voltage: 40, 37 Volt
Volt Adjustable Output: Yes
On/Off Pin: No
Power Good / Error Flag: No
Temperature Min: -40, 0 deg C
Temperature Max: 125 deg C
Reg Type: Linear Regulator