Regulated Power supply

Discussion in 'The Projects Forum' started by Norfindel, Mar 20, 2008.

  1. Norfindel

    Thread Starter Active Member

    Mar 6, 2008
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    9
    Hello,

    I'm about to make a regulated power supply using a LM317 and a pass power transistor. I had seen circuits like this:
    [​IMG]
    However, why not to just remove R1 and change R2 for a direct connection? To me it seems like the regulator would try to keep the output voltage, modifying the current thru the transistor's base. It's also more straightforward.
    Probably there's something i'm not seeing, but why to use R1 and R2? To me it seems like the circuit would run just fine without them.
    I built the circuit in a protoboard, and the only ill effect seems to be that the power supply output voltage goes up (1V approx) with no load. Probably this is because the ADJ current of the regulator is forcing the transistor to conduct. Any load (for example: a led using 20ma current) would lower the voltage to normal levels.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    896
    The max current for the LM317 is 1.5A but a few go to 2.2A.
    The max output with the extra power transistor is about 10A.

    R1 senses the current and turns on the transistor when the current becomes faily high.
    R2 limits the base current of the transistor when the output is shorted.

    R4 in this circuit and a circuit without the transistor should be 120 ohms or less for an LM317 and can be 240 ohms or less for the more expensive LM117. If R4 is too high then the output voltage rises when there is no load.
     
  3. Norfindel

    Thread Starter Active Member

    Mar 6, 2008
    235
    9
    Ok, R2 saves the transistor if the out is shorted. So, if the PS isn't going to have other means of current limiting, it wouldn't be too wise to remove it.
    R4 is 120 ohms, but what i think is happening is this: the transistor is always on, so it's always suppliying current to the load, and thus, the load of the LM317 isn't 10mA anymore, but much less.
    Thank you for such a quick response.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    You don't show any parts values.
    If R1 is the correct value (3 ohms?) then it keeps the transistor turned off until the load current rises high enough to develop enough voltage across R1 to turn on the transistor.

    The idle current of an LM317 is 10mA max. With R4 as 120 ohms then the current must be more than 10.4mA to cause the output voltage to rise without any other load.
     
  5. Norfindel

    Thread Starter Active Member

    Mar 6, 2008
    235
    9
    Oh, sorry, i should had been a lot clearer. The higher output was happening when removing R1 and shorting R2, that's why i said that the transistor was always on. I was just testing what happened if instead of using that 2 resistors i let the regulator to modify the base current without any other component in the middle. But as you said, should a short circuit occur at the output, the regulator would be ok, but not the transistor.

    The circuit is at the bottom of this page, in case you want to take a look: http://www.redcircuits.com/Page123.htm

    Thank you, again.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You removed R1 so the 10.4mA idle current of the LM317 turned on the transistor.
    R1 is supposed to be there to turn off the transistor until enough load current turns it on.
     
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