Re: TI-H: Automatic power switch


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Re: TI-H: Automatic power switch




A simple solution here is to use two diodes. Since a diode will pass
current in only one direction, you'll need a diode for the battery pack
and a diode for the AC supply jack:

adapter line in ------|>-----+
                             |
     Battery in ------|>-----+----- power out to circuit

The negative (or common) line from both sources should be tied together at
the negative supply terminal of the circuit.

Here's an explanation of why this circuit works:

Diodes conduct electricity only when they are forward-biased. Battery
power will probably always be connected to the circuit, so we'll go ahead
and make that assumption now. When there is no power from the AC adapter,
the battery-side diode is forward biased and conducts to power the
circuit.

When a (higher) voltage is connected to the adapter side of the circuit,
its diode will become forward-biased. However, since the adapter voltage
minus the .6v drop of the diode is still greater than the battery voltage
(it better be, or this circuit won't work), the battery-side diode gets
reverse-biased. It stops conducting electricity to power the circuit, and
it prevents power from flowing backwards to charge the batteries. This
leaves the adapter powering the circuit, with the batteries sitting in
reserve. If the adapter voltage dropped below the battery voltage, the
circuit would switch transparently to battery power.

Check the diode direction before you build the circuit.. sometimes I go
dyslexic and draw them backwards.


On Thu, 11 Dec 1997, MaxxKills wrote:

> Can anyone please instruct me on how to make a circuit
> that can be operated by both battery and ac adapter, and when
> it notices the ac adapter is not hooked, then it switches to batter power?

-- 
Greg Hill
greg-hill@bigfoot.com
www.comports.com/link




References: