You Just Plugged an LED Into a 5V Power Supply and It Burned Out Instantly
LEDs are incredibly common, in hobby electronics kits, as indicator lights, in flashlights, and everywhere in modern technology. But without a current-limiting resistor, an LED will burn out the moment you apply power. The LED's internal resistance is too low to protect itself. You need to calculate the right resistor value to set the current at a safe level (usually 10–20mA) and control brightness.
What This Calculator Does
This calculator instantly finds the resistor value you need. You enter your supply voltage, the LED's forward voltage (which depends on color), and your desired current, and the calculator tells you the exact resistor value. It also suggests standard resistor values you can actually buy and calculates the power dissipation so you know what wattage resistor to choose.
How to Use This Calculator
Supply Voltage: Enter the voltage of your power source. Common values are 3.3V (Arduino), 5V (USB, typical breadboard), 9V (battery), 12V (power supply), or other values you're using.
LED Forward Voltage: This is the voltage drop across the LED when conducting. It depends on the LED color:
If you don't know the exact value, typical red is 2.0V, and white is 3.2V. Check your LED's datasheet for precision.
Desired LED Current: Enter the current you want to flow through the LED, in milliamps (mA). Typical values are 5–20mA. Brighter LEDs often need 10–20mA. Dim indicator LEDs might only need 2–5mA. Some high-power LEDs can handle 100mA or more, but standard hobby LEDs use 20mA.
The calculator will show the ideal resistor value, the nearest standard resistor value (if different), and the actual current and power you'll get with the standard value.
The Formula Behind the Math
The formula for a current-limiting resistor is derived directly from Ohm's Law:
R = (Vs − Vf) / If
Where:
The voltage drop across the resistor is (Vs − Vf). The resistor must drop this voltage while limiting current to If.
Power dissipation in the resistor is:
P = (Vs − Vf) × If = (Vs − Vf)² / R
Worked Example:
You have a 5V supply, a red LED (Vf = 2.0V), and you want 15mA current.
A 200Ω resistor will work perfectly. A standard 220Ω resistor (the next available value) will reduce current slightly to about 13.6mA, which is fine. You need at least a 0.125W (1/8W) resistor; a 0.25W resistor gives you margin.
Our calculator does all of this instantly, but now you understand exactly what it's computing.
Choosing the Right LED Color and Current
Red and green LEDs are brightest per unit current. Blue and white LEDs need more current to appear equally bright but draw higher forward voltage. For typical indicator use, 5–10mA is sufficient and saves power. For flashlights or work lights, 20mA or higher gives good brightness.
Don't assume "brighter is better." Higher current draws more power from the battery, generates more heat, and shortens the LED's life. Choose a current that's bright enough for your application, then stop.
Multiple LEDs: Series vs. Parallel
Series: LED1 → Resistor → LED2 → Ground. Total forward voltage = Vf1 + Vf2. Use one current-limiting resistor for the whole chain. Current is the same through all LEDs.
Parallel: Two separate branches, each with its own current-limiting resistor. This prevents one LED from hogging current. Always use individual resistors; a single resistor in parallel with multiple LEDs leads to unequal brightness and one LED burning out first.
Working with Power Supplies and Different Voltages
The lower the supply voltage, the smaller the voltage drop across the resistor and the larger the resistor value. A 3.3V supply driving a white LED (3.2V) leaves only 0.1V for the resistor, nearly impossible to control current reliably. Use a blue or green LED instead, or step up to 5V.
A 12V supply driving a red LED gives a 10V drop across the resistor, allowing precise current control. But the resistor dissipates significant power: 0.150W at 15mA. Choose a 0.25W or 0.5W resistor to stay safe.
Tips and Things to Watch Out For
Don't skip the resistor. LEDs have nearly zero voltage drop until they conduct, then the voltage clamps at Vf. Without a resistor, the full supply voltage appears across the LED, and it burns out instantly. This is the number one reason beginners kill LEDs.
Check the resistor's power rating. A 1/4W (0.25W) resistor is standard in hobby kits. If your calculation shows 0.5W, use a 1/2W resistor. Oversizing is safe; undersizing causes overheating and failure.
Forward voltage varies with temperature and age. The values given (2.0V, 3.2V) are typical at room temperature. In freezing conditions, forward voltage drops slightly and current rises. In hot conditions, it's the opposite. For precision circuits, measure your actual LED's forward voltage with a multimeter in diode mode.
Standard resistor values matter. You can't buy a 200Ω resistor at most hobby shops. Standard values in the E12 series are 220Ω, 270Ω, 330Ω, etc. Use the next standard value higher than your calculation (to reduce current and brightness slightly) or lower (to increase them). Our calculator shows both options.
Modern high-power LEDs have different rules. Some LEDs are rated for 100mA, 500mA, or even 1A. They often come in 3-watt or 10-watt packages and require heat sinks. These are not typical hobby LEDs. Always check the datasheet.
Frequently Asked Questions
What happens if I use a resistor that's too large?
The LED dims. Too large a resistor means too little current. At extreme values (megohms), the LED might not light at all. A slightly larger resistor is safe, it just dims the LED. Use it as a brightness control.
What happens if I use a resistor that's too small?
The LED draws too much current, gets hot, and burns out. A slightly smaller resistor (like using 180Ω instead of 200Ω) is usually okay. Using zero (no resistor) kills the LED instantly. When in doubt, go larger.
How do I know the LED's forward voltage?
Check the datasheet. Typical values are: Red 1.8–2.2V, Green 2.0–2.5V, Blue 3.0–3.4V, White 3.0–3.5V. You can also measure it with a multimeter in diode mode, though this is less precise. Our calculator assumes typical values; you can adjust.
Can I use the same resistor for multiple LEDs in parallel?
No. Use one resistor per LED in parallel branches. A single resistor splits its current among the branches unevenly (the first LED gets slightly more). Over time, current concentrates, one LED burns hotter, and fails first. After it fails, all current goes to the next LED, and it fails too.
What's the difference between a 0.25W and 0.5W resistor?
A 0.25W resistor can safely handle 0.25 watts continuous power. A 0.5W resistor handles twice as much. For safety, use a resistor rated for at least twice your calculated power. If you calculate 0.1W, a 0.25W resistor is ideal. If you calculate 0.3W, jump to 0.5W.
Can I adjust LED brightness with the resistor?
Yes. A larger resistor dims the LED; a smaller one brightens it. This is how old analog lights worked before PWM (pulse-width modulation). Adjust your resistor value to achieve the brightness you want, as long as the current stays above about 5mA (or the LED might not light) and below the LED's max rating.
Related Calculators
Use our Ohm's Law Calculator to verify your resistor calculations or to explore "what if" scenarios. The Resistor Color Code Calculator helps you identify resistor values by their color bands. For more complex circuits with capacitors and inductors, explore our Capacitor and Inductor Calculators.