Calculator
Example Data Table
| Case | Method | Voltage | Resistance | Time | Expected Result |
|---|---|---|---|---|---|
| Timer delay | RC charging | 12 V to 7.58 V | 10 kΩ | 1 s | 100 µF |
| Discharge test | RC discharging | 10 V to 3.68 V | 5 kΩ | 2 s | 400 µF |
| Ripple filter | Load resistance | 12 V | 120 Ω | 100 Hz ripple | 1000 µF near estimate |
Formula Used
Charging capacitor: Vc = Vs × (1 - e-t/RC)
Capacitance from charging: C = -t / {R × ln(1 - Vc / Vs)}
Discharging capacitor: Vc = V0 × e-t/RC
Capacitance from discharging: C = -t / {R × ln(Vc / V0)}
Time constant: τ = R × C, so C = τ / R
Current method: C = I × t / ΔV
Ripple estimate: C = I / (f × pulses × Vripple)
Stored energy: E = 0.5 × C × V²
Stored charge: Q = C × V
How To Use This Calculator
- Select the method that matches your circuit test.
- Enter source voltage and capacitor voltage values.
- Enter resistance and choose the correct resistance unit.
- Add time, current, ripple, or frequency when needed.
- Use tolerance and bank fields for real capacitor planning.
- Press Calculate to show results above the form.
- Use CSV or PDF export for project records.
Capacitance From Voltage And Resistance Guide
What This Tool Solves
Capacitance cannot be found from voltage and resistance alone. A time, current, ripple, or voltage change is also needed. This calculator handles those practical cases. It supports RC charging, RC discharging, time constant work, current based voltage change, and ripple filtering. The result is shown in farads and common smaller units. That helps you select a real part quickly.
Why RC Timing Matters
A resistor controls how fast a capacitor charges or discharges. The product of resistance and capacitance is the time constant. One time constant reaches about 63 percent during charging. Five time constants are often treated as nearly complete. This makes RC networks useful in delays, filters, sensors, soft start circuits, and reset circuits.
Charging And Discharging
Charging uses the source voltage and target capacitor voltage. The target must be lower than the source. Discharging uses the starting voltage and remaining voltage. The remaining voltage must be lower than the start. The calculator checks these limits. This prevents impossible logarithm values. It also keeps the answer useful for circuit design.
Current And Ripple Options
Some circuits are easier to model with current. If current is known, capacitance equals current times time, divided by the voltage change. If current is not known, the calculator can estimate it from voltage divided by resistance. Ripple mode estimates a smoothing capacitor. It uses load current, ripple frequency, pulse count, and allowed ripple voltage.
Practical Result Checks
Real capacitors have tolerance. Their value can be lower or higher than printed. The calculator gives low and high estimates. It also estimates stored energy and charge. These values matter in power circuits. High energy can be dangerous. Always choose a voltage rating above the circuit voltage. Add margin for temperature, ripple current, and aging. For mains or high voltage work, follow safety standards. Use a qualified designer when risk is high.
FAQs
Can capacitance be calculated from only voltage and resistance?
No. Voltage and resistance alone are not enough. You also need time, current, ripple voltage, frequency, or a measured voltage change.
What is the main RC formula?
The main relationship is τ = R × C. If the time constant is known, capacitance equals τ divided by resistance.
Which method should I choose for charging?
Choose RC charging when you know source voltage, target capacitor voltage, resistance, and elapsed charging time.
Which method should I choose for discharge testing?
Choose RC discharging when a charged capacitor falls from an initial voltage to a lower measured voltage through a resistor.
What does five time constants mean?
Five time constants means 5 × R × C. It is commonly used as a near complete charge or discharge estimate.
How does ripple capacitance work?
Ripple capacitance uses load current, ripple frequency, pulse count, and allowed ripple voltage to estimate a smoothing capacitor.
Why is capacitor tolerance included?
Tolerance shows the possible low and high capacitance range. Real capacitors rarely match their printed value exactly.
Is the result safe for high voltage design?
The result is a calculation aid only. High voltage circuits need correct ratings, discharge paths, insulation, and expert safety review.