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Topic 19:Electric Circuits
A concise comparison of incandescence, LED, and OLED:
| Feature | Incandescence | LED (Light Emitting Diode) | OLED (Organic LED) |
|---|---|---|---|
| Working Principle | Heats a metal filament (usually tungsten) until it glows | Electrons recombine with holes in a semiconductor, releasing energy as light | Same as LED but uses organic (carbon-based) compounds |
| Material | Metal filament | Inorganic semiconductors | Organic molecules or polymers |
| Light Type | Broad-spectrum (warm, natural light) | Monochromatic (can be tuned by material) | Tunable, rich colors with high contrast |
| Efficiency | Low (most energy is lost as heat) | High | Very high |
| Lifespan | Short | Long | Moderate (shorter than LED, longer than incandescent) |
| Flexibility | Rigid (glass bulb) | Rigid (but small) | Flexible and can be printed on surfaces |
| Brightness Control | Not easily controlled | Easily dimmed | Easily dimmed |
| Power Consumption | High | Low | Very low |
| Applications | Traditional bulbs, heaters | TVs, indicators, lighting | Displays in phones, TVs, wearables, lighting |
Summary:
Incandescence: Generates light by heat. Inefficient and outdated.
LED: Efficient, durable, and used widely in modern lighting and screens.
OLED: Offers better contrast and flexibility than LED, mainly used in high-end displays.
Components:
DC power supply: Powers the relay coil.
Switch (manual): Turns the relay coil on or off.
Relay: Electromechanical switch that isolates the DC and AC circuits.
Coil: The electromagnet that activates the switch.
COM (Common): Central terminal of the switch.
NO (Normally Open): Open (disconnected) when coil is off; connected when coil is energized.
NC (Normally Closed): Closed (connected to COM) when coil is off; disconnected when coil is energized.
AC power supply: Provides power to the load.
AC Lamp (Load): The device being controlled.
How It Works:
When the DC switch is open (coil is OFF):
No current flows through the relay coil.
The coil does not generate a magnetic field.
The internal contact remains in its default position — COM is connected to NC.
Since the lamp is connected via COM and NO, the circuit is open, and the lamp stays OFF.
When the DC switch is closed (coil is ON):
Current flows from the DC supply through the relay coil.
The coil becomes electromagnetized, producing a magnetic field.
This magnetic field attracts a metal armature inside the relay — it is pulled toward the coil (not repelled).
The armature is mechanically linked to the switch contacts.
As it moves, it breaks contact with NC and makes contact with NO, connecting COM to NO.
This completes the AC circuit, allowing current to flow through the AC lamp — the lamp turns ON.
When the DC switch is opened again:
The current to the coil stops.
The magnetic field collapses.
A spring mechanism pushes the armature back to its default position, reconnecting COM to NC and disconnecting COM from NO.
The lamp turns OFF.
Summary:
The relay coil uses magnetic attraction (not repulsion) to move the internal contacts.
It allows a low-voltage DC control circuit to safely switch a high-voltage AC load.
The NO contact is closed when the coil is energized — by the pulling force of the electromagnet on the internal switching mechanism.