LM317T Voltage Regulator with Pass Transistor

Posted on Feb 6, 2014

The LM317T output current can be increased by using an additional power transistor to share a portion of the total current. The amount of current sharing is established with a resistor placed in series with the 317 input and a resistor placed in series with the emitter of the pass transistor. In the figure below, the pass transistor will start con

LM317T Voltage Regulator with Pass Transistor

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ducting when the LM317 current reaches about 1 amp, due to the voltage drop across the 0. 7 ohm resistor. Current limiting occurs at about 2 amps for the LM317 which will drop about 1. 4 volts across the 0. 7 ohm resistor and produce a 700 millivolt drop across the 0. 3 ohm emitter resistor. Thus the total current is limited to about 2+ (. 7/. 3) = 4. 3 amps. The input voltage will need to be about 5. 5 volts greater than the output at full load and heat dissipation at full load would be about 23 watts, so a fairly large heat sink may be needed for both the regulator and pass transistor. The filter capacitor size can be approximated from C=IT/E where I is the current, T is the half cycle time (8. 33 mS at 60 Hertz), and E is the fall in voltage that will occur during one half cycle. To keep the ripple voltage below 1 volt at 4. 3 amps, a 36, 000 uF or greater filter capacitor is needed. The power transformer should be large enough so that the peak input voltage to the regulator remains 5. 5 volts above the output at full load, or 17. 5 volts for a 12 volt output. This allows for a 3 volt drop across the regulator, plus a 1. 5 volt drop across the series resistor (0. 7 ohm), and 1 volt of ripple produced by the filter capacitor. A larger filter capacitor will reduce the input requirements, but not much.

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