Electrical Engineering Stack Exchange
Electrical Engineering Stack Exchange

Questions tagged [time-constant]

A parameter which quantifies the time a circuit takes to reach one value from another (i.e. a step response).

A parameter which quantifies the time a circuit takes to reach one value from another (i.e. a step response). It is typically denoted using the Greek letter \$\tau\$. Examples of time constants include \$\tau = RC\$ for a first order circuit composed of a resistor and capacitor, or \$\tau = L/R\$ for a first order circuit composed of a resistor and inductor.

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Why is the time constant 63.2% and not 50% or 70%?

I am studying about RC and RL circuits. Why is the time constant equal to 63.2% of the output voltage? Why is it defined as 63% and not any other value? Does a circuit start working at 63% of output voltage? Why not at 50%?
Bala Subramanian
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Why do farads multiplied by ohms produce a result that has a unit of seconds?

Why is the time constant (RC) measured in seconds even though the units are farads x ohms? This is to fulfill my own curiosity as I haven't had much luck at finding the answer. I would be most grateful if someone could give me a solid answer or send…
user37216
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How to calculate for how long an LED emits light when in series with a capacitor?

Given the following DC circuit with a fully discharged capacitor (\$V_c(0) = 0 V\$): How can I calculate for how long the LED emits light until it "turns off"? The LED is a red one with a forward voltage of 2.0 V and a forward current of 20 mA. It…
seven
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Calculating Slew Rate from a time-constant

I have to choose an op amp mainly based on slew rate. I am using the op amp as a differential comparator with dynamic (time) hysteresis with a time-constant \$3\tau=500\mu\text{s}\$. My upper threshold of the hysteresis \$|U_{\text{th}}|= \pm…
atkristin
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RC time constant: theoretical vs LTSpice

I was preparing this lab sheet and the circuit we are analyzing is The theoretical time constant of this circuit is given by $$\tau=R_{th}C_1=\left(R_1 // R_2 + R_3\right)C_1 \simeq 207.3 \,\, \mu s $$ Nice and straightforward. Now to analyze the…
Granger Obliviate
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Time constant of a high-order RC circuit

I have an unknown system that can be modelled as a high-order RC circuit: I performed the impedance spectroscopy measurement of it and fitted the equivalent components. Now I should calculate the time constants of the circuit, three in this…
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Why is each time constant in this transfer function equal to "open circuit" time constant not "short circuit" time constant?

From this lecture notes, the open circuit time constant approximation: How to prove the yellow part? The lecture note mentioned Richard Adler at MIT did prove this but I could not able to find his proof and also don't have access to…
emnha
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why does RC time constant happen to equal 1/e?

I understand that the time constant in AC RC circuits is defined the value of resistance (in ohms) x the value of capacitance (in farads). Indeed, ohms(R) x farads(C) yields time (seconds). $$\tau = RC $$ I also understand that when discussing…
Kendall V
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Opamp relaxation oscillator (single supply)

I simulated and built current circuit. What I got was a rectangular wave generator with a duty cycle significantly lower than 50%. Than I made R3 = R4 and got 50% duty cycle. I managed to think that RC constant is defined only by C1 and R1. It…
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Time constant of this circuit

Can someone tell me on why the time constant of the below circuit is not R2*C? simulate this circuit – Schematic created using CircuitLab My understanding is that, the Capacitor C1 is getting charged only through R2 and I feel that R1 isn't playing…
user220456
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Relaxation oscillator with two time constants

I have the following circuit and I have to find the duty cycle of the output signal and its frequency: I figured out that if \$v_{cond} > 0\$, then D4 will be forward biased and D3 reverse biased, so it's basically a relaxation oscillator with the…
Laura_F
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Effective resistance seen by capacitor

I needed to find time constant of the above circuit. I guess we should first draw small signal analysis. The book found time constant as (RD+RL)*Cc. It says that we can find time constant using the effective resistance seen by the capacitor. I…
Uygar Uçar
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Calculating pulse width

The following circuit is taken from The Art of Electronics book. Assuming the base-emitter voltage drop of 0.6V for both the transistors, the C1 charges to approximately 4.4V (left-to-right) when the input is low. When the input goes high, the…
om sai
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Determination of System poles through open/short circuit time constant technique

I was reading about the open and short circuit time constant technique for estimating the dominant and non-dominant poles of the system respectively. For the dominant pole (open circuit time constant) we short all the independent voltage sources and…
sarthak
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What will be the time to charge a capacitor if there is no resistance?

A capacitor charges to 63% of the supply voltage that is charging it after one time period. After 5 time periods, a capacitor charges up to over 99% of its supply voltage. Therefore, it is safe to say that the time it takes for a capacitor to charge…
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