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Hi, i would like to know at what ratio of time constant does the voltage across a capacitor reach 1/2 of the source. In two decimal places would be perfect, thank you
Show us what you've tried so far. It's very obviously a homework problem, and you're not going to convince anyone here otherwise. Show your work. We'll go from there.
I am not sure what is happening in this thread. Do we not help with homework problems or something? Please fill me in.
You should know that if it is a homework problem however and we simply spit out the answer and you do not know how to get to it yourself then you will have trouble passing any in-class tests that come up, and you will also deprive yourself of the learning of how you get to this kind of answer.
So i'll start off and we'll see how this goes...
The capacitor with zero initial voltage charges to a voltage Vc with an applied source Vs after a time t according to:
Vc=Vs*(1-e^(-t/RC))
where RC is the resistance times the capacitance, also known as the RC time constant or just the time constant.
Now if you wish to solve to see how long it takes to get to one half Vs, then substitute Vs/2 in for Vc. We then get:
Vs/2=Vs*(1-e^(-t/RC))
Now this is solved for the time t, and that will result in an expression with RC in it and a constant, and the constant will be the ratio of t1/RC where t1 is the time it takes to charge to one half Vs.
See if this makes sense, and also you may want to state why you are after this answer.
We don't just give out answers. If someone comes in with a homework question, they must first attempt it on their own and show all their work. From there we can point out where they went wrong and point them in the right direction to solve it on their own. Just giving them the answer, even if you give it step by step, won't teach them as well as having them do the work themselves.
We don't just give out answers. If someone comes in with a homework question, they must first attempt it on their own and show all their work. From there we can point out where they went wrong and point them in the right direction to solve it on their own. Just giving them the answer, even if you give it step by step, won't teach them as well as having them do the work themselves.
I agree with that. I just dont want to be too judgmental in determining what is a homework question that the OP is too lazy to answer themselves and what is a genuine cry for help. Some people really have trouble with this stuff and they honestly would given anything to find out how to do these kinds of problems.
But i have to admit you are probably right in your judgment, but i just dont want to be rude or judgmental until i know all the facts admitted by the OP themselves.
We do and IMHO we should help with homework questions by giving clues and hints to the OP.
Its so easy to get a mental block on a particular point when studying a subject, by us giving a little nudge it can often help the student grasp the concept.
Some of our older members seem to have forgotten how much help they were given in their earlier years, [ and some are still asking for ]
Hi, i would like to know at what ratio of time constant does the voltage across a capacitor reach 1/2 of the source. In two decimal places would be perfect, thank you
If you read the original post, it wasn't a request for help, nor a quest for understanding. It wasn't a "I don't understand how to solve this equation" or a "How do I figure this out" question. What was asked for is a number to two decimal places. The question clearly demonstrates a lack of understanding, as the answer to the question as asked can only be a function of the time constant, so no decimal places are involved, at least not until some values are available.
If the original poster had shown some amount of effort in finding an answer, the reply would have been different. It only took about a minute with Google to find the appropriate equation. Form that point, I could understand some difficulty in solving for the answer. If the original poster had gotten to that point, I think any number of people would have provided assistance.
The question was for an arduino project i was working on to read capacitance of an unknown capacitor i desoldered off an old pcb. When the analog value reached 512, it would divide the time it took by a ratio of RC to approximate the capacitance.
The question was for an arduino project i was working on to read capacitance of an unknown capacitor i desoldered off an old pcb. When the analog value reached 512, it would divide the time it took by a ratio of RC to approximate the capacitance.
Oh ok, well now we know the true reason for the question, that's nice to know.
For what it is worth, for a project like this you dont need to solve for the 50 percent voltage point you can solve for the 63.2 percent point and that makes the solution a whole lot simpler because that is equal to exactly one time constant. So for an analog to digital converter with top count of 1024, that means you wait for a count of 647 instead of 512. The time 't' it takes to get an ADC result count of 647 is then equal to R*C, so if you know R then C is equal to t/R.
Also, if R is made equal to a positive integer power of 2 then the value of C can be found using shifts instead of an actual divide routine.
don't get me wrong, but if you had said that in the first place you may have got more useful help. i see a lot of questions on here posted by people who think what they're doing is so secret, even they don't know what they're doing.... really.... even if the question is related to a patentable idea, remember that most patents contain a phrase similar to "obvious to anyone skilled in the art" which is a sort of disclaimer that means "anybody with the right knowledge could come up with this or something like it on their own". one other possibility requiring secrecy would be if you were working on a top secret government project, and if that were the case, your non-disclosure agreement would probably frown on you asking your question on an open public forum like this one, even if you posted no details about it. a lot of information could be gleaned just from the nature of the question, and while by itself is innocuous, could reveal secret details when coupled with other snippets of information.
one reason it's a good idea to give some detail when asking a question, is that often, the answer to the question requires some context. take, for example the question "i'd like to make an oscillator, can anybody help me?"...... oscillators are a huge open-ended subject. what is the intended use of the oscillator? what kind of waveform? what frequency range?..... there could be many details that would help you get an answer quicker, rather than leaving everybody guessing or having to prod you for details one-by-one....
rephrasing the question like "i'd like to make an RF oscillator tunable from 10Mhz-20Mhz for a direct conversion receiver" might get you a lot more useful answers, rather than playing "20 questions" for several days just to find out what it would have taken 3 seconds to type.
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