Please help me with LM2917 F/V converter.

Discussion in 'General Electronics Chat' started by evo.motorsport, Jun 20, 2007.

  1. evo.motorsport

    Thread Starter Member

    Jun 20, 2007
    21
    0
    I’m building digital tachometer to display spindle speed on my hobby CNC machine.

    I’m using optical censor to pick up the RPM signal. Signal from the optical sensor connected to LM2917. I’m reading output from the LM2917 using digital voltmeter built on ICL7107 to display the output voltage. When I’m increasing the spindle RPM the voltage increases and when I’m decreasing spindle RPM voltage decreases.

    But here is the problem:
    The output voltage increases only to 3.6V. Then it stays on 3.6V, does not get any higher.

    The supply to LM2917 is 6V

    Please help. What am I doing wrong?
     
  2. kender

    Senior Member

    Jan 17, 2007
    263
    0
    May be your frequency is too high for your selection of C1. The last formula on page 7 of the LM2917 datasheet shows how to calculate the maximum frequency that LM2917 can convert.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,142
    1,790
    Can you post a schematic diagram of your circuit?
     
  4. evo.motorsport

    Thread Starter Member

    Jun 20, 2007
    21
    0
    My frequency is 0Hz to 670Hz.
    Can you help me to calculate? How can I output 5V or even better 10V on max frequency?
     
  5. kender

    Senior Member

    Jan 17, 2007
    263
    0
    I'd be happy to help you with calculations, although I need to know your circuit. Please post a schematic or tell us the document (datasheet, app note, web site etc.) and a figure where the circuit diagram can be found.
     
  6. evo.motorsport

    Thread Starter Member

    Jun 20, 2007
    21
    0
    Here is the schematic.
     
  7. kender

    Senior Member

    Jan 17, 2007
    263
    0
    I think, the capacitor connected to pin 2 is too big. You should be able to go to 670 rpm, if you put a 0.033uF capacitor or smaller.

    By the way, at what RPM does your output reach 3.6V an stop changing in your original design?
     
  8. evo.motorsport

    Thread Starter Member

    Jun 20, 2007
    21
    0
    Actually my full RPM is 16000 but I'm dividing frequency by 24 usig decade counter. So my maximum frequency is 667Hz. The output reaches 3.6V at 480Hz
     
  9. evo.motorsport

    Thread Starter Member

    Jun 20, 2007
    21
    0
    Tried 0.033uF.... now voltage stops at 2.8 on 177Hz :(
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Can you post a more complete schematic? We need to see the rest of the circuitry to be able to help out. Generally, 2917's are pretty straightforeward to set up.
     
  11. evo.motorsport

    Thread Starter Member

    Jun 20, 2007
    21
    0
    That's the complete diagram or the the module. Frequency go In to 2917 and digital voltmeter connected to the Vout.

    is it any way to calcullate the pars values base on input parameters?

    The frequency in from 0Hz to 667Hz I need the output voltage to be 0V to 10V
     
Loading...