What Is a Decoupling Capacitor? (Values, Placement, and Why Your Chip Resets Without One)

The circuit works on the breadboard. Then you move it to a proper board, make the wires tidy, and the microcontroller starts resetting itself every few seconds. Nothing in the code changed. What changed is that the chip lost the small, unglamorous capacitor that was sitting next to it by accident.

What is a decoupling capacitor?

A decoupling capacitor is a small capacitor wired between a chip’s power pin and ground, as close to the pin as you can get it. Its job is to be a local reservoir of charge. When the chip suddenly needs current, the capacitor supplies it from a few millimetres away, instead of the chip having to pull it through the whole length of the power trace from the regulator.

That’s the entire idea. The reason it matters is time.

Why can’t the power supply just keep up?

A digital chip doesn’t draw a steady current. Every time its clock ticks, thousands of transistors switch at once, and for a few nanoseconds the chip wants a sharp spike of current, then almost none, then another spike. A microcontroller running at 16 MHz does this 16 million times a second.

The copper trace between the regulator and the chip isn’t a perfect wire. Every centimetre of trace has a little inductance, and inductance resists changes in current. Ask it for a spike that rises in a nanosecond and it simply can’t deliver in time. For that nanosecond the voltage at the chip’s pin sags. If it sags far enough, the chip’s brown-out detector decides the power has failed and resets. Or an ADC reading comes back noisy. Or a bit flips in RAM and nothing makes sense.

A capacitor right at the pin doesn’t care about the trace. The charge is already there. The spike is served locally, the trace tops the capacitor back up at its leisure, and the pin never sees the dip.

Decoupling capacitor vs bypass capacitor: is there a difference?

In practice they’re the same part doing the same job described from two directions. “Decoupling” describes it from the chip’s side: the chip is decoupled from the supply’s slowness. “Bypass” describes it from the noise’s side: high-frequency noise on the supply rail is given a low-impedance path to ground, bypassing the chip. One capacitor, both effects. Don’t lose sleep over the terminology.

What value should a decoupling capacitor be?

The near-universal answer is 100 nF (0.1 µF) ceramic, one per power pin. It isn’t a magic number; it’s a good compromise. A ceramic capacitor in a small package has very little inductance of its own, so it can respond at the tens-of-MHz speeds where the spikes live. Larger values in bigger packages respond more slowly, which is exactly the wrong trade.

For chips that draw bigger gulps (motor drivers, radio modules, anything with a USB port) you add a bulk capacitor of 10 µF or more alongside, not instead. The 10 µF handles the slower, larger demands in the microsecond range; the 100 nF handles the nanosecond edges. Datasheets will tell you when the chip wants something specific. Most of the time: 100 nF ceramic, and a 10 µF nearby if the chip is hungry.

Where should decoupling capacitors be placed?

As close to the power pin as physically possible, with the shortest possible path to ground. This is the rule people break most, and it’s the one that matters most. A 100 nF capacitor three centimetres away has three centimetres of trace inductance between it and the pin, which defeats the purpose. On a PCB that means the capacitor sits next to the pin, with a via to the ground plane right at its ground pad. On a breadboard it means the capacitor’s legs go into the same row as the VCC pin and the nearest ground.

If a chip has several VCC pins, each one gets its own capacitor. They’re not sharing one tank; each pin has its own trace inductance to defeat.

Which side of the capacitor goes to ground?

For the ceramic capacitors used for decoupling: it doesn’t matter. They’re not polarised; either leg can go to ground. The question comes from electrolytic capacitors, the cylindrical ones used for bulk decoupling. Those are polarised: the leg marked with a stripe (and usually the shorter leg) is negative and goes to ground. Fit one backwards and it will heat up, and in the worst case vent with a pop.

Are decoupling capacitors necessary?

Yes. The reason beginners doubt it is that circuits often work without them, for a while, on a breadboard, on a lucky day. The failures are intermittent: a reset every few minutes, a sensor reading that jitters, a crash that only happens when the motor starts. Those are the signature of a chip starved for a nanosecond at a time. Adding a 100 nF capacitor next to the VCC pin is the cheapest fix in electronics, and it’s cheaper still to put it there before you need it.

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Topics: what is decoupling capacitor · decoupling capacitor values · decoupling capacitor placement · decoupling capacitor vs bypass capacitor · decoupling capacitor purpose · 100nF capacitor