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Voltage Drop in DIY Power Supply

Discussion in 'Power Electronics' started by dwn, Feb 22, 2015.

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  1. dwn

    dwn

    7
    1
    Feb 22, 2015
    Hello everybody, I have constructed a small DC power supply with a variable resistor (10K pot) and the LM317. The circuit supplies the correct upper limit load ( ~16V from 18V supply) however, the lower limit only reaches about 7V when the pot is turned all the way up. The other problem I am having is with the output under a load. I hooked up an LED to test the circuit output, but quickly realized the voltage is plummeting to ~1 volt after the LED did not light up. I'm beginning to wonder if I have the potentiometer hooked up incorrectly, and if whether or not that could cause a voltage drop under a load. I used several resistors of various sizes to test to output voltage and my DMM read a voltage output you might expect given the resistance. I'm at a loss here. This is the first circuit I've ever really soldered together and its been heck of a learning experience. I have attached a simple schematic and a couple pictures of the project. If you could offer any advice, I would greatly appreciate it.

    Note: I have a resistor connection drawn over top of the lm317 and not connected to a 4th port. Poor planning on my part when drawing out the schematic. I thought it would be helpful to label the PCB layout.
     

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,397
    2,777
    Jan 21, 2010
    The 1k resistor you're using is way too large. Check the datasheet, I think it recommends 240 ohms.

    Changing this resistor will mean you require a (roughly) 2.5k pot (which may be hard to find).
     
  3. dwn

    dwn

    7
    1
    Feb 22, 2015
    Hello Steve, thanks a lot for your input. I'm waiting for the iron to heat up now. In regards to the pot, should all 3 terminals be connected to something? At the moment, I am not using the Vout terminal.
     
  4. davenn

    davenn Moderator

    13,596
    1,875
    Sep 5, 2009
    yes they should ...the Vout has the 240 Ohm resistor that Steve mentioned connected to it, not going to work without that
     
  5. dwn

    dwn

    7
    1
    Feb 22, 2015
    Hello again Dave. On my circuit, I have one end of the resistor connected to the ADJ and the other end to the Vout terminals of the LM317. hhhmmmm. Somehow the circuit is producing the correct output, now with the min voltage 1.25 and the max a couple volts let than the input.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,397
    2,777
    Jan 21, 2010
    It may be producing the correct voltage if you have a load, or if you had a bad soldered connection. If that fixed resistor is too high in value the output voltage will not be stable.
     
  7. dwn

    dwn

    7
    1
    Feb 22, 2015
    Thank you guys for your help. I don't know what people did without the internet..whew. Anyways, I only have about a max output of 1.40 mA under a small load (1K resistor and led). When I hook up the fan The wall wart I am using has a current rating of 300mA. How can I increase the current value? In order to do this, would it require a small transformer?

    The PWM emits regulated square waves to control the voltage and is typically used to control motors... as opposed to the pot, is this generally a better method to use for a power supply?
     
    Last edited: Feb 23, 2015
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