regulators

Discussion in 'General Electronics Chat' started by duxbuz, Apr 3, 2014.

  1. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    hi
    I am after using a 9v battery to power a digital circuit that needs only 5v
    My supplier is out of 9v to 5v regulators

    I had a couple of questions

    1. Can i accomplish the same regulation using resistors and capacitors? Is it better to use a voltage regulator?

    2.It seems that there are lots of 5v regulators but they are minimum 10v. I presume this is no good if I am using a 9v battery, or is it acceptable?

    There seem to be quite a few of these types
    I wonder if this would be ok
    http://cpc.farnell.com/texas-instruments/lm2594n-5-0/switching-reg-0-5a-5-0v-2594-dip8/dp/SC08553

    Thanks
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Whats your location.?

    There are lots of regulators that will work with a 9V battery and give 5V output.

    What is the 5V current load.?

    E
     
  3. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    To be totally honest, impatient as I am, I have not got really paid lots of attention to the current, I was just buying parts off a crib sheet and was hoping to learn about the current as I got started. My location is UK

    There is a mention in document about the circuit consuming 60mA at 5.0Volt
     
  4. ericgibbs

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    Jan 29, 2010
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  5. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Thanks I will look on here.

    I was reading an article where the chap used a
    LM7805CT
    whilst using 9v dc battery

    Is that ok?
     
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    A 7805 will work down to +7.5V input for currents upto ~ 250mA

    Always add a 100nF cap on both the input and output of the 7805.
     
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  7. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Just wondered why capacitors are needed either side?
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    How much current does your circuit require?
    If the current requirement is low enough you can use a 78L05 or even a 5.1V zener diode (with a current limiting resistor).

    Note that the LM7805CT while readily available and inexpensive, it is not the only solution.

    Voltage regulators are servo feedback mechanisms. If you get a perturbation it is possible that the regulator could oscillate. Sometimes the datasheets will call for a 1 to 10μF tantalum electrolytic capacitor on the output.
     
  9. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    One more great question

    http://www.fairchildsemi.com/ds/LM/LM7805.pdf

    when I look here I see it says input voltage as low as 5v

    Am I reading that correctly?

    Where would I find a datasheet that might state capacitance?

    Thanks

    Just trying to educate myself so I don't have to keep asking the same questions
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    You are not reading the specs correctly.

    It says typical dropout voltage is 2V. Hence the input voltage > 7V.
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The datasheet you linked to in post #9! See Fig 9, page 19.
     
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  12. duxbuz

    Thread Starter Member

    Feb 23, 2014
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    Ok Thanks

    I have (a little) more understanding about dropout voltage now.

    but....

    I wonder why the datasheet has
    Input Voltage​
    VO = 5 V to 18 V
    mentioned if the output is 5v and the dropout is 2v

    Unless the output can possibly be 3v at anytime
     
  13. geko

    New Member

    Sep 18, 2008
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    The datasheet is for the LM78XX regulator series.
    It comes in variants with output voltages of 5, 6, 8, 9, 10, 12, 15, 18 and 24 volts

    The data sheets shows the maximum input voltage for regulators with outputs from 5-18 volts is 35 volts, and for the 24 volt type, 40 volts.

    For any of them the dropout voltage is 2 volts, so the input must be at least 2 volts more than the output and never more than the maximum input voltage.

    For the LM7805 that is an input voltage in the range 7-35 volts
     
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  14. tracecom

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    Apr 16, 2010
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  15. Alec_t

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    Sep 17, 2013
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    You can input 5V if you want, but that won't give 5V out.
    Here's a sim of output versus input:
     
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