LED Driver and LED Strip Power Supply Calculator

Two ways to feed LEDs

LEDs are powered in two ways, and the choice of driver depends on which one applies.

• Constant voltage: LED strips and ready made modules run from a fixed voltage such as 12 V or 24 V. Resistors on the strip limit the current. The supply must match the strip voltage and be rated above the power the strip draws.
• Constant current: power LEDs are wired in series and driven at a fixed current such as 350 mA or 700 mA. The driver current must equal the LED current, and its output voltage range must include the total string voltage.

Power supply for an LED strip

P = power per metre × length, P_min = P × (1 + headroom), I = P / V

Five metres of 14.4 W/m strip draws 72 W. With 20 percent headroom you need at least 86.4 W, so a 100 W supply is chosen. On a 24 V strip the current is 72 / 24 = 3 A.

Why add headroom?

Running a supply at full load all the time makes it hotter and shortens its life, so an allowance is added on top of the load. Vendor guides quote anything from 10 to 30 percent. The real yardstick is the load limit in the driver's own data sheet and its derating curve against ambient temperature.

Constant current drivers

V_string = n × V_f, P = V_string × I

Ten LEDs with a 3.1 V forward voltage in series make a 31 V string. At 350 mA the power is 31 × 0.35 = 10.85 W. The driver must have a 350 mA output and an output voltage range that includes 31 V.

How many LEDs per driver?

The driver label states its output voltage range. A driver that works from 25 to 42 V can run between 9 and 14 LEDs with a 3 V forward voltage in series. The third tab gives these limits.

Limits

The tool covers power, current and voltage. Voltage drop along long strips, dimming compatibility, ingress protection and thermal conditions are assessed separately. For the series resistor of a single LED use the LED resistor calculator.