Replacing a Solid State Relay Resistance Voltage Regulator with a 4-20mA SSR controlled by a PID

Discussion in 'The Projects Forum' started by larold01, Sep 28, 2017.

  1. larold01

    Thread Starter New Member

    Sep 28, 2017
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    I have a still with 5 heating elements controlled by 5 SSR regulators which are adjusted using potentiometers. I would like to replace this setup with one Omega PID controlling 5 4-20mA SSR regulators. I have some experience with the standard on/off SSR's but have not done anything with the regulated ones. My question is can one PID control 5 4-20mA SSRs wired in series or do i need to add some way of splitting the 1 PID single into 5.

    This is whats in there now.
    [​IMG]

    This is what i want to use.
    [​IMG]

    THX
     
  2. crutschow

    Expert

    Mar 14, 2008
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    You do understand that the 4-20mA input just turns the relay full on or full off and that the output is not proportional to the input current?
    I don't know what you mean by SSR "regulated ones". :confused:

    Yes, you can put all the inputs in series as long as you have adequate control voltage and the proper resistor in series to have an on-current of between 4ma and 20ma.
    You need to allow about 1.5V drop for each relay, so the total drive voltage for 5 units in series would be at least 7.5V, so say 10V including the resistor drop.
    If you control voltage is not that high, you can put them in series-parallel with a resistor to limit the input current for each group.
     
  3. larold01

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    Sep 28, 2017
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    From what i have read the output of the 4-20mA SSR is proportional to the input. The first one I listed defiantly does based on what the pot is set to. I know because they are what is in the system now. Here is something from omegas website "In addition to electromechanical and solid state relay outputs, proportional controllers are also available with proportional analog outputs, such as 4 to 20 mA or 0 to 5 Vdc. With these outputs, the actual output level is varied, rather than the on and off times, as with a relay output controller." The link to the article

    https://www.omega.com/temperature/z/pdf/z110-114.pdf

    Like I said the SSR regulators or "proportional controllers" are new to me. Until now i assumed all SSRs where just on/off.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Okay.
    But from what I could determine, the SSR you selected is on/off not proportional.
    Do you have information that says otherwise?
     
  5. larold01

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    Sep 28, 2017
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  6. larold01

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    Sep 28, 2017
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    For now all i have is the description on amazon.
    uxcell ASH-25LA 4-20mA to 380VAC 25A Single Phase Solid State Intelligent Relay
     
  7. larold01

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    Sep 28, 2017
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    The link i posted from omega is the best read I have found so far on how they work. I am having a lot of trouble finding info on this type of SSR.
     
  8. larold01

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    Sep 28, 2017
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    The second page covers it pretty well.
     
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
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    What is the reason why you need 4-20mA proportional control? This kind of tight control is usually only needed for applications where there is very little thermal mass. For example a hot wire cutter; the wire starts to cool instantly when current is removed. For applications like a still, I assume you have a huge thermal mass (a vat of liquid) so I see no reason why you would need 4-20mA proportional control (this type is phase angle controlled, so your heat elements get 120 voltage pulses per second assuming 60hz supply).

    This application would typically be controlled with a much cheaper and much simpler on/off system where the controller will apply full voltage via the SSRs you already have, for a few seconds , and then off for a few seconds. You can still have PID control of the heating process using the on/off control.

    The only thing you need to buy is the omega PID controller with on/off output. Use your existing SSRs (all 5 in parallel) ... unless you have some specific reason for going 4-20ma.
     
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  10. Picbuster

    Well-Known Member

    Dec 2, 2013
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    Crutschow is correct not proportional just as the spec said solid state relay on or off.
    Look at:

    https://www.cambridgelsat.com/bookstore/uxcell-ash-40dd-3-32vdc-to-5-60vdc-40a-single/B01LY8RV5B/

    I do not want to be rude but it is not possible to build to quality standard without proper component documentation. Without a project will fail.

    Picbuster
     
  11. strantor

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    4-20mA "proportional" SSRs are just regular SSRs with a little PCB screwed to the input which fires the SSR once per half cycle (phase angle firing).
     
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  12. larold01

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    Sep 28, 2017
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    Thank you for the response.
    My original SSRs are proportional but use pots to trigger them. I ordered the standard on/off SSRs and relay controller first but after some more reading I discovered that they become an issue during the distillation process when you need constant low heat to maintain a precise temp. I now have both setups which cost the same so I need to know if one PID can run 5 proportional SSRs. The proportional setup seemed like it would do the same as the on/off one with more flexibility. I did find a diagram for a 3 phase setup that had them wired in series on the control side from the PID but it was only using 3. I was not sure if running 5 was possible without an additional piece of circuitry and can not test until tomorrow so I wanted to find out in order have what I need sent overnight.
     
  13. larold01

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    Sep 28, 2017
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    I know but that is not the one im using. This is.
    uxcell ASH-25LA 4-20mA to 380VAC 25A Single Phase Solid State Intelligent Relay
     
  14. larold01

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    Sep 28, 2017
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  15. larold01

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    If you go to amazon and search for that you can get the specs.
     
  16. strantor

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    That should work with your existing SSRs if they are 4-20mA input.
     
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  17. larold01

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    Sep 28, 2017
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  18. ebeowulf17

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  19. larold01

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  20. strantor

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    Yep
     
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