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Why 3 terminal regulator operate only 1 power supply if used in dual?

Latest Updated:12/01/2011

Question:

If a 3-terminal regulator is used in a dual power supply circuit, only one of the power supplies operates.Why?

Answer:

The IC does not operate if a power is supplied to its output pin from an external source.
In the case of a dual power supply circuit, a reverse voltage is supplied to one IC with the load if the other is activated first.
In this state, the IC does not operate.

Once the IC has fallen in this condition, it does not recover until the voltage applied to all the pins reaches zero.
Protect the IC with a diode (such as Schottky barrier diode) with a low forward voltage so that a reverse current does not flow through the output pin.

Suitable Products
3 Terminal Regulators