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Current to current converter

  1. Aug 13, 2010 #1
    Hi Guys,

    I am in bit of a spot here. I do programming for hydraulic presses. If anyone is unfamiliar this entails programming PLCs to automate a press or a machine.

    The problem I face is that the output current from the PLC is between 4mA - 20mA. I have a proportional valve that needs a signal between 4mA - 800mA. Basically it is off at 4mA and 100% open at 800mA.

    I can't find a circuit that might be able to amplify the output I am getting from the PLC so that I can get the proper functionality from the proportional valve.

    Anything that can linearly change the output current from the PLC and obviously max out at 800mA when the output from the PLC is 20mA.
  2. jcsd
  3. Aug 13, 2010 #2


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    Science Advisor

    It should be pretty easy but a few more details are needed.

    1. What voltage does the valve need at 800mA?

    2. Is the valve control differential or does one of the terminals have to be grounded?

    3. Do you also have a spare analog voltage output on the PLC, 0...10V for example.

    4. What DC power supply is available (is the PLC using). Eg, do you have 12V or 24V available?
  4. Aug 13, 2010 #3
    Can you do Pulse Width Modulation with the PLC at a high enough frequency? Then you can just use a simple common collector transistor to drive the valve.
  5. Aug 13, 2010 #4


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    Staff: Mentor

    It is pretty unusual for a control valve to not accept a 4-20 signal. Are you sure that's not the power requirement (as opposed to the signal type) you're reading?
  6. Aug 13, 2010 #5
    oops...two mis-apprehensions in my previous reply...

    First, it's a common-emitter transistor you'd use to do PWM -- I got the names mixed up when I was about 12yrs and am still trying to correct myself.

    Second, if it's just a DC current amp you are interested in the --err, correct taxonomy this time I hope-- common collector transistor circuit could do you. Basically an NPN power transistor with the collector at the plus supply rail and the load between the emitter and ground. It will need to be well heat-sinked as it directly dissipates the excess power.
  7. Aug 13, 2010 #6
    i heard about ac to dc converter
    what is current to current converter?
    http://provillusinfo.org" [Broken]
    Last edited by a moderator: May 4, 2017
  8. Aug 14, 2010 #7

    1. 24V DC
    2. The only thing we give the valve is a power supply. Nothing else
    3. yes
    4. Seperate power supply 24V DC
  9. Aug 14, 2010 #8


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    Staff: Mentor

    Well it sounds like that 800ma isn't a control signal, but just the amperage required to actuate the valve. So you don't need to do anything special here: the PLC should have a 24v digital (on/off) output that you can use directly and that amperage shouldn't be out of its range. It appears the valve does not accept an analog input.

    Here's an example of a device that works the same way: http://www.smarthome.com/307107/6-I...ir-Duct-Damper-with-Power-Supply-ZO106/p.aspx

    If the valve is drive open, spring return, the only thing you can do is a regular digital signal, but if it is drive open, drive closed, you can use timed-pulses (not pulse width modulation) to drive it to a particular position. That requires periodic calibration, though, if you want to have a good idea of the actual position. If you only care about controllability and not actual position, you don't have to worry about calibration.

    Getting the model number/spec sheet of the actuator and the specifics of the application would help...
    Last edited: Aug 14, 2010
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