Capacitor Charge Time Calculator
Calculate the time it takes for a capacitor to charge to a specific voltage
Microfarads (µF)
Ohms (Ω)
Volts (V)
Volts (V) - must be less than supply voltage
How to Use This Calculator
Enter Capacitance
Enter the capacitor value in microfarads (µF). This is the capacitance value of your capacitor.
Enter Resistance
Enter the charging resistor value in ohms (Ω). This limits the charging current.
Enter Supply and Target Voltages
Enter the supply voltage (charging voltage) and the target voltage you want the capacitor to reach. Target voltage must be less than supply voltage.
Get Results
Click calculate to see the RC time constant and the time required for the capacitor to charge to the target voltage.
Formula
τ = R × C
V(t) = V × (1 - e-t/RC)
t = -RC × ln(1 - Vtarget/V)
Where:
- τ (tau) = Time constant (seconds)
- R = Resistance (Ω)
- C = Capacitance (F)
- V(t) = Voltage at time t (V)
- V = Supply voltage (V)
- t = Time (seconds)
- ln = Natural logarithm
Key Points:
- After 1 time constant (τ), capacitor charges to ~63.2% of supply voltage
- After 2τ, capacitor charges to ~86.5%
- After 3τ, capacitor charges to ~95.0%
- After 5τ, capacitor charges to ~99.3% (considered fully charged)
Example 1:
Capacitance = 100 µF, Resistance = 1000 Ω, Supply = 12 V, Target = 10 V
τ = 1000 × (100 × 10-6) = 0.1 s
t = -0.1 × ln(1 - 10/12) = -0.1 × ln(0.167) = -0.1 × (-1.79) = 0.179 s
Example 2:
Capacitance = 470 µF, Resistance = 2200 Ω, Supply = 5 V, Target = 4.5 V
τ = 2200 × (470 × 10-6) = 1.034 s
t = -1.034 × ln(1 - 4.5/5) = -1.034 × ln(0.1) = 2.38 s
About Capacitor Charge Time Calculator
The Capacitor Charge Time Calculator is a free online tool that helps you calculate how long it takes for a capacitor to charge to a specific voltage through a resistor in an RC (resistor-capacitor) circuit. This is essential for timing circuits, delay circuits, and power supply design where you need to know when a capacitor reaches a certain voltage level.
When to Use This Calculator
- Timing Circuits: Design delay circuits and timers using RC networks
- Power Supply Design: Calculate soft-start times for power supplies
- Filter Design: Determine RC filter response times
- Pulse Circuits: Design monostable and astable multivibrator circuits
- Educational Purposes: Learn about exponential charging and RC time constants
Why Use Our Calculator?
- ✅ Accurate Calculations: Uses the correct exponential charging formula
- ✅ Time Constant Display: Shows the RC time constant for reference
- ✅ Easy to Use: Simple interface requiring capacitance, resistance, and voltages
- ✅ Multiple Units: Displays results in seconds or milliseconds as appropriate
- ✅ Free Tool: No registration or payment required
- ✅ Educational: Includes formulas, examples, and key charging percentages
Common Applications
Delay Circuits: Many electronic circuits use RC networks to create delays. For example, a power-on reset circuit might need a capacitor to charge to 2.5V before activating, creating a delay that ensures the system is stable before starting.
Power Supply Soft-Start: Power supplies often use RC circuits to gradually increase voltage at startup, preventing inrush current and protecting components. This calculator helps determine the soft-start duration.
Filter Timing: RC filters need time to respond to input changes. This calculator helps determine how quickly a filter reaches its target output voltage.
Pulse Generation: Monostable multivibrators use RC timing to generate pulses of specific duration. This calculator helps select the right RC values for desired pulse width.
Tips for Accurate Results
- Enter capacitance in microfarads (µF) - the calculator converts to farads internally
- Target voltage must be less than supply voltage (capacitor never charges above supply)
- Remember that charging is exponential - it takes infinite time to reach exactly 100%
- For practical purposes, 5 time constants (5τ) is considered "fully charged"
- Larger capacitance or resistance values result in longer charge times
- Consider capacitor tolerance - actual charge time may vary by ±10-20%
Frequently Asked Questions
What is the RC time constant?
The RC time constant (τ = R × C) is the time it takes for a capacitor to charge to approximately 63.2% of the supply voltage. It's a fundamental parameter that characterizes how quickly an RC circuit responds.
How long does it take to fully charge a capacitor?
Theoretically, a capacitor never fully charges (reaches 100% of supply voltage) in finite time. However, for practical purposes, after 5 time constants (5τ), the capacitor is considered fully charged at 99.3% of the supply voltage.
What happens if I use a smaller resistor?
A smaller resistor allows more current to flow, resulting in faster charging. The time constant (τ = RC) decreases, so the capacitor charges more quickly. However, ensure the resistor can handle the power dissipation.
Does this work for discharging capacitors too?
The formula is similar but inverted. For discharging: V(t) = V₀ × e^(-t/RC), where V₀ is the initial voltage. The time constant is the same, and after 5τ, the capacitor is considered fully discharged (0.7% of initial voltage remains).
Can I charge a capacitor instantly?
No, charging a capacitor instantly would require infinite current, which is impossible. There's always a resistor (or equivalent resistance) that limits current, creating an exponential charging curve. Even with very small resistance, there's still a finite charging time.