LM317, Filter cap issues

Discussion in 'The Projects Forum' started by MorRobots, Aug 9, 2013.

  1. MorRobots

    Thread Starter New Member

    Aug 8, 2013
    So I have built the LM317 at least 7 different fundamental ways and the moment I put a filter cap of 1000μF across the voltage coming out of the bridge rectifier the output goes to max voltage.

    Sorry for the lack of details but if you want a schematic just Google LM317 and pick a random one, I've used it. I am trying to produce a fixed voltage and the value of R1 and R2 are not important however I have tried 3 orders of magnitude IE 10Ω, 100Ω, 1kΩ, I have used varying caps between ground and the input and the output and ground.

    I found the higher overall resistance on the voltage divider feeding the adjuster and the load would effect output voltage. however at the lower Resistance it's harder to get the desired output voltage within 1% due to variations in resistance. (I know I know Trim pot)

    Everything works fine until that dam 1000uF filter cap is added.

    What am I doing wrong?

    my Transformer outputs 24v AC @ 60 Hz and it is getting rectified to 22v DC @ 120Hz.
  2. richard.cs


    Mar 3, 2012
    The values of R1 and R2 are important, particularly their ratio. It sounds like you are setting (with those resistors) a target output voltage greater than the input voltage so the 317 just feeds whatever is on the input straight through. That or you've connected the wrong pins on the 317, or have a damaged one. The voltage at the input should go up when a filter capacitor is added but when the circuit works correctly the output shouldn't change.
  3. MorRobots

    Thread Starter New Member

    Aug 8, 2013
    The Supply voltage is greater than the target by >7v The resistors ratio is calculated for the exact output

    1.25 * (1+(R2/R1)) + 5μ*R2

    I have used two different LM317s one from TI the other from Fairchild and I double checked the pin layout on both data sheets.

    Also if the filtercap is removed the voltage goes DOWN to the target output. however the output is at 120Hz yet with the filter cap its no noise DC just at Max voltage.

    diode across the output and input pin (tried it)

    perhaps I have two damaged LM317's I just find it odd that if the Filter cap is removed the circut works just fine or at least on the meter. only thing I can think of is the Cap on the out voltage might be to small in relation to the input side.
  4. SgtWookie


    Jul 17, 2007
    You'll probably be surprised at how high the voltage is on your filter capacitor; it'll be around 33v when the transformer is under its' rated load, and significantly higher than that with no load.

    The LM317 has a maximum input to output differential of 40V, and with a light load, you may exceed the maximum specification of the LM317 and damage it.

    Use 120 Ohms for R1, as this will provide a 10mA current flow from the output terminal, which will cause the regulator to perform within the guaranteed regulation range, as 10mA is the minimum required load for guaranteed regulation.

    Then, use the datasheet formula for calculating the value for R2.
    Vref is usually around 1.26v.
    Iadj is usually around 50uA.
    Griz likes this.
  5. MorRobots

    Thread Starter New Member

    Aug 8, 2013
    So I did a re-attack on the bread board when I got home and managed to get it to work by starting with a resistance of 120Ω going to the Adj and grounding out. - Bam 1.26v output GOOD!

    I then added resistance between ground and Adj knowing the more resistance the more Vout will go up. After tuning the resistances to within 1% of the target voltage I ended with 60Ω 668Ω and no issues with or without the 1000μF cap as a filter. I just find it strange I had to play around with the board to get it to work rather then the formulas showing me the right answer. this also dose not help the fact that 60 is kinda a low resistance and would be feeding extra current to Adj.
  6. crutschow


    Mar 14, 2008
    The formulas will work if you use 120Ω for R1. Why did you use 60Ω?