Jan 022021

5A Variable Power Supply with LM338 voltage regulator

The 5A Variable Power Supply uses the LM338 voltage regulator. This is a three terminal adjustable regulator that is capable of delivering up to 5A at a voltage range of 1.2 to 30 volts.

This regulator has the special characteristic that it allows current peaks of up to 12A, for a short period of time. A thermal overload protection is also included in the regulator.

The LM338 regulator works the same as the lm317 adjustable voltage regulator circuit, known to all.

5 A adjustable Voltage Supply Operation

The signal from the secondary winding, now reduced in voltage, is applied to a diode bridge (DB) that rectifies it, and is then flattened by the electrolytic capacitor C1 (filter).

5A Variable Power Supply with LM338 Voltage Regulator

At this time the signal at the positive terminal of capacitor C1 is about 33VDC unregulated. At this point a LED (D1), with its limiting resistor, is placed in order to indicate the operation of the power source.

The signal on the capacitor is applied to the input of the voltage regulator (In). The voltage at the regulator output (Out) will depend on the value of the voltage that exists in the adjustment pin (Adj) of the regulator. This voltage can be changed by means of the VR potentiometer connected to the adjustment pin (Adj).

The output voltage is obtained by the following formula: Vout = 1.25 (1+ VR / R3)

LM338 Variable Voltage Regulator Packages

LM338 Variable Voltage Regulator Packages

5A Variable Power Supply Component List

  • 1 120/240 to 24VAC transformer (T)
  • 1 LM338 variable voltage regulator (U1)
  • 1 10 A, 400 V diode bridge (DB)
  • 1 10,000 uF (microfarads) electrolytic capacitor (C1)
  • 1 2,200 uF (microfarads) electrolytic capacitor (C3)
  • 1 0.1 uF (microfarads) capacitor (C2)
  • 1 2.7 k resistor (R1)
  • 1 12 k resistor (R2)
  • 1 220 ohm resistor (R3)
  • 1 10k potentiometer (VR)
  • 2 1N4007 rectifier diodes (D2, D3)
  • 1 red LED (D1)

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