Inrush current

Discussion in 'General Electronics Chat' started by Dritech, May 30, 2014.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi,

    I am using a sensor to monitor the current of a motor. The output of the sensor is connected to a voltage comparator which is compared with a fixed set-point. To prevent the interrupt (which is connected to the output of the comparator) from being activated with the in-rush current, I planned to use an RC delay. Attached is the circuit. Will it work this way please?

    Edit: I use a potentiometer instead of a fixed resistor to vary the delay time.

    Edit 2: Also, is 200ms enough for prevent the interrupt from being triggered from start-up in-rush current?
     
    Last edited: May 30, 2014
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    There are lots of ways to do this. Using your network might work, but it need to have a long charge time, which might affect the response of your circuit. Another idea might be to bypass the sensor circuit while your motor is starting up. Again, there would be many options to accomplish this.
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks for the reply. Before the interrupt pin, the RC circuit will be connected to an AND gate. Will the RC circuit have any effect on the gate IC input pin, especially when the pot is fully rotated?
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Yep. It adds the same delay as in the interrupt input
     
  5. Brownout

    Well-Known Member

    Jan 10, 2012
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    If you use the AND gate as an enable to the interrupt pin, and connect the RC to the power rail, so that it is activated on power up ( one-time ) then you don't effect the operation of your sense circuit.
     
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi,

    Yoy said that "your network might work". In which conditions it may not work please? are there any other practical options which make use of few components?

    Thanks.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    My preferred method is to detect the motor over-current situation at all times, so the software is always aware that the motor is drawing too much current.

    Then if the situation is a startup situation, the software just ignores the over-current signal.

    And that uses "less components" like you asked. :)
     
  8. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks for the reply. At the input of the interrupt I will be using an AND gate to trigger the interrupt by either the current sensor or a limit switch. For this reason I cannot use that method because I need the delay for the current sensing only.
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    Well, it depends on how long it takes the motor to start up and how long the time constant of your network is. As well, where is the switch point for your AND gate. To insure you don't get false triggers, you need a time constant longer than your startup time, by a factor of at least 3 ( I would think ) That means there is a risk of a fault during that time. You don't get something for nothing. You might also think about using a Schmitt trigger or something to give you a clean edge at the end of startup.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Just poll the over-current input pin.

    If you use a fast timer interrupt you can use it to poll that pin manually (during the int) AND to generate any delays you need.

    Using "interuupt on pin change" is something I very rarely do, it can cause a heap of hassles (as you are finding out) and limits your options.

    Micros are so fast you can manually poll that input pin in an interrupt at 15625 Hz or other convenient timer rollover freq. A motor current sensor will need software debouncing anyway and the overcurrent situation will exist for >100mS at least. So manually polling is easily fast enough.
     
  11. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks for the replies. I will still be using an AND gate with an RC input for another application. Is it ok to do this? At what logic will the output of the AND gate be when the RC is in the range of 2.5V (neither logic 1 or 0)?
    I know that a schmitt trigger can be used between the RC circuit and the AND gate input, but I am very limited with space.
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
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    Look at the datasheet for your logic device. There will be a spec for input low and input high. But between the two, the binary level of the input will be indeterminant. To guarantee the logic will sense a 0, you will need to keep the input under the VIL specification. The point it switches cannot be determined.
     
  13. ScottWang

    Moderator

    Aug 23, 2012
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    Something about the motor, sometimes if just using the RC delay may not working properly, have you ever measuring the waveform?
     
  14. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi, what can cause it not to work properly?

    Regarding the AND gate with an RC input, so it is not recommended to use it that way, right? will I get random values at the AND gate IC when the input is between logic 1 and 0?

    Thanks.
     
  15. ScottWang

    Moderator

    Aug 23, 2012
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    Because the motor may destroy a device or component, when it has the higher positive voltage and negative voltage, so it depends on the waveform and decide how to solve the problem.
     
  16. Brownout

    Well-Known Member

    Jan 10, 2012
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    As long as you keep any transients caused by the motor out of the logic rails, you'll be OK.
     
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