12V and 1.5A

Discussion in 'General Electronics Chat' started by adithya.rp, Apr 30, 2011.

  1. adithya.rp

    Thread Starter New Member

    Mar 2, 2011
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    Hello.

    I am designing a pulse width controller to control a solenoid coil.

    At 100% dutycycle, i need 12V across the coil and 1.5A flowing through the coil. The inductance of the coil is not known. How do i solve this issue?

    Thank you!!!
     
  2. DigitalReaper

    Member

    Aug 7, 2010
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    V=IR, and since V and I are fixed so must R. Your coil must have a resistance of 8 ohms (12/1.5)
     
  3. adithya.rp

    Thread Starter New Member

    Mar 2, 2011
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    But its an inductor!!! It won't obey Ohm's law right? When i tried the circuit in simulation with a 10mH inductor, the voltage was around 500mV and current was over 100A. My coil will burn for that current value. It can take a max of 1.5A only.
     
  4. DigitalReaper

    Member

    Aug 7, 2010
    70
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    100% duty cycle is just a DC current, and at DC an inductor is just a length of wire. Your simulation probably didn't factor in the resistance of the coil which MUST be 8 ohms to satisfy your requirements.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    The inductance of the coil has no effect on the maximum current that will go thru it, it is the resistance of the coil limits the current. Inductance will just effect how fast this current is reached.

    I am unaware of any relay or solenoid rated to run at a certain coil voltage that need an external resistance to maintain the spec'ed current. They may exist but I just don't know of any.

    Post a link to the data sheet and we'll tell you what the device requires.

    AND... generally using a solenoid in a PWM control system is a mistake. They are just not fast devices.
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    100% PWM isnt REALLY PWM. it is just...ON. ;)
     
  7. adithya.rp

    Thread Starter New Member

    Mar 2, 2011
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    I was just mentioning the max values... The PWM dutycycle can be anything between 0 to 100%. I'm controlling the dutycycle based on a feedback signal with a microcontroller... and the person i'm working for wants 1.4A and 12V at 100% dutycycle. And the voltage and corresponding current must change according to the dutycycle...
     
  8. Wendy

    Moderator

    Mar 24, 2008
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  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    adithya.rp: Perhaps you should just tell us what your client want you to deliver, then we can help you devise a way to produce that.

    If someone asked me for "1.4A and 12V at 100% dutycycle" I would not know what they need.
     
  10. adithya.rp

    Thread Starter New Member

    Mar 2, 2011
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    My project is to control the movement of a solenoid valve through PWM. I am generating a PWM from a microcontroller based on a feedback signal. If the PWM is 100%, it must be 1.5A flowing through the coil. For 50%, i need 0.75A etc. The dutycycle can be anything between 0 and 100%. It may be 11%, 20%, 35% etc... and the current must be corresponding to that.
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I somewhat doubt you'll be able to control a solenoid's position by changing the input to it unless it has a spring assembly on it - even then it probably won't work.

    Since you're dealing with DC simply measure the DC resistance of the coil.
     
  12. retched

    AAC Fanatic!

    Dec 5, 2009
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    Are you SURE you are using a solenoid and not an ACTUATOR?
     
  13. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    By this do you mean at 50% you expect the solenoid valve to be 50% open?

    If that is true please post a link to the specs of this solenoid as no one here will believe such a control exists.
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
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    If you assume 100% (pure 12V DC) will give the proper current through the coil that represents full opening of the valve, any PWM that is less than 100% will give proportionally LESS current. The proportioning of the valve may or may not be linear with the percentage of duty cycle.
     
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