Capacitors & Timing

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 What a Capacitor Does

A capacitor is like a tiny rechargeable bucket for electric charge it stores energy when connected to a voltage source and releases it when needed.
  • Charging: Voltage across the capacitor rises as it fills with charge.
  • Discharging: Voltage drops as it gives up that stored charge.
The charging/discharging curve is not linear, it follows an exponential curve described by the time constant.

Capacitance Values & Ratings

Capacitance is measured in Farads F. Common value ranges can be seen below: -
  • µF (microfarads) 1 µF = 1,000 nF
  • nF (nanofarads) 1 nF = 1,000 pF
  • pF (picofarads)
The voltage rating is the maximum safe voltage before the capacitor can fail. Always choose at least 25–50% above your circuit voltage.

Polarised vs. Non-Polarised

  • Polarised (e.g., electrolytic capacitors): Must be connected the right way round (+ and – marked).


  • Non-polarised (e.g., ceramic): Can be connected either way.


RC Timing Basics

The time constant is:

τ=R×C

Where:

  • R = resistance in ohms (Ω)

  • C= capacitance in farads (F)

  • = time in seconds (s) for the capacitor to charge to ~63% or discharge to ~37% of its maximum voltage.

A capacitor is considered fully charged after approximately 5τ.


Example 1 – LED Fade In (Capacitor + Resistor)

LED starts dim, brightens smoothly.

Parts:

  • Resistor  10 kΩ

  • Capacitor 100 µF (100 × 10⁻⁶ F)

  • LED

  • Resistor 330 Ω

Calculation:

The time it takes for the LED to become bright can be calculated by: -

τ = R×C

τ = 10,000×0.0001

τ = 1 s

In ~1 second, the LED reaches ~63% brightness.
In ~5 seconds (5 × τ), it will be almost fully bright.

Circuit Diagram:

How to build:

  1. Connect the 10kΩ Resistor and Capacitor in series between +V and GND.

  2. Connect LED + 330 Ω Resistor in parallel with the Capacitor.

  3. When power is applied, capacitor charges through R1 and the LED brightness ramps up.


Example 2 – Power-On Delay with a Transistor

Delay turning an LED on after power is applied.

Parts:

  • Resistor 47 kΩ

  • Capacitor = 220 µF

  • NPN transistor (e.g., 2N2222)

  • LED 

  • Resistor 330 Ω

Calculation:

τ=R×C
τ=47,000×0.00022
τ≈10.34 s

  • The transistor begins to switch on when the capacitor voltage reaches ~0.7 V.

  • Delay will be shorter than 10.34 s because 0.7 V is much less than supply voltage  typically around 1–3 seconds.

Circuit Diagram:


How to build:

  1. Connect 47 kΩ Resistor from +5V to 220 µF Capacitor.

  2. Connect transistor base to the capacitor. Connect the emitter to GND.

  3. Connect LED + resistor from +V to transistor collector.

  4. When powered the capacitor charges slowly until base voltage reaches 0.7 V which causes the transistor to turn on and the LED lights up.


Tips:

  • Increasing Resistance or Capacitance increases delay.

  • You can stack capacitors in parallel to increase Capacitance.


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