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40A Power Supply Unit

40A Power Supply Unit

This Project describes a Protected 13.2V 40A Power Supply Unit which, provided the heat sinking and cooling fan are suitable, is capable of running at full power at 100 percent duty cycle.
It features soft starting, over voltage, current limiting, short circuit protection and automatic fan control. It is a substantial project that is not for the novice constructor.

The heavy current carrying connections are shown in bold in Fig 1. The mains input passes through an EMC filter, protection fuse F1 and ON/OFF switch S 1 to transformer TI. The secondary output of TI is rectified by D I but cannot pass through open relay contacts RL1a. To start the PSU switch, S2 is operated allowing limited current to pass via R1 to slowly charge the reservoir capacitor C1. As C1 charges, the RL1 a pulls in closing the relay contacts and shorting out R1 and S2 placing the PSU in the ON state


Voltage regulator IC1 is the popular 723. A 7.2V reference on IC 1:6 is fed to non-inverting ICI:5. This is compared with a sample of the PSU output voltage via RI3, RI4 and R15 to inverting input at ICI:4. The 723 can source 150mA at ICl:10, so transistor TR2 acts as a Darlington driver for the pass transistors TR3 toTR7 inclusive. Resistors R21 to R25 are current sharing resistors and the voltage across them is proportional to the current drawn by the PSU load.

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