Calculator
Formula Used
The capacitor discharge equation is:
Vt = V0 × e^(-t / (R × C))
Solving for capacitance gives:
C = t / (R × ln(V0 / Vt))
When leakage resistance is entered, the calculator uses:
Reffective = 1 / ((1 / Rdischarge) + (1 / Rleakage))
Energy is calculated by E = 0.5 × C × V². Charge is calculated by Q = C × V.
How to Use This Calculator
- Enter the initial capacitor voltage before discharge starts.
- Choose final voltage or final percent mode.
- Enter the measured discharge time and its unit.
- Enter the discharge resistor value and unit.
- Add leakage resistance if a parallel leakage path exists.
- Add measurement tolerances for a practical range.
- Press Calculate to show results above the form.
- Use the CSV or PDF button to download a report.
Example Data Table
| Initial V | Final V | Time | Resistance | Expected result | Use case |
|---|---|---|---|---|---|
| 12 V | 4.416 V | 2.2 s | 10 kOhm | 220 uF | One time constant test |
| 5 V | 2.5 V | 0.347 s | 5 kOhm | 100 uF | Half voltage test |
| 24 V | 8.832 V | 10 s | 100 kOhm | 100 uF | Slow discharge check |
Capacitance from Discharge Time Guide
A capacitor discharge test is a practical way to estimate unknown capacitance. You charge the capacitor to a known voltage. Then you discharge it through a measured resistor. The voltage falls in a predictable exponential curve. This calculator uses that curve to solve for capacitance.
Why discharge time matters
The time value tells how slowly stored charge leaves the capacitor. A large capacitor holds more charge. It needs more time to drop to the same final voltage. A larger resistor also slows the discharge. Because both values affect time, the resistance must be known with care.
Voltage ratio and accuracy
The formula uses the natural log of the starting voltage divided by the ending voltage. The ending voltage must be lower than the starting voltage. Very small voltage changes can cause weak results. Pick a clear threshold, such as fifty percent or thirty six point eight percent. Stable readings improve confidence.
Real circuit effects
Actual parts have tolerances. Resistors may differ from their label. Capacitors may have leakage. Leads, switches, meters, and probes can add small errors. The optional tolerance fields help show a practical range. The leakage option estimates an effective discharge resistance when another parallel path exists.
Useful design checks
The result includes capacitance in farads and common subunits. It also shows the time constant, stored charge, stored energy, and an estimated cutoff frequency. These values help compare timing circuits, reset networks, pulse hold circuits, and supply filters. They also help with bench notes.
Safe testing practice
Always discharge large capacitors safely. Use a resistor with enough power rating. Confirm polarity for electrolytic capacitors. Never short high energy capacitors with a tool. Measure voltage before handling the circuit. A careful setup protects parts, meters, and people.
When to use percent mode
Percent mode is useful when the final voltage is set by a comparator, timer input, or scope marker. For one time constant, final voltage is about thirty six point eight percent of the initial value. Half voltage is also popular. It is easy to read on basic meters.
Record every condition
Write down temperature, resistor rating, voltage limits, and instrument input resistance. These notes make repeated tests easier to compare later safely.
FAQs
What does this calculator find?
It finds capacitance from discharge time, resistance, initial voltage, and final voltage. It also reports charge, energy, time constant, uncertainty, and optional marked value comparison.
Why must final voltage be lower?
A discharge curve moves downward from the starting voltage. The logarithm only works for a valid ratio between zero and one. A higher final voltage means the entered test values are not a discharge case.
What is one time constant?
One time constant is R multiplied by C. After one time constant, an ideal capacitor falls to about 36.8 percent of its starting voltage during discharge.
Can I use milliseconds?
Yes. Select milliseconds, microseconds, seconds, or minutes. The calculator converts the entered time into seconds before applying the discharge equation.
What is leakage resistance?
Leakage resistance is an extra path that lets charge leave the capacitor. When entered, it is treated as a parallel path with the discharge resistor.
Why is tolerance included?
Bench measurements are not perfect. Resistor tolerance, timing error, and voltage reading error can shift the answer. The range gives a practical estimate.
Does this work for electrolytic capacitors?
Yes, for safe low voltage tests. Observe polarity, voltage rating, and discharge resistor power. Electrolytic parts often have wide tolerances and noticeable leakage.
Can I download the result?
Yes. Use the CSV button for spreadsheet records. Use the PDF button for a simple printable report of the calculated values.