# How to calculate time constant of caps in following ckts

Discussion in 'Homework Help' started by aamirali, Apr 19, 2012.

1. ### aamirali Thread Starter Member

Feb 2, 2012
415
2
1. in fig 1, how to calculate time constant for C1 & C2. What are other parameters included in C2. & what are initial current in C1 & C2 when charging just starts.
It is given by I= Vs-Vc/R (Let vc=0v initilally)

2. in fig 2, zener will keep 12 V across cap, assume R1=1M (in fig its 10k, ignore this value). While calculating initial current direct relation cannot be used. I=Vs-vc/R As current will be limited by R1 here.
How to find that

3. in fig 3 lets say I have a initial cap value =14v & voltage source=12V. In this I don't think cap will discharge as there no path left.

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2. ### THE_RB AAC Fanatic!

Feb 11, 2008
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There is a homework section of the forum. 3. ### WBahn Moderator

Mar 31, 2012
24,229
7,548

As for #1 and #2, we aren't in a position to "help" you with your homework until you describe what you have done thus far. This isn't a place to come to have other "work" your homework for you.

When you just post a problem and ask for a solution (or even just ask how to start) and don't give any indication of what your thoughts are or what specific difficulty you are having, it is like saying, "I haven't paid attention to a thing in class or read a thing in the book, so I want someone to do my thinking for me." That will seldom happen here as it does you no good.

So here is a hint on #1. What do capacitors look like when they are fully discharged?

Other hints: Be sure to express expressions correctly. Saying I = V-Vc/R means

I = (V) - (Vc/R)

since division has precedence over subtraction.

This needs to be written as I = (V-Vc)/R

In #1, when you say, "It is given by I= Vs-Vc/R (Let vc=0v initilally)", is that your answer, or is that part of the problem statement, or what? What is R? What is Vc? You have two resistors and two capacitors. Be specific!

In #2, what do you mean by "direct relation cannont be used". What is the "direct relation" you are referring to and why can't it be used?

4. ### aamirali Thread Starter Member

Feb 2, 2012
415
2
k, let me give more clear view: i have done analysis of rc,rlc,lc with simple voltage source & found der charging & discharging equation, by mathematical derivation. But I think that's so simple & I don't know how to solve dem:

1. fig,1, if der were only R1,R2 & V in the battery, it was like by intiution. But know it has r3 & c2 in the ckt which adds some comlex behaviour for now i don't understand how to proceed for it.

2. in fig 2, again zener won't let voltage to go pass by 12V, but also R1 won't let current to increase much.
Here simple RC method cannot be applied, because charging current bis limited by r1 while voltgae is limited bt zener

soo anu help would be really appreciated.

5. ### mlog Member

Feb 11, 2012
276
36
In Fig. 2, you can assume that D1 is not even there until the voltage across the capacitor reaches 12 volts.

6. ### WBahn Moderator

Mar 31, 2012
24,229
7,548
It would really help to know what level of course this is. Are you working with differential equations? Are you working with phasors? Are you working with Laplace transforms?