@▷ LM338 and LM350 Dual Power Supply | Diagram for Schematic

LM338 and LM350 Dual Power Supply

Circuit Project Description

The circuit was designed based on two types of voltage regulator which utilizes three LM338 to produce a maximum 25V at 15A and one LM350 to produce a maximum of 25V at 3A.

Terminology LM338 – an adjustable 3-terminal positive voltage regulator capable of supplying in excess of 5A used as battery chargers, constant current regulators, and adjustable power supplies due to its features such as protected output from short circuit, product enhancement tested, current limit constant with temperature, guaranteed thermal regulation, adjustable output down to 1.2V, guaranteed 5A, and guaranteed 7A peak output currentLM350 – an adjustable 3-terminal positive voltage regulator capable of supplying in excess of 3A used in battery chargers, constant current regulators, and adjustable power supplies due to the features such as guaranteed maximum 0.3% load regulation, 0.01% line regulation, and 1% output voltage tolerance, 86 dB ripple rejection, product enhancement tested, current limit constant with temperature, protection from short circuit output, guaranteed thermal regulation and 3A current output, and adjustable output down to 1.2V Circuit Explanation

The first part of the circuit consists of parallel connection of three LM338 IC1-2-3 where 5A current is being provided by each regulator output. The circuit has the stability of regulating an output voltage range of 4.5V to 25VDC. The regulation of voltage is performed by the action of trimmer TR1. TR1 can be replaced by a resistor with constant value if the desired output voltage is not achieved. This can be ensured by measuring the resistance value of TR1 for a particular output voltage while continuously regulating. A metal box with fan should enclose the bridge rectifier BR1 to prevent from excessive rise in temperature. On the other hand, the circuit with which IC1-2-3 is located, could use a TO-3 case with the same dimension heatsink. The heatsink and the circuit is sufficiently isolated with a mica material to prevent short circuit. The connection of resistors R1-2-3 are located far from the PCB in order to prevent burning the board due to their high temperature during the operation. Since the current across the circuit is quite high, it is important to take note of the cables used by ensuring a large diameter with a suitable output plug. The cable is connected with caution to prevent short circuit as well.

The second circuit utilizes LM350 that will regulate a voltage range of 1.25V to 25V with the maximum current of 3A. A TO-3 with heatsink case can also be used for IC5 for better thermic conduction. The trimmer TR2 performs the regulation of output voltage. The two circuits are being supplied with 450VA for a power transformer. Both circuits should be manufactured with a suitable dimension having well ventilation. A fan driven by 230VAC should be placed in the box in case of excess temperature. For measurement, a voltmeter and ampere meter can be placed in the outputs.

Part List R1-2-3=0.1R 5W
R4=100R 0.5W
R5=150R 0.5W
R6-8=330R 0.5W
R7-9=4K7  0.5W
R10-13=1K5 0.5W
R11-12=220R 0.5W
TR1=1K5 trimmer
TR2=4K7 trimmer
C-2=10000uF 63VC3-5-8-10=100nF 100V MKT
C4=220pF 100V ceramic
C6-11=47uF 63V
C7=4700uF 63V
C9=10uF 40V
D1-2=Led Red 5mm
D3=1N4007
BR1=Bridge Rectifier >250V 35A HS
BR2=Bridge Rectifier >250V 3A
Q1=2N2905
IC1-2-3=LM338 [TO-3 case] HS
IC4=LM308
IC5=LM350 [TO-220 or TO-3 case] HS
J1=3 terminal block computer type
J2-3-4-5=2pin Connector 3.96mm
F1=Fuse 5A slow block
T1=230Vac/22Vac 600VA transformer
HS: Connect to HeatsingApplication

This dual power supply circuit can be used for general purpose applications due to their clean and reliable outputs that is suitable for versatile benchtop testing and other bench requirements. They can be an essential feature for a tight budget individual or company. Some of the models available in the market are having separate digital panel meters for monitoring current and voltage simultaneously.

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