A tool for selecting resistors and capacitors to achieve a given RC time constant
Most examples of RC calculators do one of two things. Either they ask you to provide both component values and show how to determine the time constant (τ) from them, or they arbitrarily fix one of the values (for example, setting the resistor to 10 kΩ) to determine the other. The calculator presented below takes advantage of the fact that resistors and capacitors come in fairly standardized values, and so there is a relatively small finite pool of combinations that can be assembled.
Given the desired τ value and resistor tolerance (which determines the pool of available resistances), this calculator provides all combinations of R and C that generate time constants within an acceptable tolerance of the desired value.
Results are shown in order of the difference from the desired τ value, from smallest to largest. Where the calculated τ values are the same, the entries are organized by descending resistance value, as generally the larger resistance will give a lower current draw and a physically smaller capacitor. This is a suggestion only, and you should use components that make sense in your application.
| R | C | τ | Δ |
|---|---|---|---|
| No results. | |||
These results are not guaranteed to be error-free or suitable for any specific purpose.
Resistors are calculated using the following formula:
Here, the resistance, $R$, is derived from the decade, $d$, and the tolerance series, $N$. The value of $i$ ranges from $0$ to $N - 1$ to generate the standard resistor values. The value of $d$ ranges from 1 to 6. For the tolerance series, $N$ takes the values 96, 48, 24, and 12 for 1%, 2%, 5%, and 10% respectively.
The following values are not part of the normal series, but also exist for 1% tolerance resistors.
| 1.10e6 | 1.20e6 | 1.30e6 | 1.50e6 | 1.60e6 | 1.80e6 | 2.00e6 | 2.20e6 |
Source: Standard Resistor Values - RF Cafe
These are the standard capacitor values in Farads.
| 1.0e-12 | 1.1e-12 | 1.2e-12 | 1.3e-12 | 1.5e-12 | 1.6e-12 | 1.8e-12 | 2.0e-12 |
| 2.2e-12 | 2.4e-12 | 2.7e-12 | 3.0e-12 | 3.3e-12 | 3.6e-12 | 3.9e-12 | 4.3e-12 |
| 4.7e-12 | 5.1e-12 | 5.6e-12 | 6.2e-12 | 6.8e-12 | 7.5e-12 | 8.2e-12 | 9.1e-12 |
| 10e-12 | 11e-12 | 12e-12 | 13e-12 | 15e-12 | 16e-12 | 18e-12 | 20e-12 |
| 22e-12 | 24e-12 | 27e-12 | 30e-12 | 33e-12 | 36e-12 | 39e-12 | 43e-12 |
| 47e-12 | 51e-12 | 56e-12 | 62e-12 | 68e-12 | 75e-12 | 82e-12 | 91e-12 |
| 100e-12 | 110e-12 | 120e-12 | 130e-12 | 150e-12 | 160e-12 | 180e-12 | 200e-12 |
| 220e-12 | 240e-12 | 270e-12 | 300e-12 | 330e-12 | 360e-12 | 390e-12 | 430e-12 |
| 470e-12 | 510e-12 | 560e-12 | 620e-12 | 680e-12 | 750e-12 | 820e-12 | 910e-12 |
| 1000e-12 | 1100e-12 | 1200e-12 | 1300e-12 | 1500e-12 | 1600e-12 | 1800e-12 | 2000e-12 |
| 2200e-12 | 2400e-12 | 2700e-12 | 3000e-12 | 3300e-12 | 3600e-12 | 3900e-12 | 4300e-12 |
| 4700e-12 | 5100e-12 | 5600e-12 | 6200e-12 | 6800e-12 | 7500e-12 | 8200e-12 | 9100e-12 |
| 0.01e-6 | 0.015e-6 | 0.022e-6 | 0.033e-6 | 0.047e-6 | 0.068e-6 | 0.1e-6 | 0.15e-6 |
| 0.22e-6 | 0.33e-6 | 0.47e-6 | 0.68e-6 | 1.0e-6 | 1.5e-6 | 2.2e-6 | 3.3e-6 |
| 4.7e-6 | 6.8e-6 | 10e-6 | 15e-6 | 22e-6 | 33e-6 | 47e-6 | 68e-6 |
| 100e-6 | 150e-6 | 220e-6 | 330e-6 | 470e-6 | 680e-6 | 1000e-6 | 1500e-6 |
| 2200e-6 | 3300e-6 | 4700e-6 | 6800e-6 | 10000e-6 | |||