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Trap Door Project: DC BATTERY BACKUP for 12VDC Linear Actuator

Discussion in 'Sensors and Actuators' started by mjosbesh, Feb 27, 2016.

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  1. mjosbesh

    mjosbesh

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    Feb 27, 2016
    I've made a trap door to hide emergency supplies in my home. It is opened by a 12VDC motor connected to a 12V power supply which is plugged into a standard 120VAC outlet.
    I'd like to add a battery backup that is small and can power the motor if the power to the house goes out.
    I'd like the backup to be always active if possible... meaning i don't want to have to attach any wires or flip any switches to engage the backup. Everything will be housed behind the trap door.
    Any help is greatly appreciated!
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    12v battery with a trickle charger connected to the mains.

    One question though, how are you going to repair the setup if anything should go belly-up since you have "everything housed behind the trapdoor".
    A bit like having a safe with the spare key or the combination locked inside.
     
    mjosbesh likes this.
  3. mjosbesh

    mjosbesh

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    Feb 27, 2016
    Hahaha. What could go wrong?
    Just kidding. That's a good question. I used a shear pin on one of the actuator connections so I can break in w/ minimal damage if needed.
    Thanks for the advise. Do i need to worry about the trickle charger overcharging the battery? Also, do you recommend a specific type of battery for this use?
     
  4. AnalogKid

    AnalogKid

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    Sealed lead acid (SLA) batteries are built for long life in backup power applications. Depending on the motor energy required, you might not need anything more than a large capacitor, and at 12 V you can get a whole lot of capacitance for half the cost and 1/10th the effort of doing a small UPS system.

    If you still wanna, then a minimal effort would be 1 resistor and two diodes. The battery never will be completely topped off, but who cares? Even a small SLA will run your motor for many minutes, long enough by far in this application.

    ak
     
    mjosbesh likes this.
  5. cjdelphi

    cjdelphi

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    Trickle charge is there to keep the battery toppex up to 13.8/14.2 volts, the charger will dump a lot of current until it's nearly charged then switches to a 100ma or so to keep it topped up...

    Perfectly safe ..
     
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  6. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    .


    Sir mjosbesh. . . . . . .



    Since you are so big on the "hiding" aspect I will throw in a "hidden" tidbit for you .

    I once limited access to an area of mine with a door, wherein I had its trim nailed on with multiple common 10 # box nails.
    The "critical" nail was being placed at about shirt pocket height, on the left side of the door (hinge side) that went through the trim, but
    then it had a magnetic reed switch heat shrinked to it on its inner exposed 2" end's longitudinal axis.
    The reed relay then keyed in another POWER relay to handle the real power to a door solenoid.

    Always stored nearby was the key . . . . a strong rare earth magnet.

    The magnet when placed to the outside flat nail head, activates the relay.
    In 25+ years of use it has never been compromised, and many . . .to the power of 100's . . .times a person was standing right at my side when I opened the door.
    The stealth procedure was to have the magnet stored nearby, palm it in the left hand and move right up to the door and "brace" yourself to the left side of the door with your left hand
    to be justified by your then stretching waaaaaaaaay up to the right topmost corner of the door to "open" its "lock" with your right hand.
    In reality, the then totally ignored left hand was micro positioning the magnet up to the nail head.

    Thassssit

    Next forthcoming question . . . detected by ESP . . . to be pre answered . . . .what's a magnetic reed switch ?
    . . . .
    [​IMG]

    73's de Edd
     
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  7. mjosbesh

    mjosbesh

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    Feb 27, 2016
    Thank you all! This is all great input!
    This is a great idea. I'll order some reed switches to test out. Thank you.
     
  8. mjosbesh

    mjosbesh

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    Feb 27, 2016
    Great. I'd like to use a small SLA if possible so I'm interested in learning more about using a large capacitor. Can you expand on this a little?

    Some specs:
    The 12v actuator requires up to 3 amps for 3 seconds when opening or closing the door and I'd like to be able to open and close a few times in a row w/out issue.

    Questions:
    1) Will using a capacitor make the actuator start out strong then slow through the 3 or 4 second transition?
    2) When i reverse the direction of the motor it will reverse polarity. Will this be a problem when using a capacitor?
    3) Can you possibly recommend an example battery & capacitor combo that i can look at to help me better understand?

    I don't know much about capacitors so hopefully I am asking relevant questions :)
     
  9. AnalogKid

    AnalogKid

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    http://www.powersonic.com/images/powersonic/sla_batteries/ps_psg_series/12volt/PS-1212_11_Feb_21.pdf
    Powersonic is my fav SLA company. They have excellent products and app notes.

    I still think a traditional charger is overkill for you and this application. So what if the battery is not topped off with a 3-stage wonder-charger. A wall wart and a resistor is all that is needed; the battery will act as the system's voltage regulator. If the battery has to power the system through multiple cycles it will get discharged, and with an ultra-simple charger it will take 24 hours to recover. If that's ok, this is pretty simple.

    ak
     
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  10. mjosbesh

    mjosbesh

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    Feb 27, 2016
    Thank you. I think that would be fine. To clarify, are you saying I should use a wall wart charger plus an added resistor? What is the downside if I just connect the wall wart charger directly to the battery?
     
  11. Alec_t

    Alec_t

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    Jul 7, 2015
    The battery will quickly die from being overcharged.
     
  12. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Just use 8 D cells or two lantern batteries. These will last 5 years and a backed-up battery will only last 5 years as it will dry out.
    .
     
    mjosbesh likes this.
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