Capacitors keep their charge after the power is removed. On a laptop board the energy involved is usually small, but the input capacitors near the DC jack can hold adapter voltage for a while, and a charged capacitor discharged by a probe or tweezers produces a spark that can damage pads or parts. This calculator shows how much energy is stored and how long a resistor takes to bring the voltage down to a safe level.
How to use it
- Enter the capacitance and the voltage it is charged to.
- Enter the resistor you will discharge it through (or the bleed resistor already on the board).
- Enter the voltage you consider safe for the work you will do.
- Read the energy, the time to reach the target and the peak current and power in the resistor.
The formulas
- Energy: E = ½ × C × V²
- Charge: Q = C × V
- Discharge time: t = R × C × ln(V0 ÷ Vsafe)
- Peak current at the first instant is V0 ÷ R, and peak power in the resistor is V0² ÷ R.
Worked example: adapter input capacitors
The input side of a laptop has 220 µF of total capacitance charged to 20 V. That stores ½ × 0.000 22 × 20² = 44 mJ — not dangerous to you, but enough for a visible spark if shorted with tweezers. Through a 1 kΩ resistor it falls to 1 V in about 0.66 s, with a peak current of 20 mA and peak power of 0.4 W for a moment. A small resistor is fine for such a brief pulse; holding it there continuously at that voltage would not be.
On most laptop boards, the capacitors discharge on their own through the circuits connected to them within seconds of removing the adapter and battery. Waiting a little before working is usually enough; the calculation tells you how long when there is no load.
Where it matters on a laptop board
- Before measuring resistance or diode mode on a board that was just powered.
- Before soldering near input capacitors.
- When building a discharge tool from a resistor and two probes.
Larger energy storage — desktop power supplies, CRT or inverter circuits — is a different matter and needs proper procedures. For timing details see RC Time Constant Calculator; safe habits at the bench are covered in Multimeter Mistakes That Damage Laptop Boards (and How to Avoid Them).
FAQ
Is it safe to short a capacitor with a screwdriver?
Not a good habit. The spark can pit contacts, lift pads and stress the capacitor. Use a resistor.
Why does my meter show voltage on an unplugged board?
Capacitors still hold charge. Wait or discharge them through a resistor before resistance measurements.
How do I choose the discharge resistor?
Large enough to limit the peak current and spark, small enough to discharge in reasonable time. A few hundred ohms to a few kilohms suits laptop input capacitors.