[HELP]1.2v regulator from 7805

Discussion in 'General Electronics Chat' started by bahubali67, Apr 17, 2014.

  1. bahubali67

    Thread Starter New Member

    Mar 15, 2012
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    hi all i was making 1.2v regulator circuit for my AA rechargeable battery. I know using LM117 i can get 1.2v. but the thing is i want to make it using 7805. so tried using voltage divider circuit and got 1.29v but there is NO current at all??? what could be the reason???any help...i need current upto 100ma
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    The LED has forward working voltage requirement of at least 2V, so it is not conducting.
     
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  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The 7805 and 117 are very different internally. A 7805 is not designed to be an adjustable regulator. It has a fixed 5V output when the GND pin is grounded. You can put a resistor or zener in the ground leg to increase the output voltage, but nothing will decrease it below 5 V.

    In your schematic, the 10 ohm and 40 ohm resistors combine to add 1.25 V to the output like the gain setting resistors around an LM117, only the 7805 has a 5 V minimum so the output rises to 6.25 V. The extra .039 V comes from the static current through the 7805 ground leg.

    If you don't need much current, you can divide the normal 5V output down to a lower voltage with a 2-resistor divider using Ohm's Law, but this output will not be regulated like the main 5 V is, and will vary as the load current changes.

    ak
     
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  4. bahubali67

    Thread Starter New Member

    Mar 15, 2012
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    OK ..consider if I connected a AA battery instead led. Then does current flows?? If yes how much??
     
  5. bahubali67

    Thread Starter New Member

    Mar 15, 2012
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    OK thank u...so you mean its not possible to get 1.2v (with little current)from 7805 ??
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I think the amount of current will change as the resistance of the battery changes.
     
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  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Not directly from the Vout pin, no. But...

    In your first post you said "up to 100 mA". What is it that you are trying to power? What is LED-BIRY?

    If you connect the GND pin of the 7805 directly to ground along with the botton end of R2, then you will have 1.25 V at the voltage measuring point in your drawing, and 100 mA will flow through the two resistors.

    That will change when you connect a load to the R1-R2 junction. You will be able to draw some current from this node, but the voltage will sag as the current increases.

    ak
     
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  8. BobTPH

    Active Member

    Jun 5, 2013
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    You do not charge batteries from a voltage source. What chemistry is the battery? NiMH I presume?

    Bob
     
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  9. ian field

    Distinguished Member

    Oct 27, 2012
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  10. IcedFruits

    Member

    Jan 15, 2014
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    to charge a nickel 1.2v battery, just feed it with constant current (not volt) of C/10 (capacity /10 -> i.e. 160mA for 1600mAh battery or 210mA for 2100 mAh battery) for 12-14 hours, for a battery that is near about fully discharged (not over discharged).

    its the easiest, safest and most reliable method for prolonged battery life.
     
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  11. bahubali67

    Thread Starter New Member

    Mar 15, 2012
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    ok thank u all..
     
  12. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Building a charger for any battery is not a trivial project to do it properly. You just don't apply a voltage and expect the battery to charge. Not if you expect to keep the battery around for a while.

    Charging NiMH batteries can be very dangerous. If you don't do it properly, you can hurt yourself, others or burn down your home. Sorry but this is a foolish project. Go out and buy a proper charger. It simply isn't worth all of the hassle and risk.
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    You must be thinking of lithium cells that can vent with flaming gas if abuse leads to thermal runaway.

    Most nickel chemistry cells have over pressure blow our vents so they rarely explode with much enthusiasm. Its worth remembering that the cadmium in Ni-Cd cells is extremely toxic (rots all your bones out!).

    Over charging causes overheating, which drastically shortens the life of the cell - that's a good enough reason for most people to do it properly.
     
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