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# What frequency value to use to find the correct time constant for a 22kΩ resistor?

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#### cwible

##### New Member
Hey guys,

I have just been introduced to time constants and I'm having some trouble with this question.

What frequency value do you plan to use to obtain the correct time constant for a 22 kΩ resistor? (Hint: It takes five time constants for a capacitor to go through the full charging cycle.)

I really appreciate the help.

You're leaving something out of your question. It's like a half finished sentence.

No, I think there is enough information there ... it sounds like homework though.

t = R x C

f = 1 / (R x C) or f = 1 / t

Your answer is going to be more in terms of the relation between f and C since you know what R is

Example:
say you have a 34k resistor and a capacitor value of 1uF

t = 0.034 seconds

if you want five time constants, then that would be 0.17 seconds .... 0.034 x 5 = 0.17

your frequency would then be 1/t or 5.88 Hz

So in terms of f and C ..... f will be about 5880000 times C ... This will change depending on the value of R

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Wouldn't 5 time constants only be half the wave or just the charging cycle? to account for the other half of the "decharging" cycle would the equation become 10t=1/f.

t=RC

Plug in the values and solve for f.

I'm having some trouble with this question.
I'm having some trouble with your question.
What frequency value do you plan to use to obtain the correct time constant for a 22 kΩ resistor? (Hint: It takes five time constants for a capacitor to go through the full charging cycle.)

Your question has some serious flaws. First, I don't plan to use any. Second, "correct" for what? Is the circuit an oscillator, a filter, a switch debouncer, a phase shifter, a timer, or or or or or ...

Hint - no, it doesn't. It takes five time constants for a fully discharged capacitor to charge up to approx. 99.3% of the source voltage. Whether or not that is considered "full" charge depends of the rest of the circuit and its application. 3 time constants (95%) is considered close enough to full charge for many applications. Any voltage above that level is unpredictable because of component tolerances. You cannot predict what the voltage will be to 1% accuracy when the capacitance value has a +/-10% tolerance.

And, what "cycle" are you talking about? If the R-C is part of a power-on reset circuit, there is no cycle or repeating; the capacitor charges up and then just sits there until power is turned off.

What is it you are trying to ask? Is this a homework assignment?

ak

As others have mentioned, it's obviously a school question, and set by someone who doesn't know anything at all about electronics, or understand the question themselves. The questions are probably set by a History or Geography teacher?, who was 'lumbered' with the assignment.

Although to be fair, at college it was no better, with questions in term papers often making no sense either

And going further, I was twice winner of the 'Sharp Electronics UK Engineer of the Year' (and was still the current holder when they stopped running it), and the final consisted of a written exam in a posh hotel (mostly just multiple choice). One year there were a number of stupid questions, which either made no sense, none of the options were correct, or both. Luckily it was a 'hands on' type of event. and the people responsible for setting the questions were actually THERE

Needless to say, we all verbally accosted the Sharp staff, who all pointed at the same guy - and said "it was him who set the questions". So we all turned on him, and he admitted to simply taking the questions off old college exam papers - and hadn't bothered checking them himself. His response was to ignore those questions on all papers, although many of argued that as many of us had corrected the questions, or added the correct answer as an extra 'option' we should score marks on those questions (we didn't though).

So even external exams from governing bodies weren't always written by people competent in the subject in any way.

Sorry guys the question was given to me by one of my professors. I guess it makes me feel a little bit better that you guys are confused by the information that is give too. That is the all the information the question gives.

I emailed my professor and he said to use a capacitance of 0.1uF.

Not sure if this helps answer the question still.

Since you are learning about Time Constants then don't you think you should be told an application of where it is used??
A power supply rectifier filter, an audio frequency filter, an audio coupling capacitor feeding an impedance, a delay circuit etc.

Using the 0.1uF capacitor and 22k resistor, one Time Constant is 0.1u x 22k= 2.2ms and five of them are 11ms.
If you have a circuit that charges the capacitor through the 22k resistor then discharges it in the 22k resistor then the frequency is 1/(2 x 11mS)= 45.5Hz.

Another problem with the question as you have posted (and why I asked for more information) is that some circuits (like the 555 timer) are designed to have switching thresholds of 63% charge or 37% discharge specifically so you don't have to fumble around with how many time constants to use. In that case, the value you get for the time constant RC is the actual period of the circuit.

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