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LED American Flag Circuit Calculations
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Why I Chose a 24V DC Power Supply for
My LED Series Circuits for Efficient, Safe Operation
​Choosing the right DC power supply voltage is crucial for safe, bright, and long-lasting operation of series LED chains (such as a patriotic red-white-blue display). The supply voltage must be higher than the total forward voltage drop of all LEDs in the longest chain, but not excessively high, to avoid wasting power.
In my LED American flag design, I started by selecting a 24 VDC power supply first, as it's a widely available standard voltage and—most importantly—the highest common low-voltage DC option that's generally considered safe from serious electric shock hazards, making it ideal for a novice-friendly soldering kit. Rather than building the circuits and then hunting for a compatible supply, I designed the various red, white, and blue series LED chains to operate as efficiently as possible around this 24 V supply.
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The white LED chain ended up with the highest voltage drop: 6 LEDs × 3.2 V = 19.2 V. This leaves a comfortable 24 V – 19.2 V = 4.8 V across the current-limiting resistor, providing good headroom for stable operation without excessive power waste or heat.
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​This approach maximizes safety for beginners while keeping the design simple, efficient, and reliable, using a widely available and safe off‑the‑shelf 24 VDC power supply.
LED American Flag Circuit Calculations
1. Stars Section – 50 White LEDs (Vf ≈ 3.2 V each)
5 rows of 6 LEDs 6 × 3.2 V = 19.2 V
Resistor drop: 23.5 − 19.2 = 4.3 V
Current per row: 4.3 V / 510 Ω ≈ 8.43 mA
Total (5 rows): 5 × 8.43 ≈ 42.2 mA
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4 rows of 5 LEDs 5 × 3.2 V = 16.0 V
Resistor drop: 23.5 − 16.0 = 7.5 V
Current per row: 7.5 V / 510 Ω ≈ 14.71 mA
Total (4 rows): 4 × 14.71 ≈ 58.8 mA
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50 Stars total: ≈ 101.0 mA
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2. Blue Border – 12 Blue LEDs (Vf ≈ 3.1 V typical)
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2 strings of 6 blue LEDs 6 × 3.1 V = 18.6 V
Resistor drop: 23.5 − 18.6 = 4.9 V
Current per string: 4.9 V / 510 Ω ≈ 9.61 mA
Total (2 strings): ≈ 19.2 mA
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3. Top 7 Stripes – 10 rows × 7 LEDs (4 red @ 2.0 V, 3 white @ 3.2 V)
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Per row: (4 × 2.0) + (3 × 3.2) = 8.0 V + 9.6 V = 17.6 V
Resistor drop: 23.5 − 17.6 = 5.9 V
Current per row: 5.9 V / 510 Ω ≈ 11.57 mA
Total (10 rows): 10 × 11.57 ≈ 115.7 mA​
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4. Bottom 6 Stripes – 16 rows × 6 LEDs (3 red @ 2.0 V, 3 white @ 3.2 V)
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Per row: (3 × 2.0) + (3 × 3.2) = 6.0 V + 9.6 V = 15.6 V
Resistor drop: 23.5 − 15.6 = 7.9 V
Current per row: 7.9 V / 510 Ω ≈ 15.49 mA
Total (16 rows): 16 × 15.49 ≈ 247.8 mA
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Power Supply & Protection
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Voltage Source: 24 V DC regulated power supply
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Reverse-polarity protection: SS34 Schottky diode (1 A, 40 V) in series with +24 V input
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Effective voltage at LED circuits: 24.0 V − 0.5 V = 23.5 V
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Total current draw: ≈ 394–400 mA max
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Power connection: Solder Pads
