LED 限流电阻计算器 — 为 LED 电路找到合适的电阻
Every LED requires a current-limiting resistor in series to protect it from excessive current. Use the formula R = (Vs − Vf) / If — subtract the LED's forward voltage from your supply voltage, then divide by the desired current. This calculator does that instantly and also shows the nearest E24 standard resistor value, power dissipation, and actual current.
Result
常见问题
LED 限流电阻计算器是如何工作的？
限流电阻阻值 R = (电源电压 - LED 正向电压) / LED 工作电流。例如，5V 电源、2V 正向电压、20mA 工作电流的 LED：R = (5V - 2V) / 0.02A = 150Ω。功耗 P = I² × R = 0.06W。
不同颜色 LED 的正向电压是多少？
典型正向电压：红色 1.8-2.2V，橙色/黄色 2.0-2.2V，绿色 2.0-3.5V，蓝色/白色 3.0-3.5V，紫外线 3.5-4.0V。实际值因型号而异，请参考 LED 数据手册获取精确参数。
如果不使用限流电阻会怎样？
不使用限流电阻会导致通过 LED 的电流无限制增大（因为 LED 正向特性曲线非常陡峭），瞬间就会损坏 LED。即使只超出额定电流 30%，长时间运行也会大幅缩短 LED 寿命。
如何选择合适瓦数的电阻？
电阻功耗 P = I² × R（或 P = V² / R）。所选电阻的额定功率应至少为计算功耗的 2 倍以确保安全余量。常用规格：1/4W（0.25W）适合 20mA 以下的 LED；高亮度 LED 可能需要 1/2W 或 1W 电阻。
LED Resistor Formula
Every LED needs a current-limiting resistor to prevent excessive current from destroying it. The required resistance is calculated by dividing the voltage available across the resistor by the desired LED current:
Basic Formula (single LED)
R = (Vs − Vf) / If
Example: Vs = 5 V, Vf = 2 V, If = 20 mA → R = (5 − 2) / 0.02 = 150 Ω
Series LEDs Formula
R = (Vs − (Vf × n)) / If
Where nis the number of LEDs. Example: 12 V, 3 red LEDs (2 V each), 20 mA → R = (12 − 6) / 0.02 = 300 Ω
Power Dissipation
P = (Vs − total Vf) × If
The power dissipated by the resistor — used to select an appropriate wattage rating.
- Vs — supply voltage (V)
- Vf — LED forward voltage (V) per LED
- If — desired forward current (A) — typically 10–30 mA for standard LEDs
- n — number of LEDs in series
- R — resistance in ohms (Ω)
How to Choose an LED Resistor
- Find your LED's forward voltage (Vf) — check the datasheet. Typical values: red 1.8–2.2 V, green 2.0–2.4 V, blue/white 3.0–3.4 V, yellow 2.0–2.2 V.
- Decide the desired current (If) — most 5 mm and 3 mm indicator LEDs run at 10–30 mA. High-brightness LEDs may allow up to 350 mA. Check the datasheet maximum rating and stay below it.
- Calculate R— use the formula R = (Vs − Vf) / If and find the nearest E24 standard value.
- Check power rating— calculate P = (Vs − Vf) × If. A standard ¼ W (0.25 W) resistor is sufficient for most low-current LED circuits. Use ½ W or 1 W resistors if P exceeds 0.1 W.
- Always round up— when selecting the nearest E24 value, prefer the next higher value over the next lower one to avoid exceeding the LED's current rating.
Common LED Forward Voltages
| LED Color | Typical Vf (V) | Typical If (mA) | R for 5 V supply (Ω) |
|---|---|---|---|
| Red | 2.0 | 20 | 150 |
| Yellow | 2.1 | 20 | 145 |
| Green | 2.2 | 20 | 140 |
| Blue | 3.2 | 20 | 90 |
| White | 3.2 | 20 | 90 |
| Infrared (IR) | 1.2 | 20 | 190 |
Note: Forward voltage varies by manufacturer and LED model. Always check the datasheet for the exact Vf at your operating current.
Series vs Parallel LED Wiring
Series Wiring (Recommended)
LEDs wired in series share the same current path, so only one resistor is needed. The total forward voltage is the sum of all individual Vf values. Series wiring is preferred because all LEDs receive the same current, producing uniform brightness.
R = (Vs − (Vf1 + Vf2 + ... + Vfn)) / If
Requires: Vs > total Vf (otherwise no current flows)
Parallel Wiring (Avoid if Possible)
Parallel LEDs each need their own current-limiting resistor because small differences in Vf between LEDs cause unequal current sharing. Never connect LEDs in parallel with a single shared resistor — one LED will take most of the current and burn out.
Power Rating Guide
The resistor must be rated to handle the power it dissipates as heat. Use the formula P = (Vs − total Vf) × If to calculate power, then choose a resistor with a wattage rating at least twice the calculated value for reliable long-term operation.
| Power Dissipated | Recommended Resistor Rating |
|---|---|
| < 62 mW | ⅛ W (0.125 W) |
| 62–125 mW | ¼ W (0.25 W) — most common |
| 125–250 mW | ½ W (0.5 W) |
| > 250 mW | 1 W or higher |
E24 Standard Resistor Values
The E24 series contains 24 values per decade (e.g., 10–91 Ω, 100–910 Ω, 1k–9.1k Ω). This calculator automatically finds the nearest E24 value to the exact calculated resistance. The E24 base values are:
1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1
× any power of 10 (0.1, 1, 10, 100, 1k, 10k, 100k, 1M Ω...)
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