Switching between main power supply and spare power supply without any time delay issue (make use of the mechanical delay property of relays)
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**Assume 3V is Vref of upper OP-AMP
3.2V is Vref of lower OP-AMP:
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When V > 3, the upper OP-AMP set HIGH
and the lower OP-AMP set LOW
H || L => H (4V is activated))
meanwhile => not H => L (deactivated 6V)
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When V < 3, the upper OP-AMP set LOW
and the lower OP-AMP set HIGH
L || H => H (4V is activated))
meanwhile => not H => L (deactivated 6V)
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When 3<V< 3.2, the upper OP-AMP set HIGH and the lower OP-AMP set LOW
H || L => H (4V is activated))
meanwhile => not H => L (deactivated 6V)
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4V = major power supply
6V = backup power supply
