LM317 Current Regulator

Discussion in 'General Electronics Chat' started by BryanBandstra, Jan 22, 2014.

  1. BryanBandstra

    Thread Starter New Member

    Jan 14, 2014
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    [Original title: "LM317 Current Regulator Dropping Under Load"]

    Hello,

    I'm attempting to use a LM317T (TO-220) linear regulator to turn a potentiometer into a 4-20 mA outputter. With my components I calculate I should get an output range from about 3.9 mA to 19 mA (good enough). My schematic currently looks like this:

    [​IMG]

    My question is this. If I put my ammeter directly between the output (essentially the adjust pin) and ground, I get the calculated current. However, if I add a load to it, for example, 400' of 18 AWG wire, I end up with about 2.5 mA less than expected (at least at 100% - the only place I've tested). Why would I see less current? Shouldn't the LM317 just work harder to output the same amount of current given the additional load?

    Thanks for you help.
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Did you put your ammeter in series with the load? Or is it still from the output to ground.
     
  3. BryanBandstra

    Thread Starter New Member

    Jan 14, 2014
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    In series with the load to ground. See the below updated schematic.

    [​IMG]
     
  4. crutschow

    Expert

    Mar 14, 2008
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    What's the value of R_load?
     
  5. BryanBandstra

    Thread Starter New Member

    Jan 14, 2014
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    It looks like my target resistance will be 233Ω +/- 5Ω in addition to 50 feet of 18 AWG wire (0.3Ω??).
     
  6. MikeML

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    Oct 2, 2009
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    You dont have it wired right. Note where the adj terminal goes?
     
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  7. crutschow

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    Mar 14, 2008
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    I don't see the difference. :confused:
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    a much simpler 4 to 20 ma source is a 10 k 10 turn pot with a 9 volt battery. the turns counting dial will read current output. not exactly 4 to 20 ma, that would take a 10 volt source. we use them here to adjust all kiinds of things.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    My 4-20mA meter won't even go full scale on a 9V source. I don't see how a 9V battery and pot could work.
     
  10. ronv

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    Nov 12, 2008
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    Make the 82 ohm a 56 ohm or so. Right now 18 ma. drops the 1.2 volts the regulator wants to see from the output to reference.
     
  11. BryanBandstra

    Thread Starter New Member

    Jan 14, 2014
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    Don't I have it the same? Connecting after the current setting resistor? The difference I see from your provided schematic is how the wiper is connected.

    @alfaclif: Using a 9V battery isn't an option. This would be on an industrial machine that is in service. The intention is not to make a test device.

    @ronv: I'll try the 56Ω resistor later when I get a chance. Thanks for the recommendation.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    What is your source voltage?
     
  13. BryanBandstra

    Thread Starter New Member

    Jan 14, 2014
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    Source voltage? Incoming to the LM317? 24 VDC. It's labelled on the schematic.
     
  14. crutschow

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    Mar 14, 2008
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    Sorry, I missed that. :p
     
  15. JoeJester

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    Apr 26, 2005
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    What is the ripple voltage or is the source a battery?
     
  16. BryanBandstra

    Thread Starter New Member

    Jan 14, 2014
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    I don't know what the ripple voltage is. The voltage source will either be an industrial 24 VDC power supply (probably from Puls) or a 10 VDC output from a Mitsubishi VFD.
     
  17. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You might also want to add a fixed load of say 360 ohms from the output of the 317 to ground. Most will work with a minimum load of 3.5 ma., but some may want a 10 ma. load as a minimum.
     
  18. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I've tested a handful of brands and different ages, they work fine as a one-resistor current regulator down to about 1mA.

    For my understanding the minimum requirement of 10mA was on very old parts, and is more to do with being able to run at all possible voltage and transient conditions etc.
     
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