Voltage Regulator set resistor/current

Discussion in 'General Electronics Chat' started by vladtess, Jun 2, 2012.

  1. vladtess

    Thread Starter Member

    Jan 5, 2011
    43
    1
    Hello there.

    I am working with a adj. voltage regulator (LM338K) and here's what it says in one of the selections:

    The question is, if I want to use a regular power transistor to control the current supplied, it needs to be placed just like in a picture, between the 240 ohm resistor and the regulator itself? Or can I just put the power transistor elsewhere in the circuit after that 240 ohm resistor.

    Thanks much in advance!!! Pdf of LM338K attached.
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Unfortunately, LM338 (LM317, etc.) don't lend themselves to current limiting using sensing on the output side. You can't put it in series with the 240Ω resistor, because the current through that resistor is constant, independent of load. You can put it between the LM338 output and the load, but your output resistance is now equal to the current sense resistance, so you will have very poor regulation.
    Attached is a circuit that simulates well. I haven't tested it in hardware. It does not lend itself to low current limiting, because R3 would have to be large (Ilimit=0.6V/R3). You would have to change R3, and possibly some of the other resistors, to adapt it to the LM338 at a higher current level.
    Also, current limiting reduces the output voltage to ≈1.25V, NOT to zero. if the output is shorted, the current will be limited by the LM338. R3 must be high enough wattage to handle that current.
    Be sure you add the requisite capacitors on the output and input.
     
  3. vladtess

    Thread Starter Member

    Jan 5, 2011
    43
    1
    Thanks much Ron. Well, one of the current sensing resistors that I am planning to use is a 0.01 ohm with 1% tolerance. Will that make a huge difference? Thanks!!! ))
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    When using a Vbe junction across the sense resistor, current limiting won't occur until the voltage drop across the resistor is high enough to cause the junction to conduct current, i.e., 0.6-0.7V. This means that your current limit will be 60-70 amps. Sounds a little high for an LM338.:eek:
    Maybe you should tell us in detail what you are trying to accomplish.
     
  5. vladtess

    Thread Starter Member

    Jan 5, 2011
    43
    1
    Maybe you are right, I need to elaborate a bit more.

    Basically, I want to achieve two things:

    1. set current limiting through the use of a high power transistor (that will be controlled via a micro with an opamp [voltage to current converter]).
    2. measure how much current is being drained from the LM338 via a shunt resistor (as I have mentioned above .01 ohm) via help of the opamp [diff. amplifier scheme].

    And the problem comes when I need to place them in correct order to make sure that regulation is proper with these things in place.

    Thanks much Ron.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What voltage and current ranges do you envision?
     
  7. vladtess

    Thread Starter Member

    Jan 5, 2011
    43
    1
    I have a supply of 36 volts so I am intending to get close to maximum capacity of LM338 which is 32 volts. Voltage range be adjustable from 1.6 - 32.

    I also intend to maximize the current of LM338 (8A tops) since I have a 10A supply. (3x 12V @ 10A each) I will change the input voltage based on the output to make heat less of a problem through the use of a micro (DAC).
     
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