Can I do this with a voltage regulator?

Discussion in 'The Projects Forum' started by rokz_2005, Dec 30, 2011.

  1. rokz_2005

    Thread Starter Member

    May 14, 2011
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    Hi guys,

    The input power ranges from 11v to 14v dc
    I want the output to be a constant 12v dc, How do I achieve this? Its for a charger that I am attempting to make.

    I have successfully made the 5v charger. Now for the 12v :)

    I am newbie so I'm not very knowledgeable, I thought that I could use a voltage regulator but when the input voltage hits 12v then my output will drop won't it?
    Can anyone advise?

    Thanks
     
    Last edited: Dec 30, 2011
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U need to tell us the Current capacity of the charger or what kinda battery is to be charged
     
  3. anotheruser1

    Member

    Dec 6, 2011
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    There are many options to choose from, but if you are charging a battery your output needs to be around 14 volts.
     
  4. rokz_2005

    Thread Starter Member

    May 14, 2011
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    1.5a max. Its for a tablet pc. Not sure of the battery type.
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    In my opinion, trying to make your own charger for a tablet is a very bad idea. Tablets often use Lithium batteries, which require careful treatment or they become extremely dangerous, that is, catch fire or explode. Even the less volatile NiMh types can overheat or burst, and require care, if they are to last a reasonable length of time. http://www.youtube.com/watch?v=vS6KA_Si-m8

    The current and voltage levels required to charge the battery will depend on the technology used and the model of battery.
    A bald statement of " about 14V" may be in the ballpark for some types, but this varies. A simple constant-voltage supply is in any case unsuitable for recharging batteries. As an absolute minimum, the charging current must be limited so that however low the battery voltage may be, the current will never be too big.

    My advice would be for you to buy a commercial in-car charger compatible with your model of tablet.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Unless you don´t want to explicitly charge the battery by yourself, but simply replace the original adaptor with yours. Then it would be doable.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I had thought of adding a rider like that, but I prefer to be very cautious. Even an adaptor needs to be designed with some care, and the OP has asked basic questions about what can be done with a voltage regulator.

    Frankly, I do not think that any kind of power adaptor is a good beginners project. The input supply sounds to be a lead-acid battery (perhaps in a car), which is not a thing to be trifled with.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    I've done something similar, it can turn knarly. The biggest problem I fought was the voltage drop in the wires. 1.5A is a relatively small current, and you can get voltage regulators that will handle it (cooling is always an issue with analog regulators), but it will generally drop around 3V to power the regulator itself. There are newer classes of IC regulators that will drop less than a volt, but I don't have much experience with them.
     
  9. rokz_2005

    Thread Starter Member

    May 14, 2011
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    The tablet makers have not launched in car charger. There are tp ones out there but I wasn't convinced on the reliability. It's from china as well.

    I was thinking of putting a 2a fuse on power so that the fuse blows if it goes over 2a instead of causing any damage.

    I don't really need to charge the battery, I just want to use it in the car. The battery life on it is really bad.

    @ Kubeek - yes it is a car.

    By the way, this is the battery type - Li-Po 1530 mAh. I do know it requires 1.5a to charge it but I don't know where to find the details of the current it uses. It charges really quick, I reckon it probably draws about 700mA.

    Say it was available from the manufacturers, How would they do it?

    The issue I'm thinking that I will come across is the voltage drop... otherwise a simple linear voltage regulator would do for me.
     
  10. anotheruser1

    Member

    Dec 6, 2011
    30
    4
    he didn't specify the type battery to be charged, so the ball park figure was for most common 12vdc chargers, car batteries, which are not as sensitive as the other batteries mentioned. I guess it would be a good idea if I ask for more specs next go around.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I have a notebook and 2 phones to charge, and none of them use the same jack.

    My solution: $40 100 Watt Plug In Lighter Inverter thingy from Black and decker that I can plug two chargers into or the notebook, or the notebook and one phone. With my new phone, I can charge all 3 using the 5VDC USB power port on the side of the inverter.

    I've been FAR happier than shelling out $40 for a car charging cable every time I end up with a new phone or notebook. Work doesn't pay for those "luxuries".

    I also only have one adapter/charging item to deal with for each device, instead of a car charger and a home charger, only to realize I brought the wrong one when I get to a hotel.
     
  12. DickCappels

    Moderator

    Aug 21, 2008
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    I am with Adjuster on this, but for an additional reason: Do you really want to perform experiments on your PC that could easily destroy it? Ask me how I found out...
     
  13. rokz_2005

    Thread Starter Member

    May 14, 2011
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    So.. the alternative to making my own charger is to buy an inverter which is more reliable compared to the TP chargers from China?

    Can I make my own inverter. I love making electronic stuff :D
     
  14. rokz_2005

    Thread Starter Member

    May 14, 2011
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    I used an online calculator and it looks as though 100watts is not enough...
     
  15. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If your tablet really uses anything near 100W, it may be unwise to rest it on your lap for long, or you may do yourself an unfortunate injury!

    Seriously though, I suppose you must be allowing for a very rapid battery charge, and fairly heavy losses along the way . I would be interested to know how you got those figures though, and what kind of tablet this is for.

    If you start with a 100W output rated inverter, and the charger was 70% efficient, delivering 70W output at 14V, you would still expect to get 5A out, which is much more than the 1.5A you initially mentioned.
     
  16. rokz_2005

    Thread Starter Member

    May 14, 2011
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    0
  17. Adjuster

    Well-Known Member

    Dec 26, 2010
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    That is rated at 3A at 12V, or 36W maximum. It is a small lighter-socket type unit, and if it drew 100W input for 36W output it would probably melt (64 W internal dissipation)

    How exactly did you calculate over 100W - would that be with a 120V mains unit - what efficiency did you assume?
     
  18. rokz_2005

    Thread Starter Member

    May 14, 2011
    68
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    I used an online calculator
    http://www.crownaudio.com/apps_htm/designtools/ohms-law.htm

    220 volts input and output = 1.5 it gave me a wattage = 330
     
  19. rokz_2005

    Thread Starter Member

    May 14, 2011
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    0
    I am just trying to find a way to keep the output voltage from the cigarette lighter to stay stable at 12v, any ideas?

    Zener diodes??
     
  20. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The electrical supply in a motor car (automobile) carries horrible transient voltage surges. Dealing with this safely is not easy.

    Among other things, an event called "load dump" can make voltage spikes whenever a major current load is switched off, as at the end of engine cranking, when a lock is opened or closed, when a blower or fan motor shuts off, when lights are switched off... http://en.wikipedia.org/wiki/Load_dump

    What you have said about the input required by an AC adaptor suggests that you need to think about this a bit more:

    Properly made AC adaptors do not just work like resistors to drop the voltage at the same input current. They use transformers (or more modern things called switched-mode voltage converters). The high voltage input current is therefore less than the low-voltage output current.

    A 1.5A converter using simple resistive dropping from a 220V supply would make as much heat as three old 100W lamp bulbs!

    Please believe me, I'm not trying to be awkward about this, but I really think that you are trying to do stuff that you are not quite ready for. Remember that a mistake could easily ruin your tablet beyond repair.
     
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