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Powering one 12v device from two 12v sources

Discussion in 'Power Electronics' started by Ray Eyre, Feb 18, 2018.

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  1. Ray Eyre

    Ray Eyre

    17
    1
    Feb 18, 2018
    Hi to all,
    Please bear with me as I try to describe my needs.
    First a bit about myself: I am wheelchair dependent as a result of failed spinal surgery.

    O.K. What I want to do is to power the dash camera in my self drive car from two separate supply points from one 12v battery.

    Supply 1: Being from the back of the cigarette lighter socket via the AUX position on the ignition to a 12v to 5v converter. I know how to wire this.

    Supply 2: Direct from the 12v battery to the 12v to 5v USB converter. I know how to wire this.

    What I don't know: is how to stop the battery direct supply to the converter and the ignition AUX to the converter from back feeding each other. (i.e. the Aux circuit becoming active when the direct supply from the battery is used and vice versa) I have been told that this could be achieved by using 2 Schottky diodes wired into the positive leads from both supplies. If this is correct what value would these Schottky diodes need to be? I have no information on what amperage/current the dash-cam draws.

    The reason is that my dash camera has WiFi for transmission of the recorded video and I want to be able to download the video without having the laborious act of entering the car and have the ignition on AUX whilst doing so. Downloading time is dependent on video capture time and this can sometimes take an hour or more.

    I had achieved this in my Van by using a remote controlled switch (but this drained the battery over a period time as I don't drive very often)

    I hope this makes sense.

    Thanks Ray
     
    Last edited by a moderator: Feb 18, 2018
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,482
    2,830
    Jan 21, 2010
    Isn't the aux connector and the cigarette lighter powered from the same battery?

    In any case, if both supplies share the same ground, simply use diodes to isolate them from each other. These will drop between half and one volt depending on the current, but if you're powering a 12V to 5V converter that won't be an issue.
     
  3. Kabelsalat

    Kabelsalat

    154
    28
    Jul 5, 2011
    I qyestion the same as Steeve - ot seems like you describe one supply that have two paths to target. Why not just using the battery directly (via fuse) and just use a simple mecanical on/off switch.
     
  4. Ray Eyre

    Ray Eyre

    17
    1
    Feb 18, 2018
    There are TWO pathways to the one device (1) direct from the cars battery (2) Via the AUX position of the ignition.

    Do you have any idea as to what value diodes would be needed.

    Thanks
     
    Last edited: Feb 18, 2018
  5. Ray Eyre

    Ray Eyre

    17
    1
    Feb 18, 2018
    Yes. there are TWO paths to the one target and each needs to be isolated from the other.

    If a switch is used to activate direct via fuse from the car's battery the power will also be fed into the Car's Aux circuit as it will not be isolated.

    I wish to retain the function of the camera automatically powering on when the car is started and also having the ability to power the camera via the second pathway to rid me of the laborious task of getting myself into the car to get at the ignition switch.

    My car is right hand drive and my entry into the car is via an automatic door on the left. This door also has an electrically operated lift for my wheelchair.

    My car needs to be parked very close to the wall of my carport to leave enough room for me to exit from the left. There is Not enough space to open the drivers side door when the car is parked.

    These photo's show my parking/entry dilemma which is why I want to be able to power the camera via 2 pathways.

    Photo 1: Entry door open and wheelchair lift fully extended.
    2006 Toyota Porte Well Cab (4).jpg
    Photo 2: Shows why I can not access the ignition (AUX position) without having to enter the car via the wheelchair lift.
    20180219_103021.jpg
     
    Last edited: Feb 18, 2018
  6. Kabelsalat

    Kabelsalat

    154
    28
    Jul 5, 2011
    Ok, you have two supplyes. Can you explain the conditions for each supplies gives output?
    Need to understand that logic, so if for example one of the supplies is always ON, then there is no point of including that because camera will just drain the battery.

    I assume you want the dash camera to be on when motor run (ignition ON) - in what other cases should it be on?

    [edit]
    Ok - assuming that questions above is not important in your case - are you capable to put the diodes before the 12V-5V converters somehow? There is two of those, right?
    Doing that, you should afford the standard voltage drop - just get an diode with same current rating as the converter.
     
    Last edited: Feb 18, 2018
  7. Ray Eyre

    Ray Eyre

    17
    1
    Feb 18, 2018
    The camera also needs to be SWITCHED ON manually for the purpose of downloading the recorded Video via the camera's WiFi connection. I can not reach the camera to remove the SD Card to download.

    Current rating of Converter output is 3 Amp. The other information on the converter says 15w Dc Dc converter.

    I have in my spare parts box some SR5100-M10 diodes and some IN4007-GW diodes are any of these suitable?
     
  8. duke37

    duke37

    5,364
    770
    Jan 9, 2011
    You need to find the current drain.
    The MR 5100 is a Schottky diode, 100V 5A
    The 1N4007 is a 1000V 1A diode.
    Either of these will do if the current is less than 1A.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,482
    2,830
    Jan 21, 2010
    I presume the camera doesn't use much power. I would grab a pair of 1N4001 diodes (actually 1N400x, where x is any digit).

    Wire a diode in series with the positive supply from each power source. The band on the diode should be at the end closest to your camera.

    As switches as necessary (they can be before or after the diode, it doesn't matter).

    Try each method of powering individually to make sure you have the side around the right way. If both work, then they can be used together with no risk of feeding power back.

    Be sure to insulate the connections to the diode. I would use heatsink tubing.
     
  10. Ray Eyre

    Ray Eyre

    17
    1
    Feb 18, 2018
    Thanks to ALL of you.

    I have the diodes in place and working correctly. Heat shrink has been used to insulate the diodes and all soldered joints..
    The next task is to actually fit it into the car... Which will take some time due to my somewhat useless lower limbs.. and a route needs to be found to run the wire's to the battery through the car's firewall...

    Thanks again for the understanding and help. Much appreciated..
     
  11. duke37

    duke37

    5,364
    770
    Jan 9, 2011
    If you are connecting directly to the battery, make sure it is fused. A battery can start a fire very easily if the wire insulation wears through.
     
  12. WHONOES

    WHONOES

    1,119
    311
    May 20, 2017
    Very valid point duke37.
     
  13. Kiwi

    Kiwi

    360
    89
    Jan 28, 2013
    Hi Ray.
    Where in NZ are you?
     
  14. Ray Eyre

    Ray Eyre

    17
    1
    Feb 18, 2018
    Hi.
    I'm in Whangarei.
    Hope cyclone GITA has not done too much damage to your property.
    Cheers
     
  15. Ray Eyre

    Ray Eyre

    17
    1
    Feb 18, 2018
    It go's without saying. Yes both inputs are fused.
     
    (*steve*) likes this.
  16. Kiwi

    Kiwi

    360
    89
    Jan 28, 2013
    No damage, just about 70mm of rain.
     
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