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LM317 Regulator Circuit: Build Your 13.6 Volt Power Source
Description
The LM317T is a variable 3-terminal positive voltage regulator capable of delivering more than 1.5 amperes across an output range from 1.25 to 37 volts. This device incorporates built-in current limiting and thermal shutdown, contributing to its robust and dependable operation. It's designed to minimize the risk of failure due to excessive current or overheating.
The output voltage is adjusted using two resistors, R1 and R2, connected as depicted below. The voltage across R1 remains constant at 1.25 volts, while the adjustment terminal current is limited to less than 100uA. The output voltage is approximately calculated using the formula Vout = 1.25 * (1 + (R2/R1)), disregarding the adjustment terminal current; however, this approximation remains accurate when the current flowing through R1 and R2 is significantly greater. A minimum load requirement of about 10mA necessitates a resistor value for R1 that can drop 1.25 volts at 10mA, which is approximately 120 ohms. Utilizing a resistor value smaller than 120 ohms ensures that the minimum current remains above 10mA. The following illustration demonstrates a use of the LM317 as a 13.6 volt regulator. The 988 ohm resistor for R2 can be constructed by combining a 910 ohm resistor and a 75 ohm resistor in series.
Upon cessation of power to the regulator, the output voltage should decline more rapidly than the input. In the event of a slower decline, a diode can be placed across the input/output terminals to safeguard the regulator against potential reverse voltages. A 1uF tantalum capacitor or a 25uF electrolytic capacitor placed across the output enhances transient response, and a small 0.1uF tantalum capacitor is suggested across the input when the regulator is situated a considerable distance from the power supply filter. The power transformer must be sufficiently sized to maintain the regulator’s input voltage at least 3 volts above the output at full load – a requirement of 16.6 volts for a 13.6 volt output.
Circuit diagram
