---
title: "Calculates RC capacitor charge timing, energy storage, and transient voltages. Given resistance, capacitance, supply voltage, and optional initial voltage, computes the RC time constant (τ = R×C), time to reach standard charge thresholds (63%, 86%, 95%, 99%), voltage at key time points, energy stored (E = ½CV²), and peak charging current. Optionally calculates time to reach a specific target voltage using t = -τ × ln((Vsupply - Vtarget) / (Vsupply - Vinitial)). Essential for timing circuits, power-on reset delays, debounce networks, and soft-start designs. Chain from rc_filter for frequency-domain analysis or timer_555 for oscillator timing."
method: POST
path: "/api/capacitor_charge"
tags: ["pcb"]
---

# Calculates RC capacitor charge timing, energy storage, and transient voltages. Given resistance, capacitance, supply voltage, and optional initial voltage, computes the RC time constant (τ = R×C), time to reach standard charge thresholds (63%, 86%, 95%, 99%), voltage at key time points, energy stored (E = ½CV²), and peak charging current. Optionally calculates time to reach a specific target voltage using t = -τ × ln((Vsupply - Vtarget) / (Vsupply - Vinitial)). Essential for timing circuits, power-on reset delays, debounce networks, and soft-start designs. Chain from rc_filter for frequency-domain analysis or timer_555 for oscillator timing.

`POST /api/capacitor_charge`

## Request

- Base URL: `https://calc.engineer`
- URL: `https://calc.engineer/api/capacitor_charge`
- Auth: none declared

## Request body

- object
  - `resistance_ohm` number, required: Series resistance in ohms (R). Controls charge/discharge rate.
  - `capacitance_f` number, required: Capacitance in farads (C). Use scientific notation, e.g. 1e-6 for 1 µF.
  - `v_supply` number, required: Supply voltage in volts. The voltage the capacitor charges toward.
  - `v_initial` number: Initial capacitor voltage in volts (default 0 for fully discharged).
  - `v_target` number: Target voltage to reach in volts (optional). When provided, calculates time to reach this voltage. Must be between v_initial and v_supply.

## Response `200`

Computed result for capacitor_charge

- object
  - `result` object, required
    - `tau_s` number, required: RC time constant (τ = R×C) in seconds.
    - `tau_ms` number, required: RC time constant in milliseconds.
    - `time_to_63pct_ms` number, required: Time to reach 63.2% of (Vsupply − Vinitial) in milliseconds (1τ).
    - `time_to_86pct_ms` number, required: Time to reach 86.5% of (Vsupply − Vinitial) in milliseconds (2τ).
    - `time_to_95pct_ms` number, required: Time to reach 95.0% of (Vsupply − Vinitial) in milliseconds (3τ).
    - `time_to_99pct_ms` number, required: Time to reach 99.3% of (Vsupply − Vinitial) in milliseconds (5τ).
    - `time_to_target_ms` number, nullable, required: Time to reach v_target in milliseconds (null if v_target not specified).
    - `v_at_1tau` number, required: Capacitor voltage after 1τ in volts.
    - `v_at_2tau` number, required: Capacitor voltage after 2τ in volts.
    - `v_at_5tau` number, required: Capacitor voltage after 5τ in volts.
    - `energy_joules` number, required: Energy stored at full charge: E = ½CV² in joules.
    - `energy_mj` number, required: Energy stored at full charge in millijoules.
    - `peak_current_ma` number, required: Peak charging current at t=0 in milliamps: Ipeak = (Vsupply − Vinitial) / R.
  - `tool` string, required
  - `version` string, required
  - `computed_at` string, date-time, required
  - `chain` object, required
    - `feeds` unknown[]
      - unknown
    - `accepts_from` unknown[]
      - unknown

## Other responses

- `400`: Validation error or invalid JSON
- `404`: Tool not found

---

[API](https://skmtc.dev/calc/apis/calc-engineer-api.md) · [All operations](https://skmtc.dev/calc/apis/calc-engineer-api/llms.txt) · [OpenAPI document](https://skmtc.dev/calc/apis/calc-engineer-api/revisions/8830480ec488?raw)