Isolated analogue output circuit

Discussion in 'The Projects Forum' started by GLec, Mar 22, 2015.

  1. GLec

    Thread Starter New Member

    Mar 22, 2015
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    0
    Hi All,

    I have a heater controller which I need to control through a PLC analogue output card.
    The issue is that the heater controller has its own 10Vdc supply which is currently regulated through a potentiometer.

    Is there any kind of isolation circuit that could be regulated using the 0-10vdc PLC output.

    Any suggestions on possible solutions would be much appreciated.

    Thanks
    G
     
  2. MCU88

    Member

    Mar 12, 2015
    360
    35
    So you need like an opto-coupler with analogue in and analogue out?
     
  3. GLec

    Thread Starter New Member

    Mar 22, 2015
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    You got it... But haven't found anything off-the-shelf
     
  4. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
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    I have never heard of a linear/analogue opto-coupler.

    You could achieve control of the heater (via an opto-coupler) using a PWM signal.

    But, does it really need to be isolated?

    More information on the heater and controller would help.
     
    Last edited: Mar 22, 2015
  5. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    Exactly what heater controller? Sounds like the heater controller has a pot. One end of the pot to heater controller 10 volts and the other end to ground. The wiper of the pot goes to the heater controller analog in. The heater controller analog in therefore sees a 0 to 10 volt signal. Now did you want to eliminate the pot? You could just eliminate the pot or eliminate the pot wiper connection and use an external 0 to 10 VDC. Try to better explain exactly what you are looking to do and any links to data sheets for what you have would be helpful.

    Ron
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Something like this?
    IsolatedControl.gif
     
  7. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Are you sure that you need the isolation? The internal supply will likely be floating, so you might be able to connect it straight to your plc.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    There are a couple of ways to do this. One is to use the opto-isolator circuit with two optos. One is the connection to the output circuit and the other is in the feedback loop of the input circuit to linearize it. Another way is to buy an isolation opamp from Analog Devices or Burr Brown (now TI). Both companies have app notes on this.

    ak
     
  11. strantor

    AAC Fanatic!

    Oct 3, 2010
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    PLC outputs are usually isolated, and I presume your heater controller would be isolated too. You only need one of them to be isolated, and I guess 90% chance at least one of them is. If that's true then you should be able to simply remove the pot from the heater controller and connect the PLC 0V to the heater 0V, and the PLC analog_out to the heater's old wiper connection.

    But if both of them are non-isolated and at different potentials, you could use the industry-accepted idiot-proof "signal conditioner" solution for guaranteed success and no risk. Or you can DIY your own isolator for a few cents using one of the optos mentioned already - that will represent a little more risk and a little less guarantee of success. No worries there, if you're confident that you're not an idiot and can connect 4 wires correctly without damaging hundreds or thousands of dollars worth of equipment.
     
  12. GLec

    Thread Starter New Member

    Mar 22, 2015
    3
    0
    Thanks to everyone for your replies.
    While the off-the-shelf signal conditioner looks tempting - with 7 channels to do, the costs start adding up.

    It looks like I'll be wiring up a few op-amps

    Thanks again,
    G
     
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