Components · #6 of 20
Diodes + LEDs
Current Limiting, Polarity, Brightness
Why it matters
LEDs are diodes that emit light. They have polarity (direction matters) and need current limiting (resistors) to prevent destruction.
The idea
What Is a Diode?
A diode is a one-way valve for electricity:- Forward bias: Current flows (low resistance)
- Reverse bias: Current blocked (high resistance)
- Diodes have a voltage drop (~0.7V for silicon, ~2V for LEDs)
<h3>LEDs (Light Emitting Diodes)</h3>
LEDs are diodes that emit light when current flows:
<ul>
<li><strong>Anode</strong> (+): Longer leg, connects to positive</li>
<li><strong>Cathode</strong> (-): Shorter leg, connects to ground</li>
<li><strong>Voltage drop</strong>: ~2V (red) to ~3.5V (blue/white)</li>
<li><strong>Current</strong>: Typically 20mA max (check datasheet)</li>
</ul>
<h3>Current Limiting</h3>
LEDs <strong>must</strong> have a current-limiting resistor:
<ul>
<li>Without a resistor, LED draws too much current → <strong>destroys itself</strong></li>
<li>Formula: R = (V_supply - V_LED) / I_desired</li>
<li>Example: 3.3V supply, 2V LED, 15mA desired → R = (3.3 - 2) / 0.015 = 87Ω (use 100Ω)</li>
</ul>
<h3>Brightness Myths</h3>
<ul>
<li><strong>Higher voltage ≠ brighter</strong> — current determines brightness</li>
<li><strong>PWM dimming</strong> — change duty cycle, not voltage</li>
<li><strong>Color affects voltage drop</strong> — blue/white need more voltage than red</li>
</ul>Demo
LEDs are simple but critical. Review polarity and current limiting before connecting.
Key takeaways
- Diodes are one-way valves — direction matters
- LEDs have polarity: anode (+) to positive, cathode (-) to ground
- Always use a current-limiting resistor with LEDs
- Brightness is controlled by current (or PWM duty cycle), not voltage
Going deeper
LEDs have a forward voltage (V_f) that varies by color. Red: ~1.8V, Green: ~2.1V, Blue/White: ~3.0-3.5V. For PWM dimming, use frequencies >100Hz to avoid visible flicker. For battery-powered devices, use lower current (5-10mA) to save power.
Math details
Current-limiting resistor:
R = (V_supply - V_LED) / I_desired
Example calculations:
V_supply = 3.3V
V_LED = 2V (red LED)
I_desired = 15mA = 0.015A
R = (3.3V - 2V) / 0.015A = 1.3V / 0.015A = 87Ω
Use standard value: 100Ω (slightly safer, I = 13mA)
Power in resistor:
P = I² × R = (0.013A)² × 100Ω = 0.017W (safe, <0.25W)
Power in LED:
P = V_LED × I = 2V × 0.013A = 0.026W