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Calculating a SET resistor on an IC

Discussion in 'Electronic Basics' started by Mike H, May 26, 2004.

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  1. Mike H

    Mike H Guest

    I have an IC (LM334z) with 3 pins. +V, -V, and Rset. This IC is designed
    to output a fixed current based on Rset regardless of input voltage.

    The spec sheet says that the total current through IC is the sum of the
    current going through the SET resistor and the bias current for the IC.

    The formula is presented as Iset = Ir + Ibias = Vr/Rset+Ibias

    The data sheet for this IC can be found here:
    http://www.national.com/pf/LM/LM334.html

    The question is, for a input voltage of 12 - 14 volts DC, what set
    resistor should be used to reach 20mA of current?

    What would the formula look like using those voltages but wanting to vary
    the output current?

    I guess I'm just a bit confused on how to calculate it and what all values
    I need to take into account when I do so.

    Any help is appreciated. Thanks.
     
  2. This chip applies a voltage of 64 millivolts to the set resistor (at
    25 degrees C, see graph of voltage across Rset versus temperature on
    page 4). I bias is typically 1/18th of the current through Rset
    (though it might vary from 1/16 to 1/26 see ration of set current to
    bias current on page 3). So something near (.064/.02)*(19/18)=3.4
    ohms would get you near 20 ma. However, the device has an absolute
    set current rating of 10 mA (for an absolute maximum output current of
    (19/18) * 10 mA = 10.6 mA, so the exercise is pretty pointless.
     
  3. Mike H

    Mike H Guest

    Sorry about that error. Yeah, in the circuit I'm working on I'm going
    to use a Transistor to bump up the current flow using the LM334 simply
    as a limiter of the total current flow. I just needed to figure out how
    to do the math with the components involved.

    (.064 / Iset) * (19/18) = Rset

    So for the max current setting of 10mA I would look for a resistor of 6
    or 7 ohms. That would get me close and give me some good range of
    current control for the circit... I hope.

    The calculation just helps me be a little more sure of what I'm doing as
    I test things.

    Thanks.
     
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