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I Have A Very Basic Question...

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jalak7

New Member
Years ago I knew a lot about electronics, but I have forgotten most of what I knew because I didn't use it.

I have a very basic question. Would R2 have any effect on the timing constant between R1 and C1? If R2 were 1ohm or 1mega-ohm, would it change the timing constant? (Sorry for the crude drawing. And maybe I should have drawn in a switch.)

3082-timingug6.jpg
 
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em2006

Member
R1 and R2 is a voltage divider.
In absence of R2, voltage on C1 is max supply voltage (E),
time constant is R1C1ln2.
If R2 is present, voltage on C1 is smaller, max E(R2/(R1+R2))
...
 

hjames

New Member
This would the correct way of looking at things:

http://en.wikipedia.org/wiki/Thevenin's_theorem

Replace the connections with an equivalent (scaled) voltage supply and a single series resistor.
 

jalak7

New Member
Ok, Thx for the responses. I added a switch in the pic below. If I pretend that R1 is replaced with a jumper I don't see how the value of R2 would matter. It would seem to me that no matter what the value of R2 (as long as it's big enough to keep the wire resistance irrelevant) the time it would take C1 to charge would be the same. R1 of course changes the time it takes the cap to charge, but I don't understand how R2 matters.

3089-timingpi3.jpg
 

kchriste

New Member
Forum Supporter
no matter what the value of R2 (as long as it's big enough to keep the wire resistance irrelevant)
Then you can imagine that if R2 = 0:eek:hm: then C1 will never charge. Hjames's link is the correct way to analyze this circuit. Basically you figure out R which is R2 in parallel with R1 and then calculate your RC time constant using R and C.
 
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