Author Topic: spice question  (Read 2076 times)

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Offline KTP

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spice question
« on: October 07, 2010, 02:41:16 pm »
Does anyone know what hold time or delay time is used in a nested dc sweep in LTspice?

Is there a way to change this value?

example:

.dc V1 0 10 .01 V2 1 3 .5

(this will sweep voltage source V1 in a ramp from 0 to 10 volts five times for values of V2 of 1,1.5,2,2.5 and 3 volts)

how long does LTspice dwell on each measurement point in the simulation? 
 

Offline Bored@Work

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Re: spice question
« Reply #1 on: October 07, 2010, 04:37:39 pm »
There shouldn't be any hold or delay time. SPICE picks a pair of values and performs a DC operating point analysis for that pair of voltages. When it has found a result it moves to the next pair and again does an operating point analysis. Until all pair combinations have been run.
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Offline KTP

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Re: spice question
« Reply #2 on: October 07, 2010, 04:58:50 pm »
I guess I don't understand.  In this application I was simulating a curve tracer for a 2n3904.  (To approach a real world simulation we have to do in our lab next week I chose to use a base and collector resistor instead of applying the voltage sources directly to the terminals).  It works fine and all, but how does LTspice know to sweep at a slow enough rate, or what timing does it use for the ramp in the inner loop such that it can perform the dc operating point analysis.  Does my question make any sense?

Here is a screenshot of the simulation results.  Note the 5 green curves are the collector current plotted against Vce for Vbe ranging from 1V to 3V.  Also note the fat blue line is a plot of Ic/Ib (beta) vs Vce on the same graph.
 

Offline allanw

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Re: spice question
« Reply #3 on: October 09, 2010, 02:21:00 am »
DC operating point analysis means that it's not time-based at all. It opens capacitors and shorts inductors and finds the dc voltages.
 


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