7805 voltage regulator oscillation

Discussion in 'The Projects Forum' started by bitrex, Dec 17, 2009.

  1. bitrex

    Thread Starter Member

    Dec 13, 2009
    79
    4
    I'm working on a tube preamp project, and I'm using a LM7805 to provide a regulated source for two filaments - it's lower than standard voltage but it has to do with the transformer winding I have....in any case I noticed a lot of hum in the circuit and looking at the +5 volt line on a scope I saw a 5 Mhz 1V pk-pk signal on the supply! My question is how to go about attempting to fix this? Should I put more capacitance on the input, or on the output? Right now I have 2200uF on the input and just the 0.1uF the data sheet recommends on the output. The total current draw is 450 mA. Thanks in advance for any advice.
     
  2. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    A circuit diagram would help.
    How are you rectifying the filament voltage prior to feeding it to the LM7805?
    What voltage are you getting out of the transformer and what voltage are you seeing at the input of the 7805?
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Most of these 3 terminal regulators require a 0.1µF cap on both the input and output. If it is very far from the DC source at all a 220µF cap is also recommended by the datasheet. I'd start off with the 0.1µF caps though.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I tested a number of 7805 regulators awhile back to see what they'd do with no caps.
    Had a couple of salvaged Motorola 7805's that oscillated at 8MHz and 10MHz.
    Placing 0.1uF caps across IN/GND and OUT/GND cleared it right up.

    They need to be right at the regulator. Otherwise, the inductance of the wiring makes an LC circuit.
     
  5. bitrex

    Thread Starter Member

    Dec 13, 2009
    79
    4
    Thanks everyone, I put a 0.1uF cap on both the input and output of the regulator and it seems to have settled down.
     
  6. peranders

    Well-Known Member

    May 21, 2007
    87
    0
    To have 0.1 uF close the regulator on both sides is common practice.
     
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