The popular Variable voltage regulator IC LM317 can handle only up to 1 ampere current and hence it is not suitable for making a high current variable power supply. This circuit uses IC LM138 to make 1.25 to 28 volts variable power supply with a current up to 5 amps.
LM 138 is the high current voltage regulator IC which can handle up to 5 amps current .Like LM317,1.25 reference voltage exists between its adjust and output pins. Preset VR is used to set the required output voltage. LM 138 requires 240 ohms resistor between its adjust and output pins and hence R1 and R2 are used in series to get the required resistance. If 240 ohms resistor is available, it can be used. Diode D1 is used to protect the output of IC when C2 discharges at power off.
Input voltage can be obtained from 30 volt 1-5 amps transformer. Output will be 2 volts less than the input voltage supplied. IC LM M 138 requires heat sink to dissipate heat.
R1-22 Ohms, R2 220 Ohms
C1 10Uf 100V
IC LM 138