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Car stable 12v for <$3 ?

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  • Car stable 12v for <$3 ?

    OK, I am not sure if this will work, but I just started looking at car PSU and thought this might be interesting ....

    How about connecting as follows :-

    . -------- --------
    Car (10-15v)---| 7809 |----------| 7812 |------ 12v regulated
    . ---+---- ^ ----+---
    . | | |
    Car ground ------+ 18v |
    . | | |
    . ---+---- v |
    Car (10-15v)---| 7909 |--------------+--------- Ground
    . --------

    Of course thow in a few capacitors to smooth it out, but will this work ?

  • #2
    Originally posted by lostbrit
    OK, I am not sure if this will work, but I just started looking at car PSU and thought this might be interesting ....

    How about connecting as follows :-

    . -------- --------
    Car (10-15v)---| 7809 |----------| 7812 |------ 12v regulated
    . ---+---- ^ ----+---
    . | | |
    Car ground ------+ 18v |
    . | | |
    . ---+---- v |
    Car (10-15v)---| 7909 |--------------+--------- Ground
    . --------

    Of course thow in a few capacitors to smooth it out, but will this work ?
    Works in theory, you still obviously need a DC - DC PSU. Also are you willing to manually switch the PC on and off everytime you start and stop.

    Comment


    • #3
      Why would I still need a DC-DC converter ?

      The idea is that out of an unregulated 12-15v DC supply I make +9v and -9v. This gives me an 18v regulated supply for feeding the 7812 and getting a 12 v regulated output. Might be able to cope with a drop in the supply voltage (starting) by putting a couple of hefty capacitors on the front end. Otherwise I dont see why the pc woulnt just start up and stop as the ignition was turned on/off.

      The only drawback I see is that i think you would have to run a 7805 off the regulated 12v so that you can have a common grounf between them. Not a big deal just have to beef up the 7809/7909/7812 to cope with the extra load.

      Comment


      • #4
        You still need a PSU so that your motherboard has something to connect to.

        I believe an ATX PSU connector has about 14 connections...

        Comment


        • #5
          +9v + -9v = 18v? (Seriously??? I have no clue here. :P)

          Comment


          • #6
            ah, thats why we call voltage, Potential Difference.
            2007 Honda Fit Sport 1.5L SOHC-VTEC

            Comment


            • #7
              OK, I refined it ... using a 7805 trimmed you can generate a regulated +6v output. Then using a 7905 you can generate a regulated -6v output. Connect these to give a regulated potential difference of 12v. Use this to drive a second 7805 and you have a regulated 5v output that shared the same ground point. (See diagram).

              I am using a single board computer that only needs +12v and +5v so I should be ok. I just need to figure out how to handle the 'power good' part of the circuit. Any ideas out there ?

              For you ATX people who need 3.3v, a third regulator hanging fron the regulated +12v line should give you everthing you need.
              Attached Files

              Comment


              • #8
                OOPS ! Wrong connection on the front end regulators, please see revised diagram.



                Originally posted by lostbrit
                OK, I refined it ... using a 7805 trimmed you can generate a regulated +6v output. Then using a 7905 you can generate a regulated -6v output. Connect these to give a regulated potential difference of 12v. Use this to drive a second 7805 and you have a regulated 5v output that shared the same ground point. (See diagram).

                I am using a single board computer that only needs +12v and +5v so I should be ok. I just need to figure out how to handle the 'power good' part of the circuit. Any ideas out there ?

                For you ATX people who need 3.3v, a third regulator hanging fron the regulated +12v line should give you everthing you need.
                Attached Files

                Comment


                • #9
                  But... those regulators are 1 amp max. 12 watts ain't enough.
                  Chrysler 300 - Fabricating
                  http://hallert.net/

                  Comment


                  • #10
                    78xx and 79xx go up to about 3A. Which will be enough for my board. If you need more there is a power transistor tweak that will take a 78/79xx up to 10A.

                    Comment


                    • #11
                      Just curious, have you tested this circuit yet? In order to get +5v out of a 7805, you put v- on the ground pin, v+ on the input pin, and output pin is 5v. To get -5v out of a 7905, you put v+ on the ground pin, v- on the input pin, and the output is -5. Works great seperately, but as soon as you hook together the grounds and try series the 5's for higher voltage, it shorts out the plus and minus of your battery. I think you should check the 7905 datasheet again, you have it hooked up backwards in your diagrams. Nice try though.

                      -Chris

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                      • #12
                        If this is possible why havenīt anybody thougt of this before, the components are just a few bucks, and an OPUS or similar is 200? Is this even possible?

                        Comment


                        • #13
                          I see the 7905 having problems when you inadvertantly connect the computer's line level output to an amp or head unit? Since the grounds are not the same thing, and it's not floating so you can't tie them together...

                          Comment


                          • #14
                            To get a negative voltage out of a negative linear regulator, you have to feed in a negative voltage. Since you are not doing this, this will not work.
                            Old Systems retired due to new car
                            New system at design/prototype stage on BeagleBoard.

                            Comment


                            • #15
                              Look like the circuit is originally powered by a rectifier with a +V/0V/-V output.

                              As rob said you do need a 0V and -V for the 7905...but you are feeding it +V on the input side and 0V on the ground, you will most likely fry that regulator since the power you are feeding it is reversed.

                              Anyhow try it if you wish and let us know the results

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