An illuminated light switch works by using a small LED (light-emitting diode) and a resistor connected in series with the switch to provide a dim light, even when the main light is off.
The Basic Principle
The key is that a small amount of current is allowed to flow through the LED and the resistor, regardless of whether the main circuit is open (light off) or closed (light on). This current is intentionally kept very low so that it's enough to illuminate the LED dimly, but not enough to turn on the main light bulb.
Circuit Configuration
Here's a breakdown of how the circuit is typically configured:
- Power Source: The switch is connected to the standard household AC power.
- Switch Mechanism: The switch has at least two terminals - one connected to the power source and another connected to the light fixture. When the switch is flipped, it either completes the circuit (light on) or breaks the circuit (light off).
- LED: A small LED is wired in series with a resistor. The LED is what provides the illumination. LEDs are diodes, meaning they only allow current to flow in one direction.
- Resistor (Ballast Resistor): The resistor (often called a ballast resistor) is crucial. It limits the current flowing through the LED. Without the resistor, the LED would burn out very quickly due to excessive current. The resistor also ensures that the current is low enough to prevent the main light bulb from illuminating when the switch is "off".
- Wiring Arrangement: The LED and resistor combination are usually wired in parallel with the switch contacts. This means that when the switch is open (light off), the small amount of current flows through the LED and resistor. When the switch is closed (light on), the current primarily flows through the light bulb circuit, offering much lower resistance and effectively bypassing the LED.
Detailed Explanation
- Switch Open (Light Off):
- The main circuit to the light bulb is broken.
- A small current flows through the resistor and the LED, causing the LED to light up dimly. The resistor limits this current to prevent damage and to keep the LED's light dim.
- Switch Closed (Light On):
- The main circuit to the light bulb is completed.
- The current now predominantly flows through the light bulb because it offers a path of much lower resistance compared to the LED and resistor combination.
- While some tiny amount of current still flows through the LED and resistor, it's negligible compared to the current flowing through the light bulb, so the presence of the LED doesn't affect the operation of the main light. The LED may appear to dim or go out completely because the voltage drop across the light bulb is significantly less than what's required to illuminate the LED brightly.
Why this works
- Diode Properties: The LED itself is a diode, meaning it only allows current to flow in one direction. This is crucial for the circuit to function correctly.
- Current Limiting: The resistor is vital for limiting the current flowing through the LED. Without it, the LED would be destroyed.
- Parallel Circuit Behavior: The LED/resistor combination is in parallel with the switch contacts. This allows the small current to flow when the switch is open and allows the current to bypass the LED/resistor when the switch is closed due to the path of least resistance.
In summary, an illuminated light switch uses a simple circuit with an LED and a resistor in parallel with the switch contacts, allowing a small amount of current to flow and illuminate the LED when the switch is off. When the switch is on, the current takes the path of least resistance through the light bulb, effectively bypassing the LED circuit.
