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Method to switch to alternate DC source? — Parallax Forums

Method to switch to alternate DC source?

T ChapT Chap Posts: 4,223
edited 2010-07-15 14:39 in General Discussion
I have an AC transformer that outputs unregulated 26VDC. I want to add a batter backup that kicks in if the main supply fails. The back up supply will be an unknown 3rd party battery with it's own charging system. Just curious if anyone knows what would be the simplest way to implement the switch to battery without crashing the system with a gap. The backup will be a 24VDC battery backup.

Comments

  • FranklinFranklin Posts: 4,747
    edited 2010-07-14 23:05
    You may be able to use the backup as the primary power like a ups. The backup should have a switchover circuit built in.

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    - Stephen
  • kwinnkwinn Posts: 8,697
    edited 2010-07-15 02:01
    A diode between the battery and the load, and a diode between the DC supply and the load. The source with the highest voltage will provide the current automatically. If the AC powered supply puts out a higher voltage than the battery it will supply current as long as it is powered.
  • ercoerco Posts: 20,260
    edited 2010-07-15 02:04
    24v is too high to apply directly to a Stamp's Vin pin. If you can switch to a 6-12V battery, just leave it hooked up to Vin with a floating charger indefinitely and you're done.

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    ·"If you build it, they will come."
  • davejamesdavejames Posts: 4,047
    edited 2010-07-15 03:01
    ...do the diodes as kwinn suggested; easy & semi-dirt cheap depending on the current requirement.

    DJ

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  • T ChapT Chap Posts: 4,223
    edited 2010-07-15 04:47
    Thanks guys.

    I use 2 amp slo blo fuses on the motor driver board that has the unregulated 26VDC off the rectifier. On a rare occasion under unusual heavy load, the 2amp fuse will blo but it is rare, a 3 amp doesn't usually blow, but the current sense resistor on the input to the board will get super hot under unusual load with the 3amp fuse. That being the case that 3 amp is typically a good maximum fuse choice, would a 4 amp Schottky be a good option for the diodes to be safe? I am making the assumption that the diodes need to be rated higher than the maximum current that the fuse will burn up at.

    Here is something I dug up online that I think represents your diode idea:
    batterybackup.jpg
    408 x 195 - 14K
  • kwinnkwinn Posts: 8,697
    edited 2010-07-15 13:25
    A 4A Schottky diode should be fine as long as the fuse is not a slow blow. The peak current during heavy or unusual load may be high enough to blow both the fuse and diode.

    PS - make sure the 26VDC is a DC level and not just a rectified AC waveform, otherwise the battery will provide the current when the rectified AC voltage drops below the battery voltage. This will result in the battery being discharged.

    Post Edited (kwinn) : 7/15/2010 1:33:30 PM GMT
  • davejamesdavejames Posts: 4,047
    edited 2010-07-15 14:39
    Best bet would be to check the diode's data sheet and see how it's rated for Peak Current and for How Long it can maintain it. If that's not possible to do (no data sheet), then go 2X your expected max current.

    When dealing with power-oriented subjects, 2X is not a bad rule-of-thumb.

    DJ

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