Battery Backup.
PLJack
Posts: 398
I need to design a circuit that will switch to a battery if the power feed goes out.
This is on a vehicle. The scenario is listed below.
Both feeds are 5v.
One feed is coming from the 12v car battery via an inverter.
The other feed is coming from a "AA" battery pack.
Basically we need to watch the first feed, if that feed stops it needs to switch to the second feed.
My first attempt was with a 7404 quad inverter. Basically if the first feed stopped the second feed would fire off
a power transistor because the 7404 can not sink the servo current.
Anyway the circuit worked but there was not enough power to control the servo. And I feel the circuit was MUCH more complicated than it needed to be.
My confusion is with the grounds. The battery pack is a power source in its own right while the first feed is using the chassis ground.
Do I need to switch power and grounds?
Anyone have a simple solution for me?
Jack.
This is on a vehicle. The scenario is listed below.
Both feeds are 5v.
One feed is coming from the 12v car battery via an inverter.
The other feed is coming from a "AA" battery pack.
Basically we need to watch the first feed, if that feed stops it needs to switch to the second feed.
My first attempt was with a 7404 quad inverter. Basically if the first feed stopped the second feed would fire off
a power transistor because the 7404 can not sink the servo current.
Anyway the circuit worked but there was not enough power to control the servo. And I feel the circuit was MUCH more complicated than it needed to be.
My confusion is with the grounds. The battery pack is a power source in its own right while the first feed is using the chassis ground.
Do I need to switch power and grounds?
Anyone have a simple solution for me?
Jack.
Comments
Why wouldn't a pair of diodes work? Tie both grounds together, and tie both diode cathodes together as your positive supply to your circuitry. Tie each
diode anode to each supply source ... one from car and the other from the battery pack.
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Beau Schwabe - Mask Designer III
National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071
Both power sources can share a common ground (ie both negative terminals can be connected).· One easy solution is to use a NC (normally closed) relay that is feed by the primary power supply.· Normally closed means it is a closed circuit until it is switched "on" with an input current.· Just make sure that it's rated for the current you need, and that the primary power source can supply enough current to operate it and keep it open. Once your primary source quits, the relay will close and connect your alternate supply.· When the primary source goes back on, it disconnects the alternate source again.
You can either use a mechanical relay (and you'll need a shunt diode for that), which usually require a high amount of current (usually more than the BS2 can source), so you'll probably need a transistor too ... or you can use a solid state relay, which can operate on as little as 5mA and source up to 4.5Amps (but they're pricier).· PVN012 is a good, low resistance SSR, but is up around $10 now I think.
Hope that helps...
Dave
Dave
Easiest thing "mentally" I'd say would be a relay that uses the normally closed switch to send power from the battery pack to the circuit you needed the 5V for.
When there is power from the car it could keep the relay powered and have the SPDT relay contact give power from the car to the circuit. When the power from the car is gone the relay de energizes and closes the contacts to give 5V form the battery pack.
Or you can use some sort of fet or something. But the relay would be easy and only need one transistor and a resistor (and maybe a diode to shunt the high voltage from the transistor when the relay coil is de energized).
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http://www.paysonarizona.net/
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XYZZY...PLUGH...*poof*
Post Edited (Paul Baker) : 2/4/2005 10:08:02 PM GMT
Thank you for taking the time to reply.
Seems everyone would agree about connecting the grounds together.
I like the relay or "normally closed" approach.
Although I really like IC's. [noparse]:)[/noparse]
I let you know what I settle on.
Actually there is an inverter on this machine. Now that I think about it we switched
to a 12V to 5V regulator (modified cigarette lighter type).
So you are right [noparse]:)[/noparse]
Thanks again.
Post Edited (PLJack) : 2/4/2005 10:48:16 PM GMT
The only way I see that both power sources would be drained is that if both power sources are sitting at the same voltage potential. (See attachment)
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Beau Schwabe - Mask Designer III
National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071
Post Edited (Beau Schwabe) : 2/4/2005 11:31:12 PM GMT
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--==<{Chris}>==--