Negative Voltage for LM338 Current Regulator circuit

Discussion in 'General Electronics Chat' started by maw455, Jul 27, 2016.

  1. maw455

    Thread Starter Member

    Jul 20, 2016
    30
    1
    I'm new at this, so bear with me. I need to make a negative DC voltage circuit, and while negative/positive voltage seems straightforward at times, other times it doesn't, at least to me.

    I'm building my first power supply and I'm incorporating the LM338 adjustable current regulator circuit attached below, which is from the LM338 data sheet http://www.ti.com.cn/cn/lit/ds/symlink/lm338.pdf

    For the negative voltage portion of the circuit, I've drawn up the Negative Voltage Circuit attached below. Will this work? See any problems or concerns?

    Thank you.

    Mod edit: Same question posted in the Projects forum. Those posts have been moved here. One thread per question please.
     
    Last edited by a moderator: Jul 28, 2016
  2. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    No, I don't think that will work.
    What is the purpose of the resistor?
    If you have an isolated transformer source for the negative voltage, then just make another LM338 circuit but use the positive output as the sole common point.
    The fact that you are using the positive output as the common point will not affect the output. I did a similar thing on the power supply I built myself and it works fine.

    Otherwise use the LM337 for the negative voltage.
     
  3. maw455

    Thread Starter Member

    Jul 20, 2016
    30
    1
    Here is what I'm trying to figure out. I'm incorporating the circuit design below, posted by SgtWookie in response to another thread on an LM338 power supply. I need a -5V supply for the input shown at the bottom of the schematic. SgtWookie suggested the following:

    "In the version that requires a negative supply, you will need another small transformer, that outputs around 6VAC, or if center-tapped, each side 6v. A 0.1VA rating would be more than enough power. You will also need a small rectifier bridge, and a small capacitor to keep the -5v more or less stable. It is not important for the negative voltage to be regulated."

    So yes, I'm planning on using an isolated transformer for the negative voltage, and drew up the circuit attached above.

    I put in the resistor to avoid shorting out the capacitor. Is that wrong? More importantly, please help me understand why this circuit won't work?
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    SgtWookie's circuit is not to generate a negative output, it's too allow the positive output to go to 0V (normally the output of a LM317 or LM338 is 1.2V minimum).
     
  5. maw455

    Thread Starter Member

    Jul 20, 2016
    30
    1
    Thanks for your reply. My understanding from SgtWookie's post is that the purpose of tying negative voltage to the LM317 in the diagram is to cause it to sink 10mA from the LM338 in order to stabilize the current regulation in the LM338, which I think I understand. In his diagram and in the one from the LM338 data sheet, the LM317's adjustment pin needs to be connected to a negative voltage source of between -5V and -10V. Ok, so where do I get a negative voltage source in this range? SgtWookie suggested using a separate, small transformer, rectifier bridge and small capacitor to accomplish this, but did not include a circuit diagram for this additional circuit. That's what I am attempting to understand and design properly. What am I doing wrong?
     
  6. maw455

    Thread Starter Member

    Jul 20, 2016
    30
    1
    I'm trying to design and build my first power supply, adjustable to deliver 1.5-12VDC and 0-1.0A. Because I am just a hobbyist, I want to start out using voltage regulator circuits to control voltage and current. To get various pre-set current limits, I'd like to incorporate the following circuit posted in another thread by SgtWookie:

    LM338 Current Limited Supply.PNG.pdf

    This is based on the following circuit from the LM338 data sheet:

    LM338 adjustable current regulator circuit.pdf

    I realize SgtWookie's circuit is for a 5A max current, so I just need to adjust the resistor values for 1A max.

    As I understand it, to have stable current regulation, the LM338 needs to sink 10mA of current through the LM317 in this design, which is accomplished by connecting the Adjustment pin of the LM317 to a negative voltage source between -5 and -10V. SgtWookie suggested in reference to the attached schematic that this negative voltage source could be supplied by a separate small transformer, bridge rectifier and cap, and that regulation was not needed for this purpose.

    I'm struggling with understanding how to generate only a negative voltage from a transformer and the only thing I can seem to come up with is modifying a split-rail design like this:

    Negative Voltage Circuit.pdf

    Will this work? If not, please help me understand why, as negative voltage circuits seem a bit confusing to me.

    Thanks in advance.
     
  7. dannyf

    Well-Known Member

    Sep 13, 2015
    1,775
    360
    Use a dc-dc converter, 3406x for example. You don't need much current on the negative rail.
     
  8. maw455

    Thread Starter Member

    Jul 20, 2016
    30
    1
    I don't think the 3406 will go below -1.5v, will it? The LM317 circuit above needs a minimum of -5V.
     
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