LM317T Is there a minimum voltage inputtoget the required output voltage?

Discussion in 'General Electronics Chat' started by RodneyB, Mar 6, 2016.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Is there a Voltage drop when making a voltage regulator when using the LM317T.

    I am trying to regulate the voltage onto a small load to 12 Volts. I have a battery with a voltage of 13.8 volts as the supply but the output never can be set at 12 Volts. I an trying to understand why
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    Voltage drop : VI – VO, Input-to-output differential voltage, 3V
    LM317 datasheet -- Please check the page 4.
     
  3. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Thank you
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    For a lower voltage difference, you will have to use a LDO (low drop out) voltage regulator.
    The LD1117 or LM2940 will do the job.

    Bertus
     
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  5. dl324

    Distinguished Member

    Mar 30, 2015
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    How small of a load and what is the required voltage regulation?
     
  6. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Sorry I don't see where it shows on Page 4 the voltage
    The load is 50mA. I would like to keep the voltage at 12 volts
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    On the fourth page, section 7.3, second line item, Vi - Vo Input-to-output differential voltage, MIN(imum) 3V, MAX(imum) 40V.
    For 12V out you need at least 15V in.
     
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  8. WBahn

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    Mar 31, 2012
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    Also, even with an LDO regulator, the 13.8 V source that you mention sounds suspiciously like a fully charged lead acid battery. That voltage will drop as the charge drops, so you will probably drop out the regulator before using very much of the battery charge.
     
  9. dl324

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    Unnecessary requirements will make for an unnecessarily complex solution. If you're powering from a battery, the voltage will drop; even an LDO regulator will be hard pressed to provide a regulated 12V when the battery voltage is in that vicinity.
     
  10. WBahn

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    If you really need 12 V from a source that is in the vicinity of 12 V, then you will probably need to use a boost/buck regulator that can handle input voltages both above and below the targeted 12 V output.
     
  11. Papabravo

    Expert

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    It is also the standard DC power supply voltage for Amateur Radio equipment.
     
  12. WBahn

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    Probably because it was historically provided by a lead acid battery.

    Note that the TS specified that he is using a battery.
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The typical dropout voltage of the LM2940 is about 0.1 Volt @ 100 mA:

    LM2940_typical_dropout.png

    Bertus
     
  14. hp1729

    Well-Known Member

    Nov 23, 2015
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    For 13.8 V in and 12 V out consider just using three rectifiers in series to drop the voltage down.
     
  15. dl324

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    The OP wanted to supply 12V, which I took to mean precise to at least two decimals, to the load...
     
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  16. hp1729

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    Yep, then LDO regulators are the way to go.
     
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  17. dl324

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    Unless the OP plans to trim the LDO, it may not give the precision he wants. Datasheet says 11.40-12.60V over the full temperature range. If the circuit will tolerate 5% supply variation, a couple diodes could be sufficient (when the battery is fully charged).
     
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  18. hp1729

    Well-Known Member

    Nov 23, 2015
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    Of course. Always a given.
     
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