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 to TR7 inclusive. Resistors R21 to R25 are current sharing resistors and the voltage across them is proportional to the current drawn by the PSU load.
When the current through R21 reaches 8A (i.e. a total PSU current output of 40A) the voltage across R11 and R12 is 0.88V and is tapped from R I I to bias a transistor inside IC1 which robs IC1: 10 of some current, forcing the PSU into a current limiting mode.
