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Thread: Car stable 12v for <$3 ?

  1. #1
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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. #2
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    Quote 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.

  3. #3
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    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.

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

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

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

  6. #6
    Maximum Bitrate binary.h4x's Avatar
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    ah, thats why we call voltage, Potential Difference.
    2007 Honda Fit Sport 1.5L SOHC-VTEC

  7. #7
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    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.
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  8. #8
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    OOPS ! Wrong connection on the front end regulators, please see revised diagram.



    Quote 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 Images Attached Images  

  9. #9
    FLAC Chairboy's Avatar
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    But... those regulators are 1 amp max. 12 watts ain't enough.
    Chrysler 300 - Fabricating
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  10. #10
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    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.

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