startup option in simulations: LTSpice and ngspice

[Bigman]

Hi,

let's have a very basic circuit like this: GND|---- voltage source ----- capacitor ---- resistor -----|GND
When I run a simulation with LTSpice, the voltage at the capacity is NOT building up like an exponential function. The reasoning I found by googeling: as long as the "startup" option is not set, the simulation would assume the full voltage is instantly available at the connected elements.

Since I am a Linux guy, I do my simulations with ngspice which doesn't have such a "startup" option (e.g. there is no ".tran 0.1m 10m [...] startup").
As an interim solution, I add to my voltage source a PWL instruction like this one: "V1 node1 node2 PWL(0 0 1e-9 3.6)"

I am wondering: how would professional people solve this issue? I also just took 1 nanosecond by random. What duration would be really meaningful?

[PSR B1257]

the simulation would assume the full voltage is instantly available at the connected elements.

Not quite. LTSpice assumes a stady state and therefor a loaded capasitor.
You have to apply a stepfunction do the RC-network, in order to see the stepresponse.
Click on the voltage source, choose ADVANCED and then PULSE.

[mrflibble]

Since I am a Linux guy, I do my simulations with ngspice which doesn't have such a "startup" option (e.g. there is no ".tran 0.1m 10m [...] startup").

Alternatively, you could just use LTspice with wine. That has worked just fine for many versions now...

[T3sl4co1l]

Most SPICEs have options for DC startup conditions; I don't know offhand what NGSpice does.

Worst case, you can indeed use a VPULSE to do the same thing with a small delay.

Tim

[Anand]

Another option is to use initial conditions for the cap (inital voltage would be Vcap = 0V), but that would be a waste for circuits with a lot of caps.