LTspice simulation of dual-SSR

Discussion in 'The Projects Forum' started by ebeowulf17, Jun 8, 2015.

  1. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    I imagine there are a lot of problems with this circuit - it's just a conceptual idea right now, that may be way off. Having said that, I can't even get LTspice to run and show me how it fails to work, blows up, whatever. The added component data for the SCR is from here:

    http://www.littelfuse.com/technical...technical-center/spice-models.aspx#question_3
    SCR SPICE Model Sxx40x
    SPICE_Littelfuse_SCR-Sxx40x_A_lib.htm

    I've only done a few other LTspice simulations, and none of the others have included any parts that didn't come with LTspice, so I'm willing to bet this is user error on my part - doing something wrong with how I included this part model. The error I get when I try to run a simulation is:
    error-timestep_2015-06-08.png

    And here's the circuit I'm trying to simulate.
    Dual-SSR_EBS_03.png

    Again I'll say that I was in a rush and just wanted to see if I could get it to run at all for now. I have yet to calculate proper values for most of the resistors. Also, I plan to use a different opto (Fairchild Semiconductor MOC3063SR2M) but couldn't find a model for it either. If I ever get this sim to run, it'll probably kill the opto immediately unless I change the operating voltages. Ignoring component values, etc. any idea why it won't even run?

    Thanks in advance for any insights!
     
  2. Bordodynov

    Active Member

    May 20, 2015
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    SCR.PNG It is not only the absence of convergence.Your electronic circuitis no good.If the make and include your scheme,it immediately fails.There is no need in four thyristors.Here's how you can build an AC switch:
     
    Last edited: Jun 10, 2015
  3. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    All right, I'll play around some more with circuit ideas. I had expected LTspice to simulate the circuit even if it was a stupid circuit, at which point I would see the results, even if it was a dead short, wild oscillation, etc. Sounds like maybe that's not the case.

    As for the 4 thyristors, that's because I want to switch both legs of power, not just one. I'm essentially trying to make a double pole SSR. The few that I've found ready-made are very expensive, more than seems appropriate considering how inexpensive regular SSRs can be. Using two separate SSRs would work of course, but is quite bulky. Back to the drawing board!

    Thanks for sharing your circuit. Maybe I can experiment with that as a starting point.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    The root cause is that there is no ground in the "isolated" part of the circuit.

    Trick: connect one of the nodes on the isolated side to ground through a 1GΩ resistor.
     
  5. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Thank you so much! I've read about that issue before and totally forgot about it. It's running now! (Of course my circuit still needs all kinds of work, but that's a different problem entirely. When I get to where I need advice on that, I'll be back.)
     
  6. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    So, I've got a new version of my idea sort of working in LTspice. The goal is to provide SSR style switching to BOTH legs of power to a 240VAC 1/3HP motor. I have absolutely no idea how to simulate the inductance (and therefore the inrush current and back EMF) of this motor. The resistor and inductor shown in my current sim were chosen pretty haphazardly and adjusted until the running current was in the right range, but I have no idea if this is anything close to representing other characteristics of the motor. I'm sure a really good simulation is a nightmarish undertaking, but if there's a simple way to at least get in the ballpark, that would be great. Motor specs, current circuit design, and LTspice files are below.

    Thanks!

    Motor specs: View attachment 87997
    Current circuit: Triac-SSR_EBS_07.png
     
  7. Bordodynov

    Active Member

    May 20, 2015
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    Every thing works one TRIAC.You probably want to increase Triac-SSR_EBS_07_new.png electrical safety?
     
  8. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    Right, I want this to be setup such that, if the motor were chopped off and its power leads were dangling in the air, there would be no shock hazard when the triacs were off. (Obviously in the chopped cord scenario, there would still be a shock hazard when the triacs are activated.)

    This circuit is intended to replace double pole mechanical relays, which provide the safety and reliability we want, but allow the motors to create huge amounts of noise when switching on and off. (The noise issues have been greatly reduced with improved grounding and the addition of snubbers, but the noise is still present at undesirable levels, so we'd prefer to eliminate it entirely, which SSRs seem to do.)

    So, any advice on how to better simulate the motor that I'm switching with this circuit?
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    In your sim, R4 and R6 are 500m. Did you intend 500Meg?
     
  10. ebeowulf17

    Thread Starter Active Member

    Aug 12, 2014
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    :p Yes! :rolleyes:
    I must say, though: V1 is a pretty incredible power source, dumping all that current to ground and still no voltage sag!
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Yes, megaAmps and megaVolts are no sweat for ideal components :).

    That 'm' is a common gotcha. Another one is using 'F' for Farad, as in '1F' (which turns a supercap into a 1 femto-Farad blob of nothing).
     
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