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Correct voltage translator

Discussion in 'General Electronics Discussion' started by Vindhyachal Takniki, May 12, 2015.

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  1. Vindhyachal Takniki

    Vindhyachal Takniki

    9
    0
    Apr 29, 2015
    1. I have a motor encode with max speed = 6000 rpm & 3600 pulses at each revolution.
    So max freq = 6000*3600 = 21.6Mhz.
    2. Encode is powered by 10-30V dc. But MCU is powered by 3.3V.
    3. So I need to step down the voltage of encoder pulses.
    4. Currently encoder is powered by 10V. I have a placed a small resistor & 3.6V zener. I am getting step down voltage but there is some rise & fall of pulse at across zener, which I see is comparable when I compare encoder output & zener out.

    5. I can use either optocoupler or any buffer IC for this.
    6. I had looked at FOD8001, 25Mhz, but it require Vcc at input also. and max is 3.3 or 5V only.
    ICPL2631 is 10Mhz.
    7. Avago HCPL-0720, is good, but it require VCC at input with max of 6V. I need to place some regulator for powering input vdd also.

    8. Any other product fit for this?
     
  2. witsender

    witsender

    22
    14
    Dec 12, 2013
    :)
    Hi
    I believe the interface circuit should probably be a Schmidt trigger. Schmidt triggers are available as 6-to-a-pack IC's. Here's a link to a datasheet for the 74HC14A... http://www.onsemi.com/pub_link/Collateral/MC74HC14A-D.PDF.
    You'll still want to use some kind of voltage division or reduction to condition the signal for the Schmidt. I suggest that a resistive divider would probably do the trick. The 7414 will give you a nice clean (inverted) output, which can easily be re-inverted, using 2 of the 6 available triggers on the IC
    There are also low power versions, and probably singing, dancing versions of Schmidt triggers.
    :)\
     
    Last edited: May 12, 2015
  3. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Your calculation is incorrect. 6000 RPM x 3600 = 21,600,000 pulse per minute or 360000 pulses per second.

    Are you sure it puts out 3600 pulses per revolution? I can't even imagine how that is done.

    Bob
     
  4. witsender

    witsender

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    14
    Dec 12, 2013
    Yeah, Bob! Good question!
     
  5. BGB

    BGB

    154
    11
    Nov 30, 2014
    maybe they gear it up, such that while the motor turns rather quickly, the encoder wheel reaches absurd speeds.

    or, maybe a GMR read-head and dense alternating north/south lines or similar.


    or, more likely, the numbers are a bit off...
     
  6. poor mystic

    poor mystic

    1,074
    33
    Apr 8, 2011
    or perhaps measurement is being done at the motor and the shaft speed is given for a low-speed output shaft, driven through a gearbox.
     
  7. poor mystic

    poor mystic

    1,074
    33
    Apr 8, 2011
    :) Hi Vindhyachal
    Could we hear a little more about the motor and what it's being used for please?
     
  8. BGB

    BGB

    154
    11
    Nov 30, 2014
    yes, this is also possible.


    as for the topic of stepping down signal voltage, one strategy I had used before was to use a voltage divider circuit (a few resistors) to step down the input voltage to levels I could use to switch a transistor, then using the transistor to get voltages at the desired output voltage (it would switch using 3.3v as the Vcc rail).

    there is a possible issue with transistors that if the base voltage goes too far out of range (in the "off" direction), then it can exceed the breakdown voltage (say, 5v), and current will flow to the emitter (can be bad if it takes the voltages out of range).

    the purpose then of the voltage divider is to both limit base current and try to keep the voltages in range of what the transistor can tolerate.
     
  9. poor mystic

    poor mystic

    1,074
    33
    Apr 8, 2011
    Still, as BobK says, 21.6 MHz is a very high pulse rate to be coming from a motor. Even with gearboxes. There's a credibilty gap,.. I can't believe it, I'm sorry...
     
  10. BGB

    BGB

    154
    11
    Nov 30, 2014
    he was off by a factor of 60 due to mixing up seconds and minutes, and 3600 pulses/rev is drastically higher than what I have generally seen from motors.

    or, IOW, stuff isn't really adding up (either the motor operates faster than any I can currently confirm as existing, or the encoder is more precise than anything I am aware of existing).

    or, otherwise, the numbers seem a bit off...
     
    poor mystic likes this.
  11. Vindhyachal Takniki

    Vindhyachal Takniki

    9
    0
    Apr 29, 2015
    1. Yes right , its 360Khz.
    2. I have encode inside it. I can't open it. But that;s for sure that for evry revolution of motor it gives 3600 pulses.
    As I had measured it with MCU
    3. NowI will using opto from avago & see what are results.
     
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