Help with lm350 /lm317 as Constant Current Source

Discussion in 'The Projects Forum' started by Broncos, Feb 27, 2012.

  1. Broncos

    Thread Starter Member

    Feb 2, 2012
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    I'm usig an lm350 and built a circuit pretty close to this
    http://www.eleccircuit.com/wp-content/uploads/2007/12/lm317-current-regulator.png
    Without the capacitor and going thru this switch (both the the power and resistor are fed thru the switch(different poles))(this switch)

    I'm using a 2ohm resistor on the vout (expecting around 625mA's). I was getting around 1.5 amps. I measured the voltage between the Vout pin and ADJ pins. The voltage was 0.04 volts! I got the same results when using a 1 ohm resistor too.
    what could the problem be?


    I pulled the lm350 and tested it to work ok at lower currents with a .1 capacitor[vin to gnd] got (1.20 volts across Vout and ADJ) Could I need a capacitor on the higher current one, and where?
    I'm using a 7.5 volt battery pack.
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The capacitor is required to keep the regulator from oscillating.
     
  3. Broncos

    Thread Starter Member

    Feb 2, 2012
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    What pins does the cap go on? and what value is recomeded?
     
  4. eagletusk

    New Member

    Jul 20, 2011
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    It's all there in the schematic that you posted.
     
  5. Broncos

    Thread Starter Member

    Feb 2, 2012
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    does the capacitor have to physically be really close to the lm350 ic or can it be 12 inches of wire away?
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The closer the better. I suspect that 12 inches is too long.
     
  7. Broncos

    Thread Starter Member

    Feb 2, 2012
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    I tried a capacitor from Vin to ground, same results. Draws 1.52Amps...
    .047 volts across the resistor, and 0.00 volts across another resistor I tested - still 1.52 amps
     
  8. Broncos

    Thread Starter Member

    Feb 2, 2012
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    Whats the minimum voltage from the battery required?
     
  9. Wendy

    Moderator

    Mar 24, 2008
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  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The datasheet guarantees that it will work if (Vin-Vout)>3V. The attached graph is of typical performance.
    Keep in mind that, when used as a current regulator, you have to add the 1.25V reference voltage. This means that the guaranteed minimum Vin is the voltage across the load, plus 4.25V. If you are fortunate enough to get a typical part, that number is about 3.25V.
     
  11. Broncos

    Thread Starter Member

    Feb 2, 2012
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    Now when I connect power, with the vout pin floating, the adj pin still delivers 1.5 amps! (7.5 Vin , 3.3v across load)
    Bad chip? I'm using that pinout with my [lm350t P+]?
     
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