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How to: automotive 12vdc regulator ?

Discussion in 'Electronic Basics' started by SAH, Jan 2, 2004.

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  1. SAH

    SAH Guest

    I need to know the best and cheapest way to regulate my cars varying 12 -
    14.5 Vdc output to a reasonably steady 12 Vdc. I would only require about 2
    or 3 amps max. I need to run some small devices that require a 12Vdc input.

    Any and all help appreciated.

  2. "the best and cheapest way"

    Often contradictory!

    I'll bet your "device" will work just fine on the "varying ... output".
    (If not, it's a crap device!)

    | __O Thomas C. Sefranek
    |_-\<,_ Amateur Radio Operator: WA1RHP
    (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz
  3. SAH

    SAH Guest

    Agreed, I am hoping that it will be the case but the question was posed for
    the worst case scenario. The device in question is a cold cathode neon tube
    used in PC Mod applications. You can pick them up for 10 bucks in a 12"
    tube. They come with a 4 pin Molex connector to provide 12Vdc to an
    inverter. Now, in a PC application, the inverter will see a pretty stable
    12Vdc, but in a car it will obviously vary up to 3 volts on the high side,
    worst case scenario. What would be the cheapest was to have a 12 Vdc limiter
    ?......or maybe somebody here has already tried this and has some experience
    with them.....?

  4. I recently ordered ( but haven't received )
    for a different 12volt application. Not sure
    what your version of cheap is.

  5. SAH

    SAH Guest

    Well, I may have to got that route. If I don't get any first hand info soon
    I'll just throw one in a car and see what happens. I'm hoping that they are
    not all that sensitive.
  6. Chan

    Chan Guest

    You can't regulate to 12V if your input is 12-14.5. This is because your
    regulated will need to drop at least 0.6V (assuming it's just one power
    transistor). So you can have a max of 11.4V.
  7. Byron A Jeff

    Byron A Jeff Guest

    -You can't regulate to 12V if your input is 12-14.5. This is because your
    -regulated will need to drop at least 0.6V (assuming it's just one power
    -transistor). So you can have a max of 11.4V.

    PNP bipolars and P-channel MOSFETs often will only drop 0.2V over low to medium
    currents. Check out the Vsat parameters.

    BTW with an auto regulator, that's very rarely the issue. There are three
    issues that need to be dealt with:

    1) Starter voltage drop out. When the car is starting, often the battery will
    drop as low as 8V. You can't keep regulation at that voltage and it'll
    often reset the equipment connected. I resolved this issue in my car
    computer by having a Lead Acid battery before the regulator and a connection
    between the car electrical system and the battery via a diode. So the
    car system would charge the battery, and when the car started the diode
    prevented the electrical system from drawing power from the extra

    2) Surges. Auto electrical systems can have spikes way above 14V. So you should
    have surge/spike protection. A MOV or a zener based power shunt should
    be at the head of the electrical system. Also a power resistor can limit
    the amount of current into the regulator.

    3) Noise. You'll need to sprinkle caps into your regulator to smooth out
    the noise typically found in the electrical system.

  8. One could with a switching regulator.
  9. Look at the LT1371. Its a switching regulator, requiring a couple of
    external inductors, diodes, etc. It'll let you draw 3A.

    Bob Monsen
  10. Greg Neill

    Greg Neill Guest

    Doesn't anyone use zener diodes anymore?
  11. I wonder how long a 12V zener would last as a shunt at 14V? He wants 3A.

    Bob Monsen
  12. SAH

    SAH Guest

    Thats an idea. 3 amps would be the max for sure, probably only half that
    most likely. Could you give me a particular zener number, maybe I can track
    one down ?
  13. Bill Bowden

    Bill Bowden Guest

    You can make a big zener diode with a NPN power transistor
    and small zener. Hook the zener from base to collector
    so that the transistor only conducts at the zener voltage
    plus 0.7 for the b/e voltage. Put the big zener in parallel
    with the load as a shunt regulator and add a 0.2 resistor
    in series with the battery. If the battery is at 12 volts you
    lose 0.6 volts at 3 amps. If the battery is at 14 volts,
    the transistor drops 12 volts at 10 amps (120 watts) and the
    resistor dissipates 20 watts.

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