Capacitors: storage, filtering, and decoupling.

A capacitor stores energy in an electric field between two conductive plates separated by an insulating material. It resists sudden changes in voltage. That one idea explains a lot of capacitor behaviour: capacitors can supply or absorb current briefly while the voltage across them changes.

Capacitance and voltage rating

Capacitance is measured in farads, but most practical electronics uses microfarads, nanofarads, and picofarads. A 100 nF capacitor is common near digital IC power pins. A 10 uF or 100 uF capacitor may be used as local bulk storage. Tiny picofarad values appear in RF, timing, and compensation networks.

Voltage rating is the maximum voltage the capacitor is designed to withstand. It should not be treated as a working target. A capacitor rated for 6.3 V on a 5 V rail may be acceptable in some designs, but 10 V or 16 V may be a better choice depending on tolerance, spikes, temperature, and reliability goals.

Decoupling capacitors

Digital ICs draw current in short bursts when internal transistors switch. The power supply cannot always respond instantly through traces, connectors, cables, and regulators. A decoupling capacitor close to the IC provides a nearby charge reservoir for fast current changes.

Placement matters. A 100 nF capacitor on the other side of the board is not the same as a 100 nF capacitor beside the power pin. The loop from supply pin to capacitor to ground and back to the IC should be short. In fast circuits, the geometry can matter as much as the value.

Bulk capacitors

Bulk capacitors handle slower changes: motor starts, radio bursts, relay coils, LED strips, load steps, and cable drops. They are often larger values such as 10 uF, 47 uF, 100 uF, or more. A board may use both: small ceramic capacitors for fast edges and larger capacitors for lower-frequency energy storage.

Polarity and capacitor type

Ceramic capacitors are usually non-polar and useful for decoupling, filtering, and small signal work. Electrolytic and tantalum capacitors are often polarised, which means they must be connected the right way round. Reverse polarity can cause failure, leakage, heating, or worse.

Type	Strength	Watch for
Ceramic MLCC	Small, cheap, low ESR, good for decoupling.	Capacitance can drop with DC bias and temperature.
Electrolytic	Large capacitance at reasonable cost.	Polarity, ESR, leakage, lifetime, physical size.
Tantalum	Compact bulk capacitance and stable value.	Surge current, derating, failure mode, polarity.
Film	Stable, low loss, useful for audio and filters.	Larger and often more expensive.

RC timing and filters

A resistor and capacitor together create a time constant:

tau = R x C

After one time constant, a charging capacitor has moved about 63 percent of the way toward its final voltage. After about five time constants, it is usually close enough to final for many practical purposes. This behaviour is used in reset delays, button debouncing, simple filters, and analogue timing circuits.

An RC low-pass filter can smooth a noisy signal or remove fast changes before an ADC input. An RC high-pass filter can remove DC and pass changing signals. The simple formulas are useful, but real filters depend on source impedance, load impedance, component tolerance, and the frequency range of interest.

ESR, leakage, and real behaviour

Real capacitors have equivalent series resistance, often called ESR. ESR can cause heating and voltage ripple, but a little ESR can also help stabilise some regulator circuits. They also have leakage, which is a small unwanted current through the dielectric. Leakage matters in low-power timers, sample-and-hold circuits, and high-impedance nodes.

Ceramic capacitors can lose a surprising amount of capacitance under DC bias, especially small high-value parts. A capacitor marked 10 uF may not behave like 10 uF at its working voltage. For important power and timing circuits, check the datasheet rather than only the label.

Common mistakes

• Placing decoupling capacitors too far from the IC power pins.
• Using polarised capacitors backwards.
• Ignoring voltage rating and transient spikes.
• Assuming an MLCC keeps its marked capacitance at DC voltage.
• Using an RC filter without considering the load connected to it.
• Forgetting that large capacitors can create high inrush current.

A practical checklist

1. What job is the capacitor doing: decoupling, bulk storage, timing, filtering, coupling, or stability?
2. What voltage can appear across it during normal use and faults?
3. Does polarity matter?
4. Does ESR help, hurt, or need to be within a regulator specification?
5. Will leakage, tolerance, temperature, or DC bias affect the circuit?
6. Is the physical placement good enough for the frequency range involved?