How to read the DC voltage output from a rectifier for MCU purposes?

Discussion in 'General Electronics Chat' started by Xavier Pacheco Paulino, Jun 5, 2018.

  1. Xavier Pacheco Paulino

    Thread Starter Active Member

    Oct 21, 2015
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    Hi,

    Let's say that I rectify 120VAC from the grid. That would be like 170VDC at the output. How can I measure that voltage so a microcontroller can process that information? I've seen that sometimes resistors dividers are used. So, with a resistor divider, can I convert the 160VDC to 0-3.3V , and use an ADC from the MCU? Right?

    The reason why I'm asking this is because I read from an App Note (fragment attached) that voltage measurement can replace an speed sensor as feedback mechanism in PMDC motor control. Is that good idea?
     
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  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It will be something less than 120v if no smoothing cap is used.
    Which sounds like the app note is doing.
    Max.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    During part of my career I designed production equipment for semiconductor manufacturers. When I needed to measure the AC line or get feedback from it I used a transformer. That way I got the necessary information and things like Neutral and Line being swapped had no effect on the measurement and technical personnel were not needlessly exposed to lethal voltages
     
  4. danadak

    Well-Known Member

    Mar 10, 2018
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  5. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    If you want an accurate representation of the speed of a motor that is delivering power to anything then reading the applied voltage will not provide it.And if the drive power comes from a PWM driver it will be even less than poor accuracy. The good news is that some BLDC drivers use an optical pickup or a hall effect pickup to read rotor position, which the pickup signal will also give an accurate representation of speed. You will need to use a high impedance circuit to sample the voltage and avoid disrupting the motor control system, though.
    Is the intention to have a good stable speed control by means of feedback?
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    According to previous posts, his motor is DC brushed.
    Max.
     
  7. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    OK, Max. But the assertion that applied voltage is not a good way to read speed still holds. I must have misread the PMDC at the tail end.
     
  8. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    With any kind of motor you can read the speed with a retro-reflective sensor and a spot of white paint on the armature. That gives a pulse rate exactly proportional to speed. OR, add an LN2917 or LM2907 IC and get a voltage linearly proportional to speed. With the right kind of filtering you can even read the commutation frequency and not need a pickup, but the brush noise on some motors makes the filtering a challenge.
     
  9. danadak

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    Mar 10, 2018
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  10. MaxHeadRoom

    Expert

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    Picmicro have a few ways of determining DC motor rpm using various feedback methods, one is to read/capture the BEMF of the motor, which can be quite accurate.
    Max.
     
  11. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    Reading the back EMF of a brush type motor is definitely a valid approach and it can work well, but it is sort of complex because the read sample must be done during the time when the PWM drive is off, and at higher drive levels that is a quite short time period. Also, with a brush type of DC motor there is still brush noise to contend with. So while it can work it is not simple, although Max may have information about a one-chip system that does it all.
     
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