Capacitor Charge Calculator
Design a capacitor precharge path, visualise the charging transient, and verify inrush current, tolerance window, pulse stress and bleed-down safety.
Equations
The blue trace follows the capacitor voltage. Amber marks the selected target and the vertical marker is the time probe.
Current and path-resistor power share a normalised view. In CC/CV mode, the plateau makes the handover to the exponential tail visible.
Set a finite target voltage and move the time probe through the selected charge profile. The circuit view and all results update together.
Set your own voltage headroom. It is a review aid, not a universal derating rule: dielectric, temperature, DC bias and surge behaviour still come from the component datasheet.
Find the largest permanent bleed resistor that discharges a fully charged capacitor to a safer voltage within the chosen time.
A permanent bleeder consumes this power whenever the bus is energised. Validate discharge requirements, resistor voltage rating and the voltage that is actually safe for the equipment.
Precharge guide
Turn a capacitor charge transient into a safe component choice
A time constant alone is not enough when a capacitor is charged from a real supply. You also need to choose the right profile—RC or constant-current to constant-voltage—check energy in the path, and plan a discharge path after power-off.
Method and assumptions
The RC profile uses a DC source, source resistance, precharge resistance, and ESR in series: VC(t) = VS − (VS − V0)e^(−t/τ), with τ = RΣC. The CC/CV profile holds ICC until VSW = VS − ICCRΣ, then follows the same exponential tail. The RC assistant intersects maximum inrush current with target time while applying R and C tolerances on the safe side.
Inputs to verify
Effective capacitance
Use the capacitance actually available at operating voltage and temperature. For some MLCCs, DC bias, ageing, and temperature can substantially reduce the nominal value.
Finite target voltage
A 100% charge is asymptotic. Choose the threshold your system actually needs: 90%, a reset threshold, a converter's minimum voltage, and so on.
Complete charge path
Include source resistance, ESR, and any switch resistance or limiter. These elements reduce current, but can also make the precharge deadline impossible.
Recommended workflow
- 1Enter the source voltage, initial residual voltage, effective capacitance, and all known series resistances.
- 2Choose the voltage that actually means ‘ready’, then inspect the voltage rise and inrush with the time probe.
- 3In RC mode, use the assistant to intersect the current limit and deadline. In CC/CV mode, check the switchover voltage and time. In both cases, validate the path with its pulse curves, working voltage, and the SOA of the switch or regulator.
Decision example
To charge 1,000 µF from 0 to 5 V through 10 Ω with 0.2 Ω of parasitic resistance, τ is about 10.2 ms. Inrush is close to 490 mA. Before selecting 10 Ω, also compare its peak power and pulse energy with the manufacturer limits.
Limits to keep in mind
The CC/CV profile assumes instantly stable regulation. It does not cover loop dynamics, wiring inductance, fast switching, leakage, dielectric absorption, regulator heating, or faults. For hazardous energy or voltage, perform a safety review and measured tests.
Frequently asked questions
Why does the tool ask for effective capacitance?
Delay depends directly on C. A nominal value that falls under DC bias or temperature can make a threshold arrive earlier than expected; conversely, higher capacitance slows precharge and increases the energy to manage.
Is the resistor's continuous power rating enough?
No. A short peak can greatly exceed continuous power without necessarily being acceptable. Compare energy, duration, voltage, and pulse shape with the manufacturer overload curves.
References to consult
- Method references: RC step response, IEC 60063 preferred numbers, capacitor datasheet, resistor pulse curves, and the SOA of the switch in use.
Original educational content, reviewed for technical clarity on 20 August 2026. Always verify datasheets, applicable standards, and your design before power-up or manufacture.