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simple DC current sensing

Discussion in 'Electronic Design' started by Rob Votin, Oct 22, 2004.

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  1. Rob Votin

    Rob Votin Guest

    I am trying to find a simple way to detect the presence of a DC
    current. Its an industrial application where I need a PLC to be able
    to know if current is flowing to several 24 volt electromagnetic
    brakes (about 1 or 2 amps each).

    Cost is not really an issue, but ideally this would be some small off
    the shelf device (inductive or hall effect?) that could directly send
    a signal to 24 volt PLC input card. I dont want to use shunts, and I
    dont want to have to add an analog input card to the PLC rack.
    Everything I've found on the web is either for AC only or puts out an
    analog signal proportional to the current. I just want a simple on or
    off. Any ideas? Thanks...

  2. You might search the key words [DC current switch].

    I found:

    Of course, you could just wrap a few turns of wire around the glass
    capsule of a reed switch.
  3. j.b. miller

    j.b. miller Guest

    If you can afford a 2 volt drop in the power line to the load, just add a
    'module' consisting of 3 power diode in series ,paralleled with an
    optocoupler an 10 r current limiting resistor.
    Feed the opto transistor to the PLC as a 'current to load' indicator.
    have used this setup for years, rock stable,small,cheap,etc. made my own
    load indicating SSRs as well......

  4. scada

    scada Guest

    I received some samples not to long ago for a hall IC device. It will take
    up to 50A DC, and gives a linear 0-5V DC output isolated to something like
    3KV. I don't remember the name, but I can get it when I return to the office
    on Monday if your interested. I received like four samples, they sell for
    somewhere around $8 if I remember right.
  5. legg

    legg Guest

    You already have a signal; the voltage on the solenoid coil. Perhaps
    you think the signal connection for this is too long, and you're
    worried about open connections to the solenoid itself, or the sensing
    voltage connection.

    I'm not sure how you envision this (these) device(s) fitting into your
    wiring. It would seem to make more sense if the PLC solenoid driver
    card could sense this directly, without external interference.

    Over-current and under-current sensors are available from most relay
    mfrs. These have all of the problems of getting accurate and
    repeatable results from any electromagnetic actuator, unless they
    include other powered circuitry for measurement and control of the
    terminals. Clare offers app notes on winding your own reed relays,
    that could produce the lowest series voltage drop.

    A diode drop is sufficient to generate a 24V signal from a bipolar
    transistor that current is flowing in the solenoid wire, using the
    same voltage that powers the solenoid to get the voltage swing that
    the PLC input module expects. Two diode drops would be enough to power
    an opto isolator, three; an fet, four; a conventional 5V relay coil.

    Having to make an assembly of these sensors, with contacts, is
    something that the use of the PLC and its available modules is
    intended to avoid in the first place.

    If you actually want to know that the solenoid has done its job,
    you're talking motion sensors, actuators or auxiliary switches built
    into the solenoid assembly.

  6. Rob Votin

    Rob Votin Guest

    Thanks, but I've come across similar devices as that already. LEM
    makes them, and possibly others. Problem is, I would then need to
    include an analog input card, or other handbuilt circuitry to buffer
    the signal and detect a threshold etc...

    The whole reason i need this is that the machinery to be controlled
    is quite a distance away from the control cabinet, in a a very
    hostile enviroment. Wiring faults are common, and it is real important
    to stop the process if brakes should fail to release properly
  7. scada

    scada Guest

    Have you looked at I have used current sensors from them
    before. Most of there devices are din rail mounted, and interface with
  8. Joerg

    Joerg Guest

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