LED Circuit - Multiple LEDs

Multiple LEDs

Strings of multiple LEDs are normally connected in series. In one configuration, the source voltage may be greater than or equal to the sum of the individual LED voltages; typically the LED voltages add up to around two-thirds of the supply voltage. A single current-limiting resistor may be used for each string.

The other configuration is to run the sum of the supply voltage at approximately 75 – 85% of the combined LED voltages. This uses the LEDs' combined inherent resistance to stop thermal run-away. While small voltage drops to each LED generally make no discernible loss of intensity or brightness, with sufficient LEDs in series a noticeable drop in brightness begins to show with enough LEDs in the circuit. However, in using the inherent resistance of LEDs as current limiters in their own right, the gradual diminution of brightness is offset with the increased number of LEDs emitting a lower level of light. In assuming that the supply voltage is 12 V, and each LED is 3 V, by using a string of either five or six LEDs whose combined voltage is 15 V or 18 V, they effectively begin to be under-driven, and thus have an almost infinite life span and very high reliability. The LEDs in the five-LED or six-LED strings will not be as bright as the 4-LED string running on 12 V, but there are more LEDs running at the lower brightness, with no power losses in terms of waste heat from resistors, and this configuration eliminates the need for complex driver circuitry.

Parallel operation is also possible but can be more problematic. Parallel LEDs must have closely matched forward voltages (V_f) in order to have similar branch currents and, therefore, similar light output. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel.