ARudzki wrote:
>
> Does someone have a simple switch to switch out a 3.6 volt battery when a 5
> volt voltage source is present?
>
> I have a card I'm making using a PIC to keep time.  The battery is 3.6
> volts, but when the card is installed, I would rather use the systems 5
> volts instead.
>
> Thanks,
>
> Tony


You could consider the use of a simple diode, cathode to the +3.6, anode
to the circuit, so any voltage higher than the +3.6 (minus the diode
voltage drop) applied directly to the circuit would cut off automaticaly
the +3.6V battery current.  A possible resistor (lets say 200 Ohms) in
parallel to the diode, would try to recharge the battery with a back
current of aprox 7.5mA from the +5Vdc.

If your circuit can works with +3.0 or +2.9Vdc, the solution above is
satisfactory, since the diode holds 0.6 to 0.7 V from the +3.6Vdc.

You have several other choices:

Use a schottky diode, voltage drop is around 0.2Vdc.
If the circuit current is lower enough, you can even eliminate the
diode, and install just the resistor, why?  Calculate a resistor value
that doesn't drop too much the +3.6Vdc without the +5Vdc, and would set
the battery recharge current in presence of +5Vdc.

Any other silicon solution would always drop some voltage (0.2~0.7 Vdc)
as a switch. If you can't affort that, unfortunately you would need to
use a small relay to remove the +3.6Vdc after few seconds in the
presence of +5Vdc.

--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
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