The value of Current Set Resistor R1 in LM317

Discussion in 'General Electronics Chat' started by bbk.wgl, Apr 9, 2007.

  1. bbk.wgl

    Thread Starter New Member

    Mar 21, 2007
    Hi guys,
    The value of the current set resistor R1 in case of LM317 is usually 240 ohms.
    what if i use larger values (say 1K or greater) ? what are the upper / lower limits of the value of this resistor (if any) ? What are the implications of using larger values (if any) ?
    AND, here is one advice for all you nice people,
    "Alcohol and calculus don't mix. Never drink and derive.":D
  2. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    you can use this formula to find the answer to your query

    Vout = Vref ((1 + (R2/R1)) + IadjR2

    Vref = 1.25v

  3. bbk.wgl

    Thread Starter New Member

    Mar 21, 2007
    Hi moz,
    That formula you gave is used to calculate the output voltage.
    i want to know what happens if the value of R1 used is not in the
    region of the usual 240ohms.
    can you use any value according to the output requirement or is there any limit ?
  4. Johann

    AAC Fanatic!

    Nov 27, 2006
    If you look carefully, you'll find that the resistance you require is part of the formula given by moziklov. Just manipulate the formula and you'll have your answer!:)
  5. Papabravo


    Feb 24, 2006
    That's pretty insensitive. Perhaps Algebra is beyond his/her capabilities. Last time I looked we had some high schoolers on this board. The answer to his/her, question, may be revealed by a careful reading of the datasheet. If you know how to find it great, otherwise I might be able to come up with a link.

    Some things to keep in mind:
    1. You want to keep the voltage, Iadj(R2), to a small fraction of Vout which places a limit on R2
    2. You want R2/R1 to be large enough that when multiplied by Vref = 1.25 V you get close to Vout which places a maximum limit on R1
    3. If R1 is too small then the regulator will draw excessive quiescent current.
    4. If R1 is too large then not enough quiescent current will be drawn and the output voltage will rise.
  6. hgmjr

    Retired Moderator

    Jan 28, 2005
    Greetings bbk.wgl,

    To expand on the replies already tendered, if you happen to be familiar with Thevenin's Theorem you can use it here to gain an understanding of the impact of increasing R1 on the ability of the LM317 to regulate accurately.

    Looking at the diagram that you attached, you can calculate the Thevenin's Equivalent Voltage at the junction of R1 and R2 for a given voltage out. The point of the regulator is to hold this voltage constant such that the difference between Vout and this junction is held to the value of Vref which is typically 1.25 volts. This infers that for a given voltage out, the ratio of R1 to R2 will be a constant.

    Now, even though Vth is set by design to a specific voltage for a given Vout, you are free to chose the resistor combination to set that output voltage. However, the penalty that you pay is that the Thevenin Impedance will vary depending on your choice of R1 and R2. Assuming that the value of Iadj is around 100 microamps, you can see by the application of Ohm's Law that for every 100 Ohms of increase in the Thevenin Impedance (R1||R2) brought on by an increase in R1, there is a corresponding decrease in the accuracy of Vout of around 10 millivolts.

    The published formula Vout accounts for this error and so you can calculate the correct values for R1 and R2. The real problem arises when the value of Iadj drifts under the influence of time and temperature.

    All that said, the manufacturer recommends the use of R1 in the range of 240 Ohms as a guide. It is a value selected in order to yield the optimum performance from the device for the range of temperatures over which the device has been specified to operate.

  7. bbk.wgl

    Thread Starter New Member

    Mar 21, 2007
    thanks hgmjr :)