voltage regulator circuit

Discussion in 'The Projects Forum' started by jboulasii, Sep 12, 2009.

  1. jboulasii

    Thread Starter Member

    Aug 16, 2009
    11
    0
    I need some help. I have a 1/2 wave rectifier feeding a LM317T voltage regulator. The input to the rectifier is 120VAC so I'm getting about 54 volt on the other side of the diode. I have this voltage feeding the Vin of the LM317T. According to my calculation if I have a 2200 ohm resistor from the adjustment terminal to ground and 200 ohms from adjustment to Vout I should get about 15volts on Vout. I'm getting 34volts. I also have a .1uf cap from Vin to ground. Any idea on what i'm doing wrong?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    It is not good to feed more then about 40 Volts at the input.
    When the output gets shorted it is the maximum allowed voltage.
    You can use a transformer 15 - 18 Volts for the input.
    Use a bridge rectifier and a nice capacitor to remove the ripple.
    The resistor between output and adjust is normaly 120 Ohms.
    The resistor from adjust to ground will be round 1300 Ohms.

    Greetings,
    Bertus
     
  3. jboulasii

    Thread Starter Member

    Aug 16, 2009
    11
    0
    Bertus,

    I thought about the transfomer but I'm trying to keep the packaging as small as possible. What about a 20 volt zener diode on the input Vin.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    You will never get it real safe when feeding the regulator from the mains.
    If some component fails the change will be there the mains will be at the output.

    Greetings,
    Bertus
     
    quadrophedia likes this.
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You MUST use a transformer to provide isolation from the mains power.

    They are far less expensive than a funeral.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The resistor from the output to the ADJ pin must be 120 ohms or less for an LM317 or the output voltage will rise when there is no load. The more expensive LM117 can use 240 ohms.
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Average 54V? What about the peak voltage? Horrible maths. :(
     
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