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Reading Current of Solenoids

Discussion in 'General Electronics Discussion' started by Omar Zeyad, Dec 18, 2015.

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  1. Omar Zeyad

    Omar Zeyad

    6
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    Dec 18, 2015
    Hi Guys,

    I'm having some issues with a circuit that I've created to measure the current going through a solenoid.

    Basically, I'm using AD8202 to measure the current being supplied to a load (solenoid),

    Here is a small description of the circuit.
    Capture.PNG

    The expected current that will go through the solenoid is about 2 Amps! I already tested the circuit with a power resistor and I was able to read the current through PIN 5. but everytime I test the circuit with the real load (the solenoid). the AD8202 gets blows!!

    What do you guys think? would this have to do with the back EMF from the solenoid? or is my design have an issue?

    Thanks
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    duke37 likes this.
  3. Omar Zeyad

    Omar Zeyad

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    0
    Dec 18, 2015
    I was thinking of adding a diode to the solenoid (to drain the back EMF). but I was afraid that it will change the property of the circuit that I'm testing.

    Any idea if there is another chip that can handle higher voltages?
     
  4. Omar Zeyad

    Omar Zeyad

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    Dec 18, 2015
    I guess everyone is out watching the new star wars movie!!:p:D
     
  5. GPG

    GPG

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    Sep 18, 2015
  6. duke37

    duke37

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    772
    Jan 9, 2011
    Your circuit does not have the clamp diode so excessive voltage is generated on switch off.
    Having a higher voltage chip may not work since the solenoid can doubtless produce several hundred volts if turned off fast enough.
     
  7. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    You need to have a completely separated power supply for the solenoid, so the back emf does not enter the chip.
     
  8. davenn

    davenn Moderator

    13,880
    1,964
    Sep 5, 2009
    his circuit and the datasheet circuit already show that fact Colin !!
    a snubber diode is still required tho to stop the large back EMF voltages entering the IC sense input pins


    D
     
  9. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    How can a large emf develop across a 100 milli ohm resistor ??????
     
  10. GPG

    GPG

    452
    66
    Sep 18, 2015
    The ground is shared so back emf is impressed on the sense terminals relative to com.
     
  11. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    If the two have completely separate power supplies, no extra protection is needed.
     
  12. GPG

    GPG

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    Sep 18, 2015
  13. Omar Zeyad

    Omar Zeyad

    6
    0
    Dec 18, 2015
    Thank you guys, I'm going to try adding a clamp diode over the load.

    I'm almost certain it's the back EMF, I tested the circuit with power resistors and didn't have an issue. but I did have the same problem when using solenoids.
     
  14. Minder

    Minder

    3,120
    673
    Apr 24, 2015
    I am wondering why you need to measure the current as if the solenoid is DC the current is predictable based on the resistance of the coil?.
    If it is a case of confirming the current is actually present, then you do not need an A/D conv to do this?
    M.
     
  15. Omar Zeyad

    Omar Zeyad

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    Dec 18, 2015
    I'm validating a circuit that powers up the solenoids (which are current controlled).
     
  16. Minder

    Minder

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    Apr 24, 2015
    If I read that right then all you need is actual current confirmation, not the value itself!
    M.
     
  17. Omar Zeyad

    Omar Zeyad

    6
    0
    Dec 18, 2015
    Well, the current supplied to the solenoid might change.
     
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