Capacitor Safety Discharge Calculator
Solve discharge time or resistor value
Required Parameters
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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.
Capacitor Safety Discharge Calculator
Calculate the time required to safely discharge a capacitor through a bleeder resistor. Essential for power supply design and safety compliance.
Discharge Formula
V(t) = V0 x e^(-t/(R x C))
Time to discharge to safe voltage:
t = -R x C x ln(Vsafe / V0)
Safety Standards
- IEC 62368-1 requires discharge to below 60V DC within 2 seconds
- UL 60950 requires discharge to below 42.4V within 1 second after disconnect
- Always use a bleeder resistor rated for continuous power dissipation
Related Tools
- Time Constant Calculator
- Reactance Calculator
- Energy Conversion Calculator
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
How long should I wait for a capacitor to discharge?
Wait at least 5 time constants (5τ = 5RC) for the voltage to drop to 0.7% of its initial value. Then verify with a voltmeter before touching anything. For safety-critical work (>50V), wait for 7-10 time constants and always verify. A 1000µF cap with a 10kΩ bleeder: 5τ = 50 seconds minimum.
Why do I need a discharge resistor?
Large capacitors in power supplies, motor drives, and flash units can hold lethal charge for hours or even days. Without a bleeder resistor, the only discharge path is through leakage current — far too slow. A bleeder resistor provides a controlled, predictable discharge path. This is both a safety requirement and often a code/regulatory mandate.
How do I calculate the discharge time?
t = -RC × ln(V_end / V_start). For discharging from V_start to a safe voltage V_end: t = R × C × ln(V_start / V_end). Example: 470µF, 400V to 50V, 10kΩ bleeder: t = 10000 × 0.00047 × ln(400/50) = 4.7 × 2.08 = 9.8 seconds. Always round up and add safety margin.
What resistance should a bleeder resistor be?
Balance between discharge speed and continuous power dissipation. P = V²/R when energized. For 400V supply with 10kΩ bleeder: P = 160000/10000 = 16W (too hot!). Use 100kΩ: P = 1.6W (manageable). Discharge time increases but is still reasonable. Typical range: 10kΩ to 1MΩ depending on capacitance and acceptable discharge time.
What is a safe voltage level after discharge?
Below 50V DC is generally considered safe for contact per most safety standards (IEC 60950, UL 60950). Some standards use 60V DC. For medical equipment, the threshold is only 25V. For capacitors in RF or pulsed power applications, discharge to below 1V to prevent unexpected energy release.
How much energy is stored in a charged capacitor?
E = ½CV², measured in joules. A 1000µF capacitor at 400V stores: E = 0.5 × 0.001 × 160000 = 80 joules. For reference, 1 joule across the heart can be fatal. Even a small 100µF cap at 400V stores 8J — enough to cause severe burns and cardiac arrest. Always respect stored energy.
Can I just short-circuit a capacitor to discharge it?
NEVER. Shorting a charged capacitor causes an enormous current spike that can weld contacts, vaporize copper traces, damage the capacitor internally, and create dangerous arcs. Always use a discharge resistor. For high-energy capacitors (>100J), use a staged discharge: first through a high-value resistor, then through a lower one.
Do capacitors discharge on their own over time?
Yes, through internal leakage current, but extremely slowly. Film capacitors can hold charge for months. Electrolytics discharge faster (hours to days) due to higher leakage. NEVER rely on self-discharge for safety. Always assume a capacitor is charged until verified with a meter, even if the equipment has been off for weeks.
What is the 5-second rule for capacitor discharge?
UL and IEC standards require that capacitors in accessible power supplies discharge to below 50V within 5 seconds of power disconnection (or 1 second for pluggable equipment). This determines the maximum RC time constant: τ_max = 5s / ln(V_start/50). For a 400V supply: τ_max = 5/2.08 = 2.4s.
How do discharge resistors affect circuit efficiency?
Bleeder resistors continuously waste power when the circuit is energized: P = V²/R. A 100kΩ bleeder on a 400V rail wastes 1.6W constantly. In battery-powered or energy-harvesting applications, this is significant. Solutions: use a MOSFET-switched discharge resistor that only activates during shutdown, or use higher resistance values with slower discharge acceptance.