lm317 and lm338 in parallel?

Discussion in 'The Projects Forum' started by CaptainPrice, Jan 29, 2012.

  1. CaptainPrice

    Thread Starter Active Member

    Aug 12, 2009
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    I have lm317t designed with constant current at 1.5A. (just inside specs)
    I have designed lm338 designed with constant current at 3A.

    This goes to an LED(T6).
    Under normal operation I will use the lm317t, however when i push a momentary switch i want it to use the lm338 (go to 3A to the LED).

    Can i just put the lm317t in parallel with lm338? then when momentary switch is closed, all the flow goes through the lm338? or will the resistor from the lm317t mess up the calculated lm338 current, or is the design tooo unreliable? [I read you cant use lm317's in parallel to boost Imax with out it getting too complicated] or is it possible the circuits work together to deliver 4.5A?
    whats an easy way to use a momentary switch to boost to 3A?
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,669
    804
    You can´t parallel voltage sources without it getting too complicated. Current sources theoreticaly should have no problem working in parallel, those can´t be put in series.
    If you want to switch between 1.5 and 3A, you could just add another lm317 in parallel, no need for the beefier lm338.
    Or if you have the 338 already, you could switch a tap on the current setting resistor and not parallel anything.
    Also you should check that you have proper heatsinking for the power that will be dissipated in the regulators.
     
  3. CaptainPrice

    Thread Starter Active Member

    Aug 12, 2009
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    I currently have a bunch of lm317t's, would have to buy lm338.
    but when working with the 1.5A lm317t I have a 3 watt pot I want to use to vary the curret... i dont have a 4 watt pot to use with the lm338...
     
  4. CaptainPrice

    Thread Starter Active Member

    Aug 12, 2009
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    perhaps I could use the lm338 as a voltage regulator instead, if so what wattage would the resistors need to be for 3A?
     
  5. k7elp60

    Senior Member

    Nov 4, 2008
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    69
    I have had good luck putting adjustable voltage regulators in parallel, by adding fractional Ω resistors on the outputs. You hook the adjust terminals directlly together. The voltage determining resistor from the adjust terminals to ground. Connect the resistors(0.1or a little higher resistors) from the output terminals of the regulators to the output of the regulator ckt. Then the 120Ω resistor between the regulator output and the junction of the adjust terminals. The 0.1Ω resistors compensate for slight differences of the regulators.
     
  6. CaptainPrice

    Thread Starter Active Member

    Aug 12, 2009
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  7. Audioguru

    New Member

    Dec 20, 2007
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    You should look at the datasheets.

    An LM317 has a minimum output current of 1.5A. its "typical' output current is 2.2A and its max current is 3.4A which might blow up your LED.

    The LM338 is the same with 5A as minimum, 8A to 12A as "typical" and they don't say how much is maximum, maybe 15A.

    You need an accurate circuit to regulate the current.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    This is a terrible idea. Just use the LM338 and have a switch connect a different resistor to set the current lower or higher. You would need to use these as a current source to limit the current, not as a voltage source.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    OK, if you have a bunch of 317's and don't need high performance: look at the 317 data sheet and see how to set it up as a current source (just one resistor) set each for 1.5A. Use a mechanical switch to connect a second 317 to the LED, but you will need some series resistors to force the 317's to share current. A one OHM, 3W resistor on each would be sufficient.
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The EASY way (but you have to buy a 338) is just use one 338 set up as a current source. It only takes one resistor and you can also add a pot to vary the current. Could not be simpler.
     
  11. Qaisar Azeemi

    New Member

    Oct 7, 2011
    10
    0
    i used 3 lm338s in parallel to get 15A maximum current at the out put according to the circuit in Page 9 of it data sheet(Attached to the link below) but the problem is that it burnt out 2k ohms variable resistor... i don't know why. can any one guide me about that? i will be very thankful... here is the data sheet in below link.

    http://pdf1.alldatasheet.com/datasheet-pdf/view/22760/STMICROELECTRONICS/LM338K.html
     
  12. Qaisar Azeemi

    New Member

    Oct 7, 2011
    10
    0
    i got the answer. Thank you very much. the current flow through the variable resistor was the sum of the current due to 3 ICs adjust pins. so the program resistor should be used of low values and high power. here the formula to find out program resistor gets wrong because then you have to use the resistance values approximately divided by four but its now exact you have to use hit and trial rule. :)

    Best regards
     
  13. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Maybe you connected the pins on the LM338 backwards.
    The current in the 2k variable resistor is only 1.25V/120 ohms= 10.4mA.
     
  14. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    National Semi invented the LMxxx line of ICs. ST Micro simply copied them.
    But ST Micro have extremely fuzzy schematics on their datasheets.
    Why not look at the very clear schematic on the datasheet from National Semi instead?
     
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