Solenoid Valve Current Monitor

Discussion in 'The Projects Forum' started by Dazza1964, May 4, 2011.

  1. Dazza1964

    Thread Starter New Member

    May 2, 2011
    2
    0
    Hi all this is my first post.
    I need help to build a circuit to detect if a 24V dc 1.5A Approx 14 - 20 Ohm Solenoid coil is active of not.

    I need an output of 24Vdc when the coil is drawing current and no output if the coil has blown.

    I then would like the same circuit to be able to have two solenoid coils to come on and be able to detect if one had blown.

    I have available a shunt 3A 75mV 0.025R &10A 75mV 0.0075R

    Input voltage 24V DC. Any Help would be appreciated.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,148
    3,058
    Welcome to the AAC forum!

    There are a lot of approaches you could take. A fancy approach would be to use a comparator such as the quad LM339 set up as a window comparator. Have it watch the voltage across your 3A shunt, which is wired in series with your solenoid and one end to ground.

    Low or no voltage across the shunt would mean no current through the solenoid, light a green LED. Current above some reference would mean your solenoid has shorted, light a red LED. Positive current below that threshold means things are working, light a yellow LED. Or blue or white or whatever; you get the point.

    There are simpler approaches if you only need to detect current "on" versus "off". With the right shunt (a bit higher resistance than the ones you have), you could turn a transistor on to light an LED whenever there is current flowing.
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Please elaborate. Are you attempting a redundant backup? In other words, there will be two solenoids performing the same task but only of them is on at any given time. If one solenoid goes west the backup will turn on to repair the fault?

    Does this mean that the circuit will be monitoring two parallel solenoids?
     
  4. Dazza1964

    Thread Starter New Member

    May 2, 2011
    2
    0
    Hi All

    i think I have attached a circuit diagram I created. I would like some feedback as to where or not this will work for 1 solenoid valve.
    I would also like to have control of adjustment of the comparator input around the R-Sense resistor. I just guest the values.
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,148
    3,058
    I'm not sure why you have those two transistors on the output, or why you'd use the oldest op-amp you could possibly find. Are you trying to get the output meter to read in proportion to solenoid current, or just be an on/off indicator?

    But anyway another problem you have is that the voltage at the inputs could very easily be right at the upper rail of the op-amp's power supply, meaning it won't give a reliable output. You should drop the input voltage to always stay within the common mode range.
     
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