dual-pole SSR

Discussion in 'The Projects Forum' started by ebeowulf17, Feb 16, 2016.

  1. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    So, this is a new design, but some of the background is covered in a previous thread:

    http://forum.allaboutcircuits.com/threads/ltspice-simulation-of-dual-ssr.112263/

    I'd like to design a dual-pole SSR circuit which will be incorporated into a larger board with custom features. The other aspects are not locked in yet, and should be manageable for me, but the high power switching is new territory for me, so I wanted advice. I've got the circuit below working as planned in simulation, and it intuitively makes sense to me, but I was hoping you all could help me in case there are problems I'm overlooking. The resistor and inductor at the far right are meant to represent a 1/3HP AC motor. The motor would run on 200-250VAC, sometimes split-phase, sometimes single-phase. The control signal into the optos would be 5VDC. The key requirement that differentiates this circuit from more standard triac/SSR circuits is that we want BOTH power legs to the motor cut off when the 5VDC signal is absent. Please let me know what I've got wrong - I imagine there's plenty!

    Thanks in advance!
    Dual-SSR_EBS_08.png
     
    Last edited: Feb 16, 2016
  2. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    79
    Now that I think about it, it's been a while since I started working this sim up, and I don't remember how I chose the values for the 500ohm resistors. I think they were meant to be placeholders and I was going to figure out real values later. I'll have to look into that. Or if that answer is obvious to anyone looking at it, let me know. I may have posted this prematurely, but I'll happily take any advice regardless.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    You have each gate connected directly to the opposite power line (through its opto and resistor). This is not what is indicated in the 3022 datasheet.

    ak
     
  4. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Hah! Embarrassing mistake. That explains a lot. I couldn't for the life of me figure out how to size that resistor in any sort of reasonable way. Now I see how this is supposed to work (I think.) With the opto active, a voltage difference develops across the gate and MT1, but only until it's high enough for gate current to flow, activating the TRIAC. At that point there is essentially no voltage difference, which prevents an unreasonable amount of current from flowing through the gate, opto, or associated resistor once the TRIAC is conducting. I think I understand this better now. Thanks!
    Dual-SSR_EBS_09.png
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Note that the datasheet gate drive circuit for an inductive load is not a simple resistor.

    ak
     
  6. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    I thought (perhaps naively) that, since I'm using "snubber-less" TRIACs plus an additional snubber circuit as insurance, it wouldn't matter. Do either of those facts change things, or would you recommend adding the modifications? Thanks for your patience and guidance!
     
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