コンデンサ放電安全 計算ツール
安全な放電時間または放電抵抗値を算出
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Quick Answer
A capacitor discharges through a resistor following V(t) = V₀ × e^(-t/RC). The discharge time to a safe voltage is t = R×C × ln(V_start/V_end). Always verify with a voltmeter — capacitors can hold lethal charge for hours.
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V(t) = V0 x e^(-t/(R x C))
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Design Notes
High-voltage capacitors are one of the most dangerous components in electronics. A 400V 1000µF capacitor stores 80 joules — enough to kill. Always include bleeder resistors in power supply designs. UL/IEC standards require discharge to <50V within 1-5 seconds of power removal. Balance the bleeder resistance: too low wastes power continuously; too high takes too long to discharge safely.
Common Mistakes
- 1
NEVER short-circuit a charged capacitor to discharge it. The instantaneous current can weld contacts, damage the capacitor, and create dangerous arcs.
- 2
Relying on self-discharge — film capacitors can hold a charge for months. Always use a discharge resistor.
- 3
Forgetting to account for the bleeder resistor's power dissipation when the circuit is energized: P = V²/R.
Engineering Handbox
1. Time constant: τ = 10,000 × 470×10⁻⁶ = 4.7 seconds 2. Voltage ratio: ln(400/50) = ln(8) = 2.08 3. Discharge time: t = 4.7 × 2.08 = 9.8 seconds 4. Stored energy: E = ½ × 470µF × 400² = 37.6 J
Knowledge Base
放電にどれくらい待つべき？
最低5時定数（5τ = 5RC）で残留電圧0.7%。テスターで確認。50V超では7〜10τ待機。
なぜ放電抵抗が必要？
大容量コンデンサは数時間〜数日間、致命的な電荷を保持します。自己放電に頼るのは危険です。
充電コンデンサのエネルギーは？
E = ½CV²（ジュール）。1000µF×400V = 80J。心臓を通過する1Jで致死の可能性。
短絡で放電してよい？
絶対不可。巨大電流で接点溶着、銅パターン蒸発、危険なアーク発生の恐れ。必ず放電抵抗を使用。
安全な電圧レベルは？
ほとんどの安全規格で50V DC以下（IEC 60950）。医療機器では25V。