Power Supply Design
LM317T Variable Voltage Regulator
Some features or the LM317T voltage regulator are:
- an adjustable 3 terminal positive voltage regulator
- capable of supplying in excess of 1.5 amps
- voltage range from 1.25 to 37 volts
- has a built in current limiting and thermal shutdown making it practically blow-out proof
In the circuit below the output voltage is set by two resistors 100R and and 900R . The voltage across the 100R resistor is a constant 1.25 volts and the adjustment terminal current is less than 100uA. The output voltage approximated from Vout=1.25 * (1+(R2/R1)) which ignores the adjustment terminal current but will be close if the current through R1 and R2 is many times greater.
The second resistor (900R in the circuit) determines the output voltage of the circuit. This can be replaced by a 1K variable resistor if variable voltage is required
The transformer should have sufficient capacity so that the regulator input voltage remains 3 volts above the output at full load, or 16.6 volts for a 13.6 volt output.
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