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5V DC Power from 6V Rechargeable Pack

Discussion in 'General Electronics Discussion' started by Revenant, Apr 14, 2010.

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  1. Revenant

    Revenant

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    Apr 14, 2010
    I'm dealing with a circuit board that requires 5V DC power and can draw as much as 2 amps.

    I want to be able to supply this power with a rechargeable battery pack (Later a wall adapter to power it and charge the batteries).

    I'm not an advanced electronics engineer at all, very new to it. I know basics essentially.

    What I was looking at doing is coupling two +5V Fixed-Voltage Regulator 7805 in a parellel circuit. Since the voltage should still be 5V output and 6V input, this should effectively double the plausible amps in my mind. Since each of those are rated to 1 amp current, would that give me ~2 amps?

    Can anyone offer input on this question?
     
  2. Revenant

    Revenant

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    Apr 14, 2010
    Actually, I might have found a better solution. 1N5401 diode seems to have a forward voltage drop of 1V, so I basically end up with 5V after the Diode, and since it's a diode, it can stop the batteries from being connected backwards anyway as a bonus :)
     
  3. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    You can not do that for two reasons.

    Two 7805 in parallel will not provide 2A. In the best case, one of them will provide a little more current than the other, putting it over 1A. In the worst case, one of them will provide almost all the current.

    The second problem is that 7805 requires more than 1V between its input and output, which you do not have.

    You need to consider using something that is rated for more than 2A and that can work with less than 1V. LP3853, LP3856, MIC29300 and MIC29500 are some of the possibilities. All of these are available in many voltages, so you need to specify the particular voltage you want to use. You can get them from DigiKey, Mouser and others in the USA.

    But that setup is not going to help you that much. As soon as the battery voltage drops to about 5.6V the output will start to sag. Depending upon what type of a battery pack you are using. the "6V" may actually be a bit lower than 6V, so you may not get much use out of fully charged pack.

    That does not address the battery charging, which is another huge project.

    ---55p
     
  4. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    It will work as long as the "6V" battery is actually 6V. If it is a lead acid battery, it may start off at close to 6.5V.

    ---55p
     
  5. Revenant

    Revenant

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    Apr 14, 2010
    LP3853 looks like the way to go. The voltage drop was too unpredictable anyway. Thanks for the info.

    Edit: Finding a place to purchase one isn't as easy though.
     
    Last edited: Apr 14, 2010
  6. Revenant

    Revenant

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    Apr 14, 2010
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
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    Jan 21, 2010
    Probably a workable combination. Just make sure that the battery doesn't fall below 7V when it is "flat".

    With Lithium Ion cells you need to be *VERY* careful not to discharge them too deeply. You should add additional circuitry to prevent this or you will likely kill the battery really fast (the same goes with charging, although I'll just assume you'll buy an appropriate charger with it.
     
  8. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    A switching regulator is not the easiest thing to build for someone who "know(s) basics essentially." I would strongly recommend sticking to a linear regulator. If you are the USA, you can get the ones I recommended at DigiKey.com or Mouser.com.
    Are you talking about the safety circuit included in the battery? I would like to draw your attention to the following from that page (highlight from the original): CAUTION: when working with Li-ion cells, they are very sensitive to charging characteristics and may explode if mis-handled. Do take that warning seriously because it is serious. You can search youtube for videos of LiIon battery fires and explosions.

    ---55p
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
    2,832
    Jan 21, 2010
    I have some experience with these regulators. As long as you use their PCB layout and follow their recommendations for the inductor (and basically all the other components) it's not much harder than a kit.

    The very strong hint (to Revenant) is to follow the manufacturer's recommendations *exactly*.

    I also second 55p's battery warnings. My warning about the input voltage not falling below 7V was not to preserve the battery but to preserve regulation.

    If weight is not a major issue, why not use a small 12V lead acid (gel) battery. Whilst you can still do damage to them by over discharging and overcharging, they are far more tolerant and tend not to expire in a pyrotechnic manner (hydrogen explosions excluded)
     
  10. Revenant

    Revenant

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    Apr 14, 2010
    Thanks for all the input, I'm definitely keeping it all in mind.

    As for me knowing basics, I do have help from a friend that's been doing electronics since the 80s for IBM and now works with me here. So he's been a great deal of help too.
     
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