OrCAD PSPICE trouble

Apr 17, 2006 4 Replies
B

Hello,



I'm new to PSPICE and it seems I need a little help. I have OrCAD 10. I'm trying to simply put a resistor in series with a capacitor and connect a 10V power supply (VDC). I set a voltage tester between the resistor and capacitor and when I run a transient alanysis, I get a straight line at 10V. No matter what time values, no matter what values I use for the resistor and capacitor, I ALWAYS get a straight line.



I changed the VDC to a VSTIM and set up a square wave. Sure enough, on each pulse, I see a curved rise and a curved fall. Why don't I see a curved rise at the start of the simulation with VDC?? I don't want to use a square wave generator, I just want a simple voltage source like a battery. What is going on??



Any help is appreciated! Thanks!



-Brad



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J

(1) If you want to observe a transient response you HAVE TO USE a transient source.

(2) Use VPULSE

...Jim Thompson

| James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | Old Latin teachers never die...they just decline
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B

I see. That seems pretty weird. How do I know the difference between a transient source and a source like VDC (whatever it's called). There is nothing in the place part dialog that suggests that these two parts are different in that respect.

Well, at least now I know I'm not doing something wrong when I use something other than VDC.

Thanks for the help, much appreciated!

-Brad

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E

Hi,

Disable the DC bias point calculation (I don't have that version so I'm not sure how they do it). At the beginning of SPICE transient simulations, a DC bias point is calculated where the capacitors are treated as open circuits. It uses this calculated point as the first point of the simulation. When this calculation is disabled, all voltages and currents (not specified with an IC statement) start at zero, and you should get the curve you are looking for.

Alex

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R

I would use the IC ("Initial Condition") parameter available in the capacitor property editor field. Just set it to "0". Otherwise it starts to simulate the situation departing from the bias calculation situation.

- René
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