Running a series circuit helps to provide the same amount of current to each LED. This means each LED in the circuit will be the same brightness and will not allow a single LED to hog more current than another. When each LED is receiving the same current it helps eliminate issues like thermal runaway.

When you wire in series, you add the forward voltages of each LED in the circuit but the current fed to each LED remains the same. If you have 3 LED COBs, each with a forward voltage of 36V at a given current, when you wire them in series, the total voltage drop of the circuit becomes 108 Volts.

An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly. The resistor value, R is given by: R = (VS - VL) / I R = resistor value in ohms (). V S = supply voltage. V L = LED voltage (2V, or 4V for blue and white LEDs).

Strings of multiple LEDs are normally connected in series. In one configuration, the source voltage must 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.