Can use LM2917 freq to voltage converter as a dc motor FEEDBACK circuit?

Discussion in 'The Projects Forum' started by wayne_lik, Dec 17, 2007.

  1. wayne_lik

    Thread Starter New Member

    Dec 17, 2007
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    I use the voltage supply to control the 24v dc motor. After tat for the feedback circuit I use optocoupler to generate a frequency and then convert voltage from 0 to 5v by using LM2917-14 then send to PIC. wat i wan is 5 v to represent 24v and 0v represent 0v. Can i use LM2917-14 as a feedback counter?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    It will be harder than that. You can calibrate the 2917 output to give 5 volts out when the motor speed, with no load, is whatever it will be when fed with 24 volts. But the motor will stop turning at some point before the applied voltage has gone down to 0. You will have to experiment to see how low the voltage has gone before the motor stops, but it will not be 0 volts.

    If this is a school experiment, things should work well. If you really need to control the motor, you will need a more elaborate set of inputs to the PIC. Loads on the motor will slow it down, even though the applied voltage stays the same. You will have to monitor current as well to be able to implement a closed-loop controller.
     
  3. wayne_lik

    Thread Starter New Member

    Dec 17, 2007
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    other than tat, can u recommend any circuit to use as feedback circuit?
     
  4. beenthere

    Retired Moderator

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    Use an optocoupler to track motor RPM. You can get them with logic-level outputs.
     
  5. wayne_lik

    Thread Starter New Member

    Dec 17, 2007
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    thanks...
    i actually use an optocoupler to track the motor RPM. but should i use lm217-14 as a tachometer to detect the RPM of the motor???
     
  6. beenthere

    Retired Moderator

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    You have a PIC. Let it count the number of pulses per second and convert to RPM. You have digital data out of the optocoupler - use it that way.
     
  7. wayne_lik

    Thread Starter New Member

    Dec 17, 2007
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    actually i have use the circuit below as my counter. but i jz want the output range from 0v to 5v that can represent 0v to 24v in different frequency level.
    From the equation i have fixed the RC value and Vcc value, but i still cant get the actual value that i wan. The problem is the output will less than 5v if i change the resistor value and fixed the capacitor value. if i change the capacitor value and fixed the resistor value it can raech 5v but not the frequecy that i wan. Example: my maximun frequency from the opto-sensor is 0 to 1250 HZ, so i wan when 1250HZ that can giv me an output 5v. so wat do i need to change in the circuit that i use???
     
  8. beenthere

    Retired Moderator

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    The formula in the article gives Vout = (R1 + R2)(Vcc)(C1)(Fin). I don't know your supply voltage. I suspect it may be 5 volts, and you are having trouble getting the output of the 2917 up there. It may not be possible unless you raise Vcc above 5 - maybe as far as 9 volts.

    It would helpful if you posted the component values you are using.

    I will also repeat this: You have digital data out of the optocoupler. Converting it to an analog representation and then back to digital will lose some precision. You will have better results, if more programming effort, by taking the pulses directly into the PIC and using the internal counters.
     
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