There aren’t many concepts in the entire field of electronics that are quite as fundamental as the RC time constant. Whether a given person is tinkering with electronics or is simply trying to build a rudimentary timer for a model railway, or they are designing a filter for an audio project, they will need to know what RC time constants are and the steps for calculating them.
So, courtesy of the electrical component supplier RS, here is a “cut-out-and-keep” explanation of RC time constants, the relevant formula, and everything else you ought to know in 2026 and beyond.
Introduction to the RC Time Constant
Let’s start from the very beginning, with what those initials mean: R and C refer to a resistor and a capacitor, respectively. These components are combined in an RC circuit, and the RC time constant tells you how quickly the capacitor charges or discharges through the resistor.
After all, when voltage is applied in an RC circuit – such as from a battery – the capacitor doesn’t charge instantly. Instead, it takes time because of the resistor limiting the current flow.
The RC time constant, then – denoted by the Greek letter tau (τ) – is the time it takes for the voltage of a charging or discharging capacitor in an RC circuit to change by approximately 63.2% of its final value.
Charging vs Discharging: What It Means for The RC Time Constant
The distinction between these two processes can sometimes be a source of confusion. So, let’s set it out quickly before we launch into the specific calculation for the time constant:
- Charging: A capacitor charged through a resistor will reach about 63.2% of its final voltage after one time constant. It is regarded as practically fully charged (over 99%) after approximately five time constants.
- Discharging: A capacitor that is discharging through a resistor will drop to approximately 36.8% of its initial voltage after one time constant. It is considered practically fully discharged after around five time constants.
What Is the Formula for Figuring Out RC Time Constant?
It has probably flashed across your mind, so we might as well mention it: yes, digital tools exist that can determine the RC time constant for a circuit within seconds. Indeed, you can simply navigate to the RS website to use our RC time constant calculator for resistance and capacitance values you already know.
However, it is still very useful to know how to calculate RC time constant manually, not least for verifying the accuracy of any calculators you do try.
Here, then, is the simple formula for calculating RC time constant: τ = R x C, where τ is the time constant in seconds (s), R is the total resistance in ohms (Ω), and C is the total capacitance in farads (F).
Step By Step: How To Figure Out the RC Time Constant
Presuming, then, you are performing the calculation manually, here are the essential steps:
- Identify the resistance (R) and capacitance (C) values in your circuit. If your circuit is a complex one with multiple resistors or capacitors, you will first need to simplify them to find the total or equivalent resistance and capacitance connected to the relevant part of the circuit.
- Make sure the units are correct. By this, we mean that resistance must be in ohms (Ω), and capacitance must be in farads (F).
- Multiply R by C to find the time constant, τ. Don’t forget to verify the accuracy of the result by another method (for example, using a trusted RC constant calculator like the one on the RS website, if you initially performed the calculation manually).
Example Calculation: Putting It All Together
While we’re on the subject of calculators, it is worth mentioning that the earlier-mentioned digital tool (linked above) allows for different units of measurement.
Let’s assume, for instance, that an RC series resistor circuit you’re working on has a resistor with a value of 47 kiloohms (kΩ), and a capacitor with a capacitance of 1,000 microfarads (μF).
The online calculator saves you from having to convert these figures to ohms and farads, respectively, so you can simply enter them into the tool “as is”.
That means they will be automatically converted to 47,000 Ω and 0.001 F, resulting in the correct reading of τ = 47,000 x 0.001 = 47 seconds.
Conclusion: Knowing The RC Time Constant Calculation Is a Vital Foundation for Learning
That, in essence, is that! Hopefully, this article will have helped you feel more confident about the process of determining the RC time constant.
Certainly, mastering the relevant calculations will help open the door to more advanced circuit projects. This, in turn, can give you a pathway to realising your greater potential in electronics.
