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Tina-TI 9 tip for users of SPST switch model

Ken Whitley
Prodigy 40 points


I was modeling some vacuum-tube circuits with rather high impedances, and the 1gig leakage resistance of the switch model was insufficient (it's insufficient in reality for point-to-point wiring on teflon turrets and ceramic tube sockets also, for what that's worth).

But if I set the switch model for infinite resistance, then I get "irregular circuit" errors for circuits that work perfectly well in real life (i.e. a series RC circuit through a switch).

I also discovered through experiment that if you set the switch model for an "off" resistance HIGHER than 1gigohm, your setting is ignored, and the switch resistance is 1gig anyways.  Lower values work fine.

My solution (one circuit had 36 switches, most of which had "irregular circuits" attached to them) was to set the switch off resistance for 1gig, and put a resistor across the switch with the value set to NEGATIVE 1gig.  Works fine - the open switch measures infinite resistance, the closed switch measures zero ohms, no leakage signal passes through them when they're open, and I get no more irregular circuit errors unless my circuit actually has an error.  Computing the appropriate negative resistor for a finite amount of leakage greater than 1 gig is simple algebra - resistors in parallel.  ab/(a+b) works fine with negative numbers. If a=-b, you have infinite resistance.

  • TI__Mastermind 41310 points

    Ken,

    Could you provide the .tsc file for the irregular circuit occurrence? I am wondering why the switch would give you that unless it is tied to a floating capacitor. If that is the case, you might find it easier to change the Shunt Conductance to a very small value instead of 0.

    Britt

  • Prodigy 40 points
    My original circuit is huge - 36 filters with switches, as well as input
    and output amplifiers. I generally duplicate problems like this in the
    simplest possible circuit before reporting it.

    The problem does, in fact, have to do with floating capacitors. My
    reported solution doesn't work for a few filters with multiple capacitors
    without a DC path (i.e. two in series to get an exact value, or elaborate
    RLC filters with multiple DC-isolated sections.

    My initial tests with capacitors suggest that the capacitor solution works
    better. Although the dropdown box only gives the choice of 1gig resistance
    or infinite, entering other values greater than a 1gig works fine (i.e. 1
    terabyte). For some reason, this doesn't work with the switch model.

    So it appears that a better solution is to make all my capacitors, or at
    least the critical ones, slightly leaky. That may cause its own problems
    (in circuits with kilowatt radio trasmitting tubes, for example), but
    that's not what I'm working on.
  • in reply to Ken Whitley
    TI__Mastermind 41310 points

    Ken,

    My suggestion was not for the RPar of the individual capacitors, it was for the Shunt Conductance found in the Analysis --> Set Analysis Parameters... If you set this value to a very small number instead of 0 (the default), the series capacitor irregular circuits will go away normally. Please give this a try to see if the irregular circuit errors go away.

    Britt