Opposite Phase AC is provided by the transformer's secondary and a grounded center tap. The single full wave bridge converts these into positive and negative DC voltages (with respect to grounded center tap). Filtering is provided by Capacitors C1 and C2.
The LM 340 IC provides regulation of the positive voltage, while the LM 320 regulates the negative voltage. It is very important to mention here that LM 320 has a different pin configuration than the LM 340.
The diodes provide protection, but ensure that they are not reversed. Diode D1 and D2 ensure that transient on the regulator outputs do not drive the outputs to a potential above their inputs and cause a damage to the regulators.
Also the two regulators may not turn on simultaneously. If this occur, the output of the slower regulator may be driven towards the potential of the faster one. Diodes D3 and D4 prevent these reverse polarities on start up.
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DC POWER SUPPLY
POWER FAILURE ALARM WITHOUT BATTERY