power supply help

Discussion in 'General Electronics Chat' started by Bryan47, Dec 1, 2010.

  1. Bryan47

    Thread Starter New Member

    Dec 1, 2010
    Hey guys,

    Desigining a small power supply, variable with an lm 317t. I haven't done this since high school
    so I was hoping someone could just look over the circuit for me? It seems to work I just
    want to make sure I didn't miss anything or if there are any improvements I can make.

    PS its in a multisim file I'm not sure how to post it!
  2. bertus


    Apr 5, 2008

    In the FAQ there is given how to post attachments:

    You can also post a screenshot of the circuit as not all have multisim.

  3. Bryan47

    Thread Starter New Member

    Dec 1, 2010
    Sorry I'm on a mac using bootcamp and couldn't get the Screen Shot to work. Here it is! Thanks!

    Transformer is 24 V on the outside 12 V center tap
  4. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    24VAC produces a rectified and filtered +32.5V. Extra input voltage causes a lot of heat in the regulator. The LED will have a current of about 30mA and might burn out unless it is a big one.
    A more expensive LM117 uses a 240 ohm resistor for R2 and an LM317 uses 120 ohms so that the voltage does not rise without a load on some LM317s. Then the value of the pot must also be reduced.

    The LM317 reduces its max output current when its input to output voltage is higher than 15V. If the input to output voltage is 40V then the max current might be reduced to only 150mA.

    Your circuit shows a pot that adjusts the output voltage down to 1.25V. If much current is drawn from the output at a low voltage then the IC will get extremely hot and will shut-down.
  5. gootee

    Senior Member

    Apr 24, 2007
    You can usually lower the output ripple significantly if you put 10 to 22 uF from the LM317's Common pin to ground. But you might then want to also install the standard protection diodes (1N4002 from ADJ (Common) to VOUT and 1N4002 from VOUT to VIN, although you wouldn't really need the one from vout to vin unless the output capacitor was larger).

    The LM117 datasheet from national.com is highly recommended.
    Last edited: Dec 1, 2010