circuit design for a specific current

[saeedazari]

hi everyone,

i want to design a circuit with a capacitor and some other elements in order to have a voltage source with 1+0.6 e(-0.4t) current.
maybe it is a RC circuit like below but i am not sure of the design.


thanks in advance

 

[albbg]

As far as I understand you want a circuit that draws a current I=1+0.6 e(-0.4t) from the input voltage generator.
We have two meshes:
Vin-R3 in which the current will be I3=Vin/R3
Vin-series R2 and C. The voltage across the capacitor is Vc=Vin*{1-exp[-t/(R2*C)]} then the current will be:
I2=(Vin-Vc)/R2, that is: I2=Vin/R2*exp[-t/(R2*C)]
The total current will be I=I1+I2=Vin/R3+Vin/R2*exp[-t/(R2*C)]

Then Vin/R3=1 that is R3=Vin/1, Vin/R2=0.6 that is R2=Vin/0.6
Furthermore 1/(R2*C)=0.4 then C=1/(0.4*R2) that is C=0.6/(0.4*Vin)=1.5/Vin

Since you choose Vin=6 then R3=6 ohm, R2=10 ohm and C=250 mF

So the only error is the capacitor 25 nF instead of 250 mF

 

[saeedazari]

Thanks for your answer, it really helped me to be sure about the design.
And i changed the capacitor to 250mF. :wink:

 

[saeedazari]

Is there any limit in choosing R and C in an RC circuit? because when i choose 10 for resistor and 250mf for capacitor... the circuit response is really unlike other regular rc circuits in pspice and proteous simulations and by changing the values of these elements it will form an exp curve.

 

[albbg]

No, there is any limit (at least in a simulation where the components are supposed to be ideal).
Could you post the result of the simulation you did ?

 

[BradtheRad]

Is there any limit in choosing R and C in an RC circuit? because when i choose 10 for resistor and 250mf for capacitor... the circuit response is really unlike other regular rc circuits in pspice and proteous simulations and by changing the values of these elements it will form an exp curve.

When you build a real circuit you must ask whether sufficient current is available to drive the combination of RC values.

A low resistance and high capacitance needs higher current to drive it.

Usually there is an unseen resistance somewhere which is part of the RC loop. It throws off the RC time constant.

Therefore it is typical for real circuits to make the resistor a high value, and the capacitor a low value, rather than vice-versa.

 

[saeedazari]

Thanks for your responses... actually the problem was with the capacitor selected in OrCAD.
The one which worked is named c/ANALOG_P and also i found that i have to allocate 15 seconds to run time because time constant of this circuit is 2.5 sec, and it takes 15 seconds for the capacitor to reach its final value but when you see results for 1 sec you can not see the exp curve. Here is the result: