# Questions about RC (time constant)

Discussion in 'Homework Help' started by TsAmE, Jun 29, 2010.

1. ### TsAmE Thread Starter Member

Apr 19, 2010
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0
I have a couple of doubts on the RC (time constant):

*Why is it that it takes 5RC to fully charge a capacitor and not RC (since total time = RC)
*Say you wanted to double or half the output pulse duration, how would you know what Vc you must apply to pin 5?

Could someone please explain by referring to my attached diagram.

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2. ### Ghar Active Member

Mar 8, 2010
655
72
The time constant is not 'total time'. It is simply a coefficient in the exponents you get:

$v_C(t) = V(1 - e^{-\frac{t}{RC}})$

When t = RC you get:

$v_C(RC) = V(1 - e^{-1}) = 0.63V$

When t = 5RC you get:

$v_C(RC) = V(1 - e^{-5}) = 0.993V$

You can figure out your second question by going through the capacitor charging equation (which I just used) though the question is a bit vague.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
In relation to either a rising or falling simple first order exponential function ....

If you consider what is the basic definition of time constant - it is the time at which the extension of line of slope of the curve at zero time intersects the final value.

The time constant simply denotes the apparent initial trend to final value at t=0. It's not the time it actually takes to reach the final value.

4. ### TsAmE Thread Starter Member

Apr 19, 2010
72
0
Sorry, perhaps if I attach a diagram and further info it will become more clear.

Attached is a monostable, where by default the output is 1.1RC (as it goes high for this time once). Now if I wanted to double the duration of this high time it would obviously be 2.2RC right? But now I want to know what voltage you must apply to pin 5 in order to do this?

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5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
This is presumably a 555 timer question.

Have you searched this forum for more info on the 555 timer?

In any case, pin 5 of the "555" timer is a means of varying the reset & trigger points for the capacitor discharge and charge threshold levels. These are normally preset inside the 555 timer as 2/3Vcc and 1/3Vcc respectively. Imposing an external input on pin 5 changes these levels according to the value of the control voltage you apply. The source resistance of the control voltage will also come into consideration if you don't have a low resistance source - compared with the "555" internal divider chain values ~ 5kΩ.

You might want to do a bit more reading on the "555" and look at some specific application notes on the web. Then you will hopefully have a better idea on how to design for the values you need.

Some more generous forum member might do the design for you ...... I'll pass on that.

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
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1,088
Look at simplified diagram of a "555" circuit

And you will note that threshold voltage is equal 2/3Vcc and if you apply a voltage to node 2 you will change the threshold voltage level.

And also you need ask yourself a question to to which the voltage capacitor will charge after 2.2RC

Last edited: Jun 30, 2010
7. ### KL7AJ Senior Member

Nov 4, 2008
2,013
273
In reality, the 5 time constant rule is a somewhat arbitrary "rule of thumb." 99% "there" is good enough for 99% of any practical application.

Eric