Increasing LM317 Current with a Transistor

Increasing LM317 Current with a Transistor could be implemented in order to contribute an integral part of the overall current.

The level of current contribution is ascertained using a resistor added in series with the 317 port in addition to a resistor installed in series with the emitter of the pass outboared transistor. In the figure below, the pass transistor should begin operating as soon as the LM317 current becomes approximately 1 ampere, because of the voltage drop across the 0.7 ohm resistor.

Current restricting transpires at roughly around 2 amps for the LM317 which is able to shake off around 1.4 volts over the 0.7 ohm resistor and then generate a 700 millivolt drop across the 0.3 ohm emitter resistor. Consequently the overall current is confined to approximately 2+ (.7/.3) = 4.3 amps.

The input voltage should always be around 5.5 volts in excess of the output at optimal load and heating dissipation at maximum load could well be approximately 23 watts, which means that a pretty massive heat sink might be essential to both of them the regulator and pass by transistor.

The filter capacitor specifications is usually estimated from C=IT/E in which I is the current, T is the half cycle time (8.33 mS at 60 Hertz), and E is the drop in voltage that could materialize in the course of one half cycle.

To retain the ripple voltage below 1 volt at 4.3 amps, a 36,000 uF or bigger filter capacitor can be used. The power transformer needs to be huge in order that the peak input voltage to the regulator continues to be 5.5 volts above the output at maximum load, or 17.5 volts for a 12 volt output.

This helps for a 3 volt decrease across the regulator, additionally a 1.5 volt drop across the series resistor (0.7 ohm), and 1 volt of ripple caused by the filter capacitor. A more substantial filter capacitor minimizes the input specifications, however not a great deal.

The complete diagram for implementing the proposed idea os Increasing LM317 Current with a Transistor can be seen below.

LM317 high current power supply using transistor