6 Volt Power Supply from 12 volts (car battery)

Discussion in 'The Projects Forum' started by fuadramsey, Jul 21, 2008.

  1. fuadramsey

    Thread Starter New Member

    Jul 21, 2008
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    Hi I have very basic understanding of circuits (college DC class long forgotten).

    I need a 6 volt power supply (no more than 1 amp) from my 12 volt deep cycle battery. I plan on using this battery to run my laptop. The 6 volt supply is to power my telescope (low current).

    What can you recomend? Thanks!

    From what I understaind two 6 ohm resistors in series with 12 volts from the battery would give me 6 volts with 1 amp on each resistor?
     
    Last edited: Jul 21, 2008
  2. SgtWookie

    Expert

    Jul 17, 2007
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    A pair of resistors provides no regulation; it's simply a voltage divider. When the battery has a 12v charge, there would be 1 Ampere of current going right through the voltage divider, doing no useful work except heating up the resistors - not too useful in July. Adding a 1A load in the voltage divider would mean another 6 Ohm resistance in parallel with one of the existing resistors, which would then act as a single 3 Ohm resistor, throwing the voltage divider network off-kilter. Unless your telescope simply has a light bulb inside of it, regulation would be very poor. As the battery discharges, the telescope would receive less and less voltage.

    You need a regulator circuit at least. An old-fashioned LM7806 regulator with a big heat sink would do it. It wouldn't be efficient, but it would work.
    http://www.fairchildsemi.com/sitese...=sitemap+id&ia=1&text=LM7806&as=1&render=1&w=

    A DC-DC buck converter would be a lot more efficient. Here's an interesting design that is nearly as easy to use as an LM7806, and is 85% efficient (much less wasted power)
    http://www.dimensionengineering.com/DE-SWADJ.htm
     
    Last edited: Jul 21, 2008
  3. Tahmid

    Active Member

    Jul 2, 2008
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    Hi,

    How about using the L4960? Very useful and there is almost no heat as it is a switching regulator.

    Thanks.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Sure, the L4960 would work fine, but it requires at least 8 external components. The buck regulator I linked to would only need a cap on the regulated output.
     
  5. Tahmid

    Active Member

    Jul 2, 2008
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    Hi,
    Yes, more components, but it would be less expensive. what you linked to costs $15, whereas the L4960 circuit that I built cost only $1.5 or so.

    Thanks.
     
  6. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I'd say if you don't need anything but 6V then the LM7806 is the way to go.
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I think, considering the experience level, the MAX8546 in the uMAX package might be stretching it a bit not to mention the added parts involved.

    I still say the LM7806 with such minimal additional parts is the best way, unless we can find a very similar Voltage regulator with better LDO properties and not a an additional handful of parts.
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
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    The LM7806 would indeed be a good choice if the current requirement is low. Regulation by dissipation can be anathema to battery operated systems.

    Fuadramsey, do you know how much current the telescope draws?
     
  10. Tahmid

    Active Member

    Jul 2, 2008
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    Hi,
    If I am not mistaken, Fuadramsey had said that he requires 6v 1amp. So the 7806 would not be very efficient considering the losses at 1amp, being a linear regulator. I would suggest a switching regulator, like what sgtwookie linked to, or L4960 or the MAX8546. Although additional parts are required, they are much more efficient and their is minimal heat.
     
  11. fuadramsey

    Thread Starter New Member

    Jul 21, 2008
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    I've estimated the average current draw to be around 1.02 amps. (run time average 20 hours on 4 D batteries-1 D Energizer battery has 20500 mah capacity-4 D batteries in series gives me 6 volts at 20500mah)

    So maybe at least 1.5amps would be ideal?

    Thanks for all the help, I just went though another set of D batteries this weekend.

    Tahmid- can you post/pm me the L4960 circuit and what I would need and where I can get it?

    If it's a half a dozen components fine, if it's dozens, then I may want to go a different route.
     
  12. Tahmid

    Active Member

    Jul 2, 2008
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    Hi,
    I have attached the circuit. I have used it for my CD player and it is very reliable.
    Input your 12v to where is labelled 'V In'. Adjust the 5k potentiometer and monitor your output till you get the 6v. It can supply as much as 2.5 Amp. Hope this will fulfill your requirement.

    You can find L4960 at Digikey, Newark or Mouser.

    Thanks.
     
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