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LM317T Voltage Regulator Circuit with Pass Transistor

This is the schematic diagram of voltage regulator circuit with pass transostor. The regulator is based regulator IC of LM317T. The LM317T output current can be raised by utilizing an additional power transistor (on circuit, it is 2N2955) to share a portion of the total current. The amount of current sharing is established with a resistor placed in series with the LM317 input and a resistor placed in series with the emitter of the pass transistor.

LM317T Voltage Regulator Circuit with Pass Transistor

In the above scheme design, the pass transistor will start conducting when the LM317 current reaches about 1 ampere, due to the voltage drop across the 0.7 ohm resistor. Current limiting happens at about 2 amperes for the LM317 which will drop about 1.4 volts across the 0.7 ohm resistor and make a 700 millivolt drop across the 0.3 ohm emitter resistor. Thus the total current is limited to about 2+ (.7/.3) = 4.3 amperes.

The input voltage will need to be about 5.5 volts higher than the output at full load and heat dissipation at full load would be about 23 watts, so a fairly large heatsink may be required for both the pass transistor and IC regulator LM317. 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 happen during one half cycle. To keep the ripple voltage below 1 volt at 4.3 amperes, a 36,000 uF or greater filter capacitor is required. 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 permits for a 3 volt drop throughout the regulator, as well as a 1.5 volt drop throughout the series resistor (0.7 ohm), and 1 volt of ripple generated by the filter capacitor. A bigger filter capacitor will minimize the input needs, although not significantly.

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