🔋Capacitors Explained for Beginners: Types, Codes (104 = 100 nF), Polarity and τ = RC (with 2 Experiments)

Open any circuit board — a phone charger, an Arduino, a TV remote — and you will find small brown discs and little blue or black cylinders next to almost every chip. These are capacitors, and after resistors they are the most common part in electronics.

A capacitor stores a small amount of electrical charge and releases it when the circuit needs it. That simple job lets it smooth power supplies, block noise, set the speed of a blinking LED and keep a microcontroller from resetting. This guide explains how capacitors work, how to read their codes, how to use them safely, and ends with two experiments you can do on a breadboard today.

What does a capacitor actually do?

A capacitor fills like a tank through a narrow pipe: quickly at first, then more slowly as the levels get closer

Inside, a capacitor is just two metal plates separated by an insulator (called the dielectric). No current can flow straight through the insulator. When you connect a battery, electrons pile up on one plate and leave the other, so a voltage builds up between them. Disconnect the battery and the charge stays there — the capacitor now behaves like a tiny, very fast rechargeable battery.

The water analogy makes this easy to picture:

The three are linked by a simple formula:

For example, a 1000 µF capacitor charged to 9 V holds Q = 0.001 × 9 = 0.009 C (9 mC) and E = ½ × 0.001 × 9² = 0.04 J. That is tiny compared with a battery, but a capacitor can give it back in a fraction of a second — which is exactly what makes it useful.

Capacitor units

One farad is huge, so you will almost always see smaller units:

Remember: 1 µF = 1,000 nF = 1,000,000 pF.

Types of capacitors

The four types in most beginner kits. Electrolytic and tantalum capacitors are polarised; ceramic and film are not.

How to read capacitor codes

First two digits are the value, the third is the number of zeros. The result is in picofarads.

Electrolytic capacitors usually have the value printed in full ("1000µF 25V"). Small ceramic and film capacitors use a three-digit code:

1. Take the first two digits as a number.
2. The third digit says how many zeros to add.
3. The answer is in picofarads (pF).

A letter after the code is the tolerance: J = ±5 %, K = ±10 %, M = ±20 %. So 104K is 100 nF ±10 %. Film capacitors may also carry a voltage code such as 2A (100 V). Save yourself the counting with our capacitor code calculator.

Polarity and voltage rating: the safety rules

Every capacitor has a maximum voltage printed on it (or in its datasheet). Exceed it and the insulator breaks down.

• Choose a rating comfortably above your supply. For a 5 V Arduino circuit use 10 V or 16 V parts; for a 9 V battery circuit use 16 V or 25 V. A higher rating is always fine — only the size and price go up.
• Fit polarised capacitors the right way round. The negative leg goes to the lower voltage (usually ground). A reversed electrolytic can get hot, swell and burst its top vent.
• Large capacitors can stay charged after power is removed. Before touching a big capacitor in a power circuit, discharge it through a resistor (for example 1 kΩ for a few seconds) — never short it with a screwdriver.

Charging, discharging and the time constant τ = RC

Every RC circuit follows the same curves; only the time scale changes with R × C

When a capacitor charges through a resistor, the current is largest at the start (the capacitor is empty, so the full supply voltage pushes current through the resistor). As the capacitor voltage rises, less voltage is left across the resistor, so the current — and the rate of charging — falls. That gives the famous curved shape.

The speed is set by the time constant:

Where capacitors are used

• Decoupling: a 100 nF ceramic capacitor right next to the power pins of every chip supplies the quick bursts of current the chip needs, and stops noise from travelling along the power wires. This is why Arduino and ESP32 boards are covered in tiny capacitors.
• Smoothing and bulk storage: a 100 µF – 1000 µF electrolytic across a supply fills in dips when a motor or servo starts. If your Arduino resets when a servo moves, this is often the fix.
• Filtering: a resistor and capacitor make an RC low-pass filter that removes fast noise. Its cut-off frequency is f = 1 ÷ (2π × R × C) — with 10 kΩ and 100 nF that is about 159 Hz. Try it in the RC filter cut-off calculator.
• Timing: the capacitor in a 555 timer sets how fast an LED blinks. Bigger capacitor, slower blink — see the 555 timer calculator.
• Coupling: a capacitor in series passes changing (AC) signals but blocks steady DC, which is how audio stages are joined together.

Experiment 1: an LED that fades out slowly

Press S1 to charge C1. Release it and C1 keeps the LED lit while it discharges.

You need: a breadboard, a 9 V battery with clip, a push button, a 1000 µF electrolytic capacitor rated 16 V or more (25 V is ideal), a 100 Ω resistor, a 1 kΩ resistor, a red LED and jumper wires.

1. Connect the battery + through the push button and the 100 Ω resistor to a free breadboard row. Call this row A.
2. Put the capacitor's long (+) leg in row A and its striped (−) leg on the negative rail.
3. From row A, connect the 1 kΩ resistor to the LED's long leg. Connect the LED's short leg to the negative rail.
4. Connect the battery − to the negative rail. Check the capacitor polarity once more.
5. Press and hold the button for a second. The LED lights up.
6. Let go. Instead of switching off, the LED fades out over about 3–5 seconds as the capacitor empties through the 1 kΩ resistor and the LED.

Try next: swap the 1000 µF capacitor for 470 µF — the fade takes about half as long. Use a 2.2 kΩ resistor instead of 1 kΩ — the LED starts dimmer but glows for longer. Can you explain both using τ = R × C?

Experiment 2: measure a time constant with an Arduino

Your eyes cannot time a capacitor accurately, but an Arduino can. Connect D7 → 10 kΩ resistor → row B, a 100 µF electrolytic from row B (+ leg) to GND (− leg), and a wire from row B to A0. The expected time constant is 10 kΩ × 100 µF = 1 second.

const int chargePin = 7;
const int sensePin = A0;
const float R = 10000.0;   // 10 kΩ charging resistor

void setup() {
  Serial.begin(9600);
  pinMode(chargePin, OUTPUT);
}

void loop() {
  // 1. Empty the capacitor and wait until it reads (almost) 0 V
  digitalWrite(chargePin, LOW);
  while (analogRead(sensePin) > 2) { }
  delay(500);

  // 2. Start charging and time how long it takes to reach 63.2 %
  unsigned long start = millis();
  digitalWrite(chargePin, HIGH);
  while (analogRead(sensePin) < 647) { }   // 0.632 × 1023 ≈ 647
  unsigned long tauMs = millis() - start;

  // 3. Report the time constant and the capacitance it implies
  float tau = tauMs / 1000.0;
  Serial.print("tau = ");
  Serial.print(tau, 3);
  Serial.print(" s,  C = ");
  Serial.print(tau / R * 1000000.0, 1);
  Serial.println(" uF");
  delay(2000);
}

Open the Serial Monitor at 9600 baud. You should see a value near 1 s and about 100 µF. Do not be surprised by 0.85 s or 1.15 s — electrolytic capacitors often have a ±20 % tolerance, and this experiment measures the real value of yours.

Common beginner mistakes

Where to get the parts

Most beginner kits include an assortment of ceramic and electrolytic capacitors, resistors, LEDs and a breadboard. If you are just starting, our breadboard basics guide shows how to place each part, and you can browse Thinking Robot kits or individual components for everything used above. To see how capacitors and PWM work together in motor projects, read PWM explained.

Frequently asked questions

What does 104 mean on a capacitor?

It means 10 followed by 4 zeros, in picofarads: 100,000 pF, which is 100 nF or 0.1 µF. It is the most common decoupling capacitor in electronics.

Can I use a capacitor with a higher voltage rating?

Yes. A 1000 µF 35 V capacitor works perfectly in place of a 1000 µF 16 V one — it is just larger. Never use a lower voltage rating than the circuit needs.

Which leg of a capacitor is positive?

For an electrolytic capacitor, the longer leg is positive and the stripe marks the negative leg. For a tantalum capacitor, the + mark or bar shows the positive leg. Ceramic and film capacitors have no polarity.

Why does a capacitor charge to only 63 % in one time constant?

The charging current falls as the capacitor fills, so it never charges at a steady rate. The maths of that curve (1 − e⁻¹ ≈ 0.632) gives 63.2 % after one time constant τ = R × C. After five time constants it is over 99 % full.

Is it safe for children to experiment with capacitors?

Yes, with small capacitors and battery or USB power, under adult supervision. Check polarity and voltage rating before powering up, and never take apart mains-powered equipment. Our workshops cover these basics hands-on.