Electronics
LEDs
The forward current of a LED depends on its color. The regular cheap 3-10 mm LED diodes range from \( 1.8-2.2 V \) (red, orange, yellow, etc) to \( 3.2-3.8 V \) (white, green, warm white, etc). They have typical a forward current of \( 20 mA \), and a maximum forward current of \( 30 mA \)1 (for reference, a GPIO pin of an ESP32 provides up to ca max \( 40 mA \)).
Resistance
Since these LEDs all generally have consistent forward currents and forward voltages, we can just use Ohms law and work out a reference table for common supply voltages.
Supply voltage \( V_{s} \) | \( V_{f} = 1.8 \) | \( V_{f} = 3.2 V \) |
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\( 3.3 V \) | \( 75 Ω \) | \( 5 Ω \) |
\( 5 V \) | \( 160 Ω \) | \( 90 Ω \) |
\( 12 V \) | \( 510 Ω \) | \( 440 Ω \) |
The resistor values are assume the typical forward current of 20 mA.
Forward voltage \( V_{f} \) | \( V_{s} = 3.3V\) | \( V_{s} = 5 V \) | \( V_{s} = 12 V \) |
---|---|---|---|
\( 1.8 V \) | \( 75 Ω \) | \( 160 Ω \) | \( 510 Ω \) |
\( 3.2 V \) | \( 5 Ω \) | \( 90 Ω \) | \( 440 Ω \) |
The voltage across the circuit is equal to the sum of the voltage across the components in the circuit, the resistor and the LED.
So for an LED witha forwar d voltage \( V_{f} = 1.8V \) that is connected to a supply voltage \( V_{s} = 3.3V \), the voltage across the resistor is \( V = V_{s} - V_{f} = 1.5V \).
>>> I = 0.02 # 20 mA == 0.02 A
>>> Vs = 3.3
>>> Vf = 1.8
>>> V = Vs - Vf
>>> V/I # R = V/I
75 # 75 Ohm
So to connect LED with a forward voltage of 1.8 V to 3.3 V, it should be protected by a 75 Ω resistor.
Wiring schematics
Schematics |
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Multiple LEDs connected in series. |
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Proper way to connect multiple LEDs in paralellel, each LED has its own current limiting resistor. |
Pinout
- The cathode leg (\(-\) or \(K\)) is the shorter leg (usually \( 17 mm \)).
- The anode leg (\(+\) or \(A\)) is the longer leg (usually \(19 mm\)).
RBG LED |
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RGB LED pinout4 |
Potentiometers
References
Wikipedia: Light emitting diode