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LM331: Resistor and Capacitor values for 5v supply and 300 Hz input

Part Number: LM331



In the above frequency to voltage converter circuit I have a supply voltage of 5V and an input frequency range of 0-300 Hz. The resistor and capacitor values above are from the example circuit outlined in your documentation for the LM331 IC, F to V which has a 15V supply. What will the ideal resistor and capacitor values be with the lower 5V supply and an input frequency range of 0 to 300 Hz? I am more concerned with accuracy than response time. I also would like to know what type of capacitor is recommended for each capacitor in the circuit if I used SMT components. The output voltage must be between 0 and 5V. Thank you for your time.

  • Hi Justin, 

    The formula for selecting components is shown in Figure 18:

    Vout = f_in x 2.09V x (RL/RS) x (RtCt) 

    Adjust values of the components so that when f_in is 300Hz,  Vout is 5V (if output must be 5V, Vs may have to be 8V), see the captured image.

    Some items that you need to be careful about with this circuit:

    Rs:  Make this value between 12k to 16k.  Making this resistor too small can take too much current from the internal circuit.  14k may be a good value to start with.

     Ct should be a NP0, Teflon or polystyrene. 

     CL (the 1uF) needs to be a mylar (best) types or ceramic (ok) type of capacitor.  This cannot be an electrolytic capacitor.



  • Thank you Raymond. Using the equation you provided I was able to manipulate the values to achieve an output voltage that I like. With that said, these are my new values, see below. RL and CL remained the same, but RT is now 10k and CT is 0.1uF. This will give me about 4.5V out at 300Hz. Am I at liberty to change RT and CT, or should I simply change RL to achieve the desired Vout? If I change RL to 1M I can achieve similar Vout results.

  • Hi Dustin,

    Please change RT and CT instead. Do not use tiny capacitor package < 0805 capacitor in size. If you can use 1206 package size.