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Retard-o SPICE EXP Question

Discussion in 'CAD' started by Mike Rocket J. Squirrel Elliott, Nov 19, 2003.

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  1. Should be obvious to me, but my lack of skill interpreting SPICE
    helpfiles is, after all, the stuff of legend. It's tough being a legend.

    I want my voltage source to climb exponentially from 0 volts to 50 volts
    after a 10ms delay and have a risetime constant of 1s.

    But EXP 0 50 10ms 1

    Creates a waveform that starts at 0 volts, then, after 10ms, plummets to
    -50v, and climbs exponentially back up to 0.

    How do you get it to sit at 0v for 10ms, then climb exponentially to +50v?
  2. Mike,
    EXP(0 50 10ms 1 1e308)

  3. 1e308? Some magic number?
  4. Mike,
    It's just a time that is longer than I thought you'd have as the stop time
    on your .tran statement.

  5. Yeah. A reasonable guess ... it's -- what? -- 3e300 years?
  6. Mike obviously believes in giving plenty of leaway. :)
  7. For good reason! There's nothing more annoying than the premature
    termination of a long transient simulation of those circuits you're
    designing to last well beyond the projected age of the universe.
  8. Mike,
    It's an easy-to-remember approximate upper limit of the
    magnitude of a 64bit floating point number. Most any
    reasonable number encountered in numerical computation would
    be between -1.7e308 and 1.7e308. Numbers beyond that
    range can be grouped with infinity. Yet, 1e308 is much longer than
    the estimated age of the Universe. So 1e308 is slightly less than
    infinity but much longer than forever.


    So many singularities, so little time.
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