Battery Backup.

PLJackPLJack Posts: 398
edited 2005-02-05 - 21:31:57 in General Discussion
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.

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,432
    edited 2005-02-04 - 20:22:56
    Jack,

    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
  • achilles03achilles03 Posts: 247
    edited 2005-02-04 - 20:25:24
    First of all, I think you mean regulator, not inverter...

    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
  • achilles03achilles03 Posts: 247
    edited 2005-02-04 - 20:28:41
    Beau Schwabe said...
    Jack,

    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.

    Good idea, but the problem with that is that both power sources can be drained, depending on the resistance paths.· That means your AA's could go kaput even without a primary power interruption, and they might die relatively quick.

    Dave
  • paysonbadboypaysonbadboy Posts: 81
    edited 2005-02-04 - 20:32:08
    I'd at least tie both grounds together so you're alwayls looking at the same potential from ground and just switch the +5V from one to the other.

    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/
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-02-04 - 21:02:16
    In this application tying the grounds together shouldn't be too much of an issue, you may want to place a choke on the battery ground to prevent noise loops. You could use a high side current monitor to monitor the car batteries connection and use the sign output to switch over to the backup battery (this of course would be powered by you backup battery). There are of course are several other way of accomplishing this.
  • Tom WalkerTom Walker Posts: 509
    edited 2005-02-04 - 21:42:14
    This sounds like a slight modification to a "battery backup" discussed elsewhere...or am I missing something?

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  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-02-04 - 21:50:51
    Your likely correct but that post was in the wrong forum, this is the proper forum for such discussions, I frequently find stampers misposting into the stamp forums when the topic belongs in the Sandbox (the SXers don't seem to do it as much though some are equally guilty). "How to control 110v AC" is a good example (no personal offense meant to poster), because he desires to control it with a stamp doesn't make it a stamp related question, hes asking a more generalized question regarding control of power lines, therefore the post belongs here. As such, while I have considerable experience interfacing external devices with microcontrollers, because I use the SX and not the stamp, there's a chance I will never see his post (I try to hop over there occasionally because of this phenomenon).

    Post Edited (Paul Baker) : 2/4/2005 10:08:02 PM GMT
  • PLJackPLJack Posts: 398
    edited 2005-02-04 - 22:41:32
    Great suggestions people.
    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.

    said...
    First of all, I think you mean regulator, not inverter...
    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
  • Beau SchwabeBeau Schwabe Posts: 6,432
    edited 2005-02-04 - 23:27:07
    Dave said...
    Good idea, but the problem with that is that both power sources can be drained, depending on the resistance paths. That means your AA's could go kaput even without a primary power interruption, and they might die relatively quick.

    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)

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    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
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-02-05 - 21:31:57
    Tom is right, there was a discussion which I started back some time ago about higher-current backups systems, like those using a Lead-Acid battery.· I cannot seem to locate the post.· A Car system would undoubtedly compare.· I believe the answer came from Tracy Allen, who also mentioned the 2 diode system.· Which also means Beau is also right.· As long as the potential from the car's system is higher, the batteries won't drain.· In fact I believe there will be a slight reverse trickling effect on the batteries which would keep them fresh, although, Tracy was the one who supplied these details. I will look once more for the thread...



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