Vehicle power supply controller

Discussion in 'The Projects Forum' started by terratoss, Nov 19, 2012.

  1. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    Hi guys, I am thinking of building a car battery power supply controller cum timer cum minimum voltage cut-off. The controller is to be used for supplying power to my in-car camera, so during parking mode, it still has power supply to keep it going for the night(8-12hours). I am at a lost how to plan the schematics. Any kind souls can help me out in this?

    Car battery: 12V 60Ah
    Camera: 5V 0.5A

    Thank you all in advance!
     
  2. wayneh

    Expert

    Sep 9, 2010
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    In-car camera? Automotive modifications are a forbidden topic here.

    How is it normally powered, and why can't you use it the normal way?
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I would use a rechargeable battery say 6 volts and use the car battery to keep it charged up, otherwise your using an LM7805 regulator which is dropping 7 volts at 0.5 amps.
     
  4. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    My apologies, maybe I wasn't specific enough. I am talking about a car video camera that captures video while you are driving. Normally the vid cams has a battery in it but it cant last the whole night. When the vid cam's battery cant supply enough power till I start the engine again that next morning, there might be a gap in recording which something crucial might happen at that moment which will not be captured on video.
     
  5. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    Thanks for your suggestion. If I will to use a set of rechargeable batteries of 6v to supply power to the camera, the batteries usually will take at least 2-4hours to be fully charged. Therefore, probably after the initial discharge of power, I wouldn't be driving long enough to charge up the batteries.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    I think this is legit. It is not an auto mod. As long as it only plugs into the 12V accessories outlet.
     
  7. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    Thanks Mr Chips for your input in this. Do you have any idea for the schematics for my project?
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Thanks for clarifying.

    Sorry if this is obvious to everyone else but I still don't grasp what the problem is.

    So, the camera runs fine off the cigarette lighter (or however it's powered) but not when the car is off? My van has a direct-to-battery receptacle and I love it for that reason. You don't need the keys to get power. Would that solve the problem?
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    0.5A at 8.5V drop is not too bad. I think a LM7805 voltage regulator should be fine.
    No need to use rechargeable batteries. Just run the camera entirely off the 12V outlet.
     
  10. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    My vehicle dosen't have a direct-to-battery receptacle. I also need something to have a safety cut-off just in case the car battery voltage drops to a certain level before it's juice runs below power level needed to crank the starter for the engine.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    OK, now I get it. :D

    I think you can get at battery power from the in-car fuse box, so let's assume you can that worked out, maybe by installing a new receptacle that is always hot.

    Now you need to drop the voltage and, as you mentioned, power down if supply voltage (the battery) drops too low.

    I agree that a 7805 on a heat sink is the way to go. Quick and easy, just look up the datasheet for a schematic. It WILL get hot when the camera is running and trying to charge its batteries, so give it some room and maybe some airflow on that heat sink.

    I can think of a few ways to do the cut-out but I think I recall someone else here had a good solution that's probably more elegant than my comparator-based idea. Can't think of it at the moment. Maybe someone else remembers.

    If not: The idea is to just divide down the battery voltage, say to 1V from 12V, and compare that to a 1V reference which you obtain by dividing down the 5V from your 7805 regulator, again down to 1V. If the battery voltage stays above 12V, the comparator will hold a power MOSFET on, allowing current to flow to your load, your camera. If it falls below 12V, the comparator turns the MOSFET off and no more juice to the camera. The comparator and regulator would continue to draw current, but at a tiny rate.
     
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  12. wayneh

    Expert

    Sep 9, 2010
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    Agreed, per #4.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    Connect a 12V zener and a resistor from 12V to GND, cathode of zener to 12V, 100k ohm resistor from anode to GND. Connect node to gate of MOSFET.
     
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  14. wayneh

    Expert

    Sep 9, 2010
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    Sweet. I knew there was a more elegant way, and that's it. ;)
     
  15. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    hm.. ok.. let me go read up on those parts.. it has been so long since i touch on electronics.. thanks for the tip!
     
  16. wayneh

    Expert

    Sep 9, 2010
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    You can use just about any NPN MOSFET, such as the IRF540N. I believe the Shack has the IRF510 in stores, which would also work. They also have a 12V zener. (I'm not a fan of the Shack, but I do enjoy the convenience.)
     
  17. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    This mosfet solve the min voltage cut-off portion.
    Is there anyway I could make a selectable voltage cut-off value e.g. 11.5v-12v?
    If I also want to add in a timer feature e.g. 8hrs to 12hrs, is it possible?
     
  18. wayneh

    Expert

    Sep 9, 2010
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    You can get a lot of different values on zeners - go shop at Mouser for instance. You really don't want to run your battery down that low, though.

    There must be a 12V timer out there but I've never used one. You would just put it upstream of the regulator and cut-out we've been discussing. I doubt you'll need to build one yourself.
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    I don't like this plan. You'll wind up with a dead battery and a car that won't start in the morning. Car batteries are designed with many thin plates for hundreds of Amperes discharge when starting the engine, and then to be charged back up immediately. Having the battery in a partially discharged state is terrible for it, and will lead to it falling apart inside.

    You really need an auxiliary deep-cycle battery to power the cam.
     
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