Retard-o SPICE EXP Question

[Mike Rocket J. Squirrel Elliott]

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?

 

[Mike Engelhardt]

Mike,

How do you get it to sit at 0v for 10ms, then climb exponentially to +50v?

EXP(0 50 10ms 1 1e308)

--Mike

 

[Mike Rocket J. Squirrel Elliott]

Mike,




EXP(0 50 10ms 1 1e308)

1e308? Some magic number?

 

[Mike Engelhardt]

Mike,

1e308? Some magic number?

It's just a time that is longer than I thought you'd have as the stop time
on your .tran statement.

--Mike

 

[Mike Rocket J. Squirrel Elliott]

Mike,




It's just a time that is longer than I thought you'd have as the stop time
on your .tran statement.

Yeah. A reasonable guess ... it's -- what? -- 3e300 years?

 

[Paul Burridge]

Yeah. A reasonable guess ... it's -- what? -- 3e300 years?

Mike obviously believes in giving plenty of leaway.

 

[Mike Rocket J. Squirrel Elliott]

Mike obviously believes in giving plenty of leaway.

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.

 

[Mike Engelhardt]

Mike,

Yeah. A reasonable guess ... it's -- what? -- 3e300 years?

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.

--Mike

So many singularities, so little time.