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I started using PSpice only yesterday and I have a problem with a really simple circuit.

Vs = 2 V

R1 = 1 kΩ

C1 = 50 nF

The switch closes at t = 0 (it's not really needed).

Charging a 0.05uF capacitor

After running a Time Domain analysis and tracing the voltage across the capacitor, I saw that it takes around ~250ms to raise the Vc to Vs.

Time Domain Analysis Trace

This seemed like too much time, so I decided to double check in MatLab by graphing a function of voltage across a charging capacitor:

\$\ Vc(t) = Vs \cdot (1- e^{-{{t}\over{RC}}}) \$

where

\$RC = 1kΩ \cdot 50 nF = 0.00005s \$

\$Vs = 2 V\$

I can't post the screenshot of the result, because I don't have enough reputation for another link. But it's easy to check that the Vc reaches 2 V shortly after t = 0.25ms.

Why do I get different results for the same circuit in PSpice?

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  • \$\begingroup\$ Hmm, something somewhere is out by a factor of 1000 in your simulation. It should indeed reach \$V_s\$ at around \$5\tau = 5RC = 250\mu s\$. I would try redoing the simulation to make sure there is nothing cached and that it is actually simulating the circuit you are intending it to simulate. \$\endgroup\$ – Tom Carpenter Oct 9 '16 at 17:56
  • \$\begingroup\$ What is the on and off resistance of the switch? Are you sure it actually closed? \$\endgroup\$ – Mario Oct 9 '16 at 18:01
  • \$\begingroup\$ @Mario Yes, it was probably because of the switch. I removed it altogether and now everything seems right. Thanks. \$\endgroup\$ – Maxim Mikhaylov Oct 9 '16 at 18:04
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The switch is the main problem. I'm not 100% sure here but I see at least 2 problems with the switch: 1. The closing time is defined as 1s, not 0s as you expected (So actually the switch opened during your whole simulation) 2. The default resistance value of the switch is 1 MOhm. So the switch itself is similar to a big resistor and your time constant in this case is: tau = R_sw * C = 1M * 50n = 50 ms That's why it took about 250ms for Vc to reach Vs value

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  • \$\begingroup\$ Yeah, the switch was causing the problem, as Mario suggested in comments. I removed it altogether and everything worked fine. BTW, welcome to EE Stack Exchange! \$\endgroup\$ – Maxim Mikhaylov Dec 21 '17 at 14:14
  • \$\begingroup\$ Thank Max. By the way, I thought you want to use that sw_tclose component to model the switch? Removing it will be helpful in this case only. You'll be in the same trouble if the closing time of the switch is at 2 seconds for example :) \$\endgroup\$ – dejoker Dec 21 '17 at 14:17
  • \$\begingroup\$ I'll keep that in mind for the future. Haven't simulated any circuits for almost a year now. \$\endgroup\$ – Maxim Mikhaylov Dec 21 '17 at 14:20

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