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Question on LM317 based high current voltage regulator

Discussion in 'General Electronics Discussion' started by Prithwiraj Bose, May 11, 2017.

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  1. Prithwiraj Bose

    Prithwiraj Bose

    5
    0
    May 11, 2017
    I am new to Electronics and have been spending time for a while now as a hobbyist. I have a fairly simple question regarding the circuit below:

    [​IMG]

    I have made the circuit but the it would only regulate between 18V-24V and sometimes the range slightly changes based on the POT used. But still, I do not get the full range starting from or very close to 0V upto 25V. What should be the voltage around R1 ideally? I am expecting to regulate the voltage without compensating the current limit using this LM317 based circuit. But somehow when the power transistor is connected the LM317 stops behaving as expected. When it is detached, I see expected range of voltage. Isn't the principle correct that Ve should be Vb - 0.7v approx and thus this circuit should output a regulated voltage across R1?

    I have been trying to figure out the problem and tried to get help from other forums. I have only received the average answers saying why linear regulators are bad, that I should use an SMPS and all other suggestions which doesn't answer my question. I am trying to learn it and this circuit has drawbacks, which I admit while asking this question. I am expecting to get some meaningful answer here which explains why the circuit doesn't behave as expected, which my newbie understanding can grasp. Please help.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,448
    2,809
    Jan 21, 2010
    You don't show where the output is. I presume it is across the 1K resistor?
     
  3. Audioguru

    Audioguru

    3,042
    678
    Sep 24, 2016
    Your circuit is missing the important input and output capacitors shown on the datasheet of the LM317.
    Your 220 ohms for R2 is for the more expensive LM117. 120 ohms is needed fore the LM317 so that the output voltage does not rise when the load current is low. Then the value of the pot must be decreased to about 2.5k.
    The datasheet says that the minimum voltage of the LM317 is about 1.2V.
    Your diode and transistor emitter-follower ruin the excellent voltage regulation of the LM317 because their output does not send negative feedback to the LM317. The emitter voltage of the transistor will drop when loaded. The datasheet of a 2N3055 transistor shows that the emitter voltage can drop 1.5V at 4A and an additional 1.5V for your diode. Much more voltage drops at higher currents.

    The datasheet for the LM317 shows that a PNP power transistor can be added in parallel with the LM317 to pass most of the current and share the heat but it is controlled by negative feedback so the voltage regulation remains excellent.
     
    bushtech and hevans1944 like this.
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