Secondary Power-On Delay

00 The circuit described here enables short-circuit protection and power-on delay to be added to a power supply. Power supplies with a large reservoir capacitor may draw such large currents on switch-on that problems occur, even at the primary of the mains transformer. Particularly when a toroidal mains trans- former is used, it may be necessary to fit a much heavier primary fuse than is desirable for normal protection. The current in the secondary is limited by a resistor, RI, in series with the reser- voir capacitor, CI. A few seconds after switch-on, RI is short-circuited by a relay contact. Compared with switching at the primary side, this method has the advantage that no separate supply for the relay is necessary and that this does not have to switch the 240 V mains. 12...15V SECONDARY POWER-ON DELAY 15V""v 8A C3 7000p 25 V cl 0 884039 SUPPLEMENT 7 EE JULY/AUGUST 1988 Operation is fairly simple. After switch- on, Ci is charged slowly via RI. After a few seconds, the output ...