Electric Calculator for hardware engineers
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Capacitance for a pulse current

Find the capacitance able to supply a pulse of current. Work out the value needed from the output current, the output voltage and how long the pulse lasts.

Diagram: Capacitance for a pulse current
A
V
s

SI prefixes accepted (4k7 / 1M / 10m / 220). Upper-case M = mega, lower-case m = milli

Capacitance
—µF
History
Press Enter in any field to keep a snapshot here.

Formula

Capacitance needed: C = I[A] × t[s] / V[V] [F]
In µF: C[µF] = I × t / V × 10⁶

Design notes

A capacitor stores energy and can hand it back as a burst of current. It covers the brief, heavy draws that the supply itself is too slow to follow.

Typical cases:
• The transmit burst of a radio module (a large, very short current)
• Decoupling next to the supply pin, keeping an MCU stable
• Inrush at motor start-up

When you need a lot of capacitance, the usual answer is an electrolytic together with an MLCC: the electrolytic supplies the bulk, and the MLCC covers the high frequencies where the electrolytic's ESL gets in the way.

When you need this

For sizing a capacitor to keep a circuit alive after the supply drops (hold-up), or to supply a burst of load current locally. Typical cases are surviving a brownout, completing a shutdown routine, and absorbing motor or relay inrush.

What is calculated

C = I × t / ΔV. Supplying current I for time t causes the voltage to sag by ΔV; this inverts that relationship to give the capacitance.

The "output voltage" field is the permissible voltage drop ΔV, not the supply voltage. If 5V may sag to 4.5V, enter 0.5. Getting this wrong is an order-of-magnitude error.

You could work from E = ½CV², but in practice it is more natural to start from how far the rail may sag. That limit comes from the dropout voltage of the downstream regulator or the brown-out threshold of the microcontroller.

Worked example: completing a shutdown after power loss

  1. A 5V-input product. After power is removed, saving to flash takes 10ms.
  2. Consumption during that window is 200mA.
  3. The downstream 3.3V regulator has 0.5V of dropout, so 5V may fall to 3.8V. ΔV = 1.2V.
  4. Entering 0.2A / 1.2V / 10ms gives 1667µF.
  5. Allowing for electrolytic tolerance (−20%), ageing and low-temperature loss, fit 2200µF or more.

What gets overlooked

  • Instantaneous drop from ESR. A capacitor is not an ideal source. 0.1Ω of ESR at 200mA loses 20mV immediately. For current pulses, ESR can matter more than capacitance.
  • Electrolytics lose capacitance with temperature and age. Expect 60–80% of the room-temperature value at −25°C, with a further 20% loss by end of life. Equipment running cold often needs close to double the capacitance.
  • Ceramic DC bias derating. High-permittivity types (X5R / X7R) lose capacitance under bias — some halve at half rated voltage. If you use ceramics for hold-up, calculate with the effective value.
  • You need a blocking diode. If the supply collapses to a short, the stored charge flows back into it. Without a diode you cannot deliver it to the load — and the diode's forward drop reduces your usable range.

Frequently asked questions

What goes in the "output voltage" field?
The permissible voltage drop ΔV, not the supply voltage. If 5V may fall to 4.5V, enter 0.5. The calculation asks how much charge you can draw before the voltage falls by that amount.
Electrolytic or ceramic?
Electrolytic where you need bulk capacitance, ceramic where you need fast response. In practice both in parallel is normal: the electrolytic provides capacity and the ceramic handles high-frequency ESR. Where long-term reliability matters, consider polymer capacitors or multilayer ceramics, which have no wear-out mechanism.
Can I use a supercapacitor?
For hold-up on the scale of seconds to minutes, yes. But ESR is high so they suit sustained draw rather than sharp pulses, and you need inrush limiting when charging. Their temperature range and lifetime constraints are often tighter than electrolytics, so read the specification carefully.
How do I deal with inrush?
Connecting large capacitance directly across a supply draws a large surge at power-up, which can blow fuses or trip the supply into protection. Limit it with an NTC thermistor, an inrush limiter IC, or a resistor-and-relay pre-charge circuit.

Standards and references

  • IEC 60384-4 — Aluminium electrolytic capacitor specification.
  • IEC 60384-21/22 — Multilayer ceramic capacitor specification.

Last updated: 2026-08-29