Aug 182020
 

How to make 4A Variable Power Supply using LM317 and transistor?

This 4A variable power supply using LM317 and transistor is useful when we want a variable power supply with a current greater than what a single lm317 regulator can deliver. The LM317 voltage regulator can deliver a maximum of 1.5 amps and in many cases it is insufficient or too tight.

4A Variable Power supply Operation

The operation of this voltage source can be understood by analyzing the values ​​of resistors R2 and R3.

Current flowing through voltage regulator LM317 passes through diode D5 and resistor R3. The diode has a voltage drop of 0.7 V., the same as the base emitter voltage (VEB) of transistor Q1. Because of this, the voltage across R2 equals the voltage across R3.

4A Variable Power Supply using LM317 & Transistor

4A variable voltage source with LM317 and current amplifying transistor

Since R2 = 0.3 x R3, three times more current flows through the transistor than through the regulator. So the capacity of the entire circuit is 4 times greater than if the LM317 regulator were alone. Also, the current limiting capability of the LM317 applies to the entire circuit.

Resistor R1 is placed before the regulator-transistor assembly, so as not to affect the output voltage, and is used to measure the current delivered to the load. I = V / R, since R = 1 Ω then I = V.

LM317 voltage regulator pinout

LM317T voltage regulator pinout

Then only the voltage at the ends of this resistor is measured, but the multimeter reading is interpreted as a current.

The voltage regulator offers the option of choosing between high and low voltage using a switch. This switch is used to choose between two different voltages at the input of the diode bridge.

When the source output voltage is very low, it is better to choose the low voltage option to avoid the regulator and transistor having to dissipate a large amount of power.

Circuit component list

  • 1  LM317T variable monolithic voltage regulator (U1)
  • 1  PNP 276-2043 transistor, NTE 219 or similar (Q1)
  • 5  common 50 Volts / 25 amp rectifier diodes (or a common 50 volt / 25 amp diode and a rectifier diode of the same characteristics)
  • 1  1 Ω, 1 watt resistor (R1)
  • 1  0.1 Ω, 5 watt resistor (R2)
  • 1  0.33 Ω, 5 watts resistor (R3)
  • 1  10 Ω, 2 watts resistor (R4)
  • 1  220 Ω, 1/2 watt Ω resistor (R5)
  • 1  2.5 KΩ potentiometer (R6)
  • 2  0.01 uF / 1 KV (1000 volts) capacitors (C1, C2)
  • 1  3300 uF / 35 volt or more electrolytic capacitor (C3)
  • 2  110 / 220 to 18 vac central tapped transformers, 4 to 5 amps
  • 1  1 amp fuse
  • 1  main switch for the circuit
  • 1  two-position switch (low voltage / high voltage). See diagram.
  • 2  heat sinks, for the LM317 regulator and the Q1 pass-through transistor.
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