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LM234 current source

Discussion in 'General Electronics Discussion' started by eem2am, May 19, 2013.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    Hello,

    I wish to make a 6.5mA current source with LM234.

    The ratio of I(bias) to I(set) isn't quoted in the datasheet for any more than 5mA, therefore, i must use two pieces of LM234 and parallel them. (3.25mA each)

    This means using a resistor of 21.2 Ohms in each LM234 circuit.

    1....Is it ok to parallel LM234's

    2...The datasheet page 3 quotes the ratio of I(set) to I(bias) as 14 for Iset= 1mA to 5mA, ...However, the graph on page 5 of the datasheet clearly shows that this ratio varies widely over the range 1mA to 5mA.....so how come they have quoted a fixed figure of "14" on page 3 of the datasheet?

    3.....Why does the datasheet indicate that LM234 can supply 10mA?....after all the ratio of I(bias) to I(set) isnt stated for any more than 5mA....so surely 5mA is the realistic maximum for LM234?

    LM234 DATASHEET:
    http://www.ti.com/lit/ds/symlink/lm234.pdf
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    1,271
    Nov 28, 2011
    Do you need to use an LM234? It doesn't look very easy to use. As you say, the data sheet is not helpful for currents above 5 mA, although it describes the device as having a maximum current of 10 mA. The ISET/IBIAS ratio graph seems to indicate that the ratio falls off very sharply around 5 mA. Where that curve goes above 5 mA is not clear, but the steep drop at the end of the curve does not bode well. In fact, it's not even a Bode plot! (Nerd pun.)

    I agree with your comments. Can't you use something else?
     
  3. eem2am

    eem2am

    422
    0
    Aug 3, 2009
    I need to use the LM234 as there's no easier way to get a current source.
    The attached shcematic shows an LT3092 current source being used, but they are too expensive.
    It will be used as in the schematic....i.e. to provide a high side bias supply to the opto-diode....it has to be a current source or else it interferes with the regulation of the led current.
     

    Attached Files:

  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
  5. eem2am

    eem2am

    422
    0
    Aug 3, 2009
    Thanks very much , i have been distracted lately and missed your response noted.....i will gratefully read it, as i have noted your replies are particularly good technically, Thanks again.
     
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