Calculator KB-30
RC circuit calculator
Turn-on delays, soft start, microcontroller reset: it's all the same formula.
Result
In brief: RC circuit calculations: time constant τ, capacitor charging time through a resistor, voltage at a given moment, initial current, and charging curve. Example: Time constant τ: 1 s.
Calculation example
| Resistance | 10 kΩ |
|---|---|
| Capacity | 100 µF |
| Supply voltage | 5 V |
| Time | 1000 ms |
| Charge to | 90 % |
| Time constant τ | 1 s |
|---|---|
| Nearly fully charged, 5τ | 5 s |
| Voltage at a given moment | 3.16 V |
| Time to reach the desired level | 2.3 s |
| Initial charging current | 500 µA |
How it is calculated
U(t) = U · (1 − e−t/τ)
t = −τ · ln(1 − x)
In one time constant, the capacitor charges to 63% of the voltage; after five, it is almost fully charged. Discharging follows the same curve, only downward.
What to look for
- Electrolytic capacitors have a wide capacitance tolerance, up to ±20%. You can't get precise delays with them.
- High resistance values are sensitive to capacitor leakage current and board humidity.
Frequently asked questions
What is the time constant of an RC circuit?
τ = R · C. It's the time it takes a capacitor to charge to 63% of the supply voltage. 10 kΩ and 100 µF give 1 second, while a practically full charge takes 5τ, or 5 seconds.
How do you calculate the time it takes a capacitor to charge to a given voltage?
t = −τ · ln(1 − x), where x is the fraction of the supply voltage. A capacitor takes 2.3τ to reach 90%: with τ = 1 s, that's 2.3 s.
What current flows at the start of charging?
The maximum current: I = U / R, as if the capacitor were shorted. At 5 V and 10 kΩ, that's 500 µA. In high-power supplies, the inrush current of large capacitors is limited with a thermistor.
