Powering LED lights correctly comes down to matching your power source to the voltage and current your LEDs need. Get it right and they’ll run efficiently for tens of thousands of hours. Get it wrong and you’ll burn them out in seconds. The basics are straightforward: figure out what your LEDs require, pick a power source that delivers it, and protect against overcurrent.
What Every LED Needs: Voltage and Current
Unlike traditional bulbs, LEDs don’t simply plug into any power source and regulate themselves. Each LED has a “forward voltage,” the minimum voltage needed to turn it on, and a rated current it’s designed to handle, usually measured in milliamps (mA). Exceeding either one shortens the LED’s life or kills it immediately.
The forward voltage depends on the LED’s color. Red, orange, and yellow LEDs need about 1.8 to 2.2 volts. Green LEDs run between 2.0 and 2.5 volts. Blue and white LEDs require more, typically 3.0 to 3.5 volts, because of the different semiconductor materials used to produce those colors. Most standard indicator LEDs (the small 3mm or 5mm types) are rated for 20 mA of current.
This means you can’t connect a bare LED directly to a 9V battery or a 12V power supply. The excess voltage would push far too much current through the LED, destroying it almost instantly. You need either a resistor to limit the current or a purpose-built LED driver.
Using Resistors for Simple LED Circuits
For individual LEDs or small projects, a resistor is the simplest way to control current. The formula is basic: subtract the LED’s forward voltage from your supply voltage, then divide by your target current. If you’re running a red LED (2V forward voltage) from a 9V battery and want 20 mA of current, that’s (9 – 2) / 0.020 = 350 ohms. Grab the nearest standard resistor value above that number (390 ohms works) and you’re set.
For a gentler, longer-lasting setup, aim for 5 to 10 mA instead of the full 20 mA. Most LEDs are still clearly visible at lower currents, and you’ll dramatically extend battery life. Simply plug your lower target current into the same formula.
Pre-assembled LED strips skip this step entirely. Standard 12V LED strips (like the common 5050 SMD strips used for accent lighting) have resistors built into the circuit board. Every group of three LEDs on the strip already has a resistor soldered in. You connect these strips directly to a 12V power source with no additional components.
Choosing the Right Power Supply
LED power supplies fall into two categories: constant voltage and constant current. The type you need depends on what you’re powering.
Constant voltage supplies output a fixed voltage, most commonly 12V or 24V DC. These are what you use for LED strip lights, under-cabinet lighting, and any pre-built LED product that specifies a voltage rating. The strips themselves handle current regulation through their onboard resistors. Constant voltage supplies are widely available, affordable, and familiar to anyone who’s used a laptop charger or phone adapter.
Constant current drivers output a fixed current (specified in mA or amps) while adjusting voltage as needed. These are designed for high-power LEDs, like the individual emitters used in flashlights, grow lights, or custom fixtures. Because high-power LEDs are extremely sensitive to current fluctuations, a constant current driver prevents thermal runaway, a destructive cycle where rising temperature causes the LED to draw more current, which raises temperature further until the LED fails. If you’re building a fixture with bare high-power LEDs, a constant current driver is the safer and more reliable choice.
Sizing Your Power Supply
To size a constant voltage supply for LED strips, multiply the strip’s power consumption per meter (listed by the manufacturer) by the total length you’re installing. Then add 20% headroom so the supply isn’t running at maximum capacity. A strip rated at 14 watts per meter, run for 3 meters, draws 42 watts. A 50W power supply gives you comfortable margin.
Wire gauge matters too, especially on longer runs. For low-power setups under a few amps, 18 AWG wire handles the job. Once you get above 5 to 10 amps (common with longer strip installations or multiple strips), step up to 14 or 12 AWG copper wire, which can safely carry 15 to 20 amps. Undersized wire creates voltage drop, meaning LEDs at the far end of a run appear dimmer, and in extreme cases the wire itself can overheat.
Powering LEDs With Batteries
Batteries work well for portable LED projects. Common options include AA batteries (1.5V each, combined in series for higher voltage), 9V batteries, and rechargeable lithium cells (3.7V). The key consideration beyond voltage is runtime.
Runtime is straightforward to estimate: divide your battery’s capacity in milliamp-hours (mAh) by the total current draw in milliamps. A single RGB LED pixel running white at full brightness draws about 60 mA (20 mA for each of its three internal LEDs). On a 2100 mAh rechargeable battery pack, that one pixel would last roughly 35 hours. Scale that up to 10 pixels at 60 mA each and you’re drawing 600 mA, cutting runtime to about 3.5 hours.
A practical trick: most LED projects don’t run at full white brightness the entire time. Animated color patterns, mixed hues, and reduced brightness all lower average current. A reasonable estimate for typical use is about 75% of maximum draw. Ten pixels at 75% average brightness would pull around 450 mA, giving roughly 4.5 hours on that same 2100 mAh pack.
Powering LEDs Through USB
USB ports are a convenient power source for small LED projects, desk lights, and short LED strips. A standard USB 2.0 port delivers 5V at up to 500 mA (2.5 watts). USB 3.0 bumps that to 900 mA (4.5 watts). USB-C without power delivery negotiation provides up to 5V at 3 amps (15 watts), enough to run about a meter of standard-density LED strip.
For larger setups, USB-C with Power Delivery can supply up to 20V at 5 amps (100 watts), but tapping into those higher voltages requires proper PD negotiation circuitry. For most people, a simple USB connection at 5V is best suited to small, low-power LED projects rather than full room lighting.
Wiring LEDs in Series vs. Parallel
When connecting multiple bare LEDs (not pre-wired strips), how you wire them changes what your power supply needs to deliver.
In a series circuit, LEDs are daisy-chained end to end. The same current flows through every LED, and the required voltage is the sum of all forward voltages. Three white LEDs in series need about 10.5V (3.5V × 3) and still draw only 20 mA. Series wiring ensures uniform brightness because every LED gets identical current.
In a parallel circuit, each LED connects independently across the same voltage. Every LED sees the full supply voltage, but the total current is the sum of all individual LED currents. Three white LEDs in parallel need only 3.5V, but draw 60 mA total (20 mA × 3). Parallel wiring requires a lower voltage source but demands more current, and ideally each parallel branch should have its own resistor to prevent current imbalances.
For most DIY projects with a handful of LEDs, series wiring is simpler and more reliable. Use parallel when your supply voltage isn’t high enough to stack more LEDs in series.
Dimming Your LEDs
There are two ways to dim LEDs. Analog dimming simply reduces the current flowing through the LED. It’s straightforward but has a significant drawback: at lower brightness levels, LED color shifts noticeably, and dimming accuracy drops off quickly. Below about 10% brightness, the results become unpredictable.
Pulse-width modulation (PWM) is the preferred method. Instead of reducing current, PWM rapidly switches the LED on and off at full current, varying how long it stays on during each cycle. A 50% duty cycle means the LED is on half the time, producing roughly half the perceived brightness. Because the LED always runs at its rated current when it’s on, there’s no color shift at any brightness level. PWM dimming can achieve dimming ratios of 3000:1 or higher, meaning you can take an LED from full blast down to a barely visible glow with consistent color throughout. Most commercial LED dimmers and smart controllers use PWM.
Heat Management for High-Power LEDs
Small indicator LEDs and standard LED strips produce minimal heat and need no special cooling. But high-power LEDs, defined as those consuming 350 milliwatts or more per emitter, generate significant heat at the junction where light is produced. That heat doesn’t radiate forward with the light. It conducts backward through the LED’s base, and if it has nowhere to go, the LED’s efficiency drops and its lifespan plummets.
For LEDs above about 1 watt, a metal heat sink is necessary. Aluminum is the standard choice. The LED mounts to the heat sink with a thin layer of thermal paste or a thermal pad to ensure good contact. For arrays above 15 watts, use a high-conductivity thermal interface material that keeps thermal resistance below 0.2 K/W across the junction. At higher wattages still (30W and up in enclosed fixtures), an active cooling fan may be needed to keep temperatures in a safe range. If your LEDs are dimming on their own after being on for a while, inadequate heat sinking is almost always the cause.