Dozens of and even hundreds of LEDs need to be connected together to form a light emitting device in commercial practices. The connection mode of LED load circuit will directly influence the reliability and life-span of the LED. The LED circuit which can work safely has the following 2 basic functions: to keep the current constant and to maintain low power consumption for the circuit itself. Only in this way can the LED system efficiency stays in a higher level.

1. Series connection: In the series circuit, the positive pole of each LED is connected to the negative pole of another LED in series. The circuit is often configured with resistances in series to restrict the current. The operating current is the same everywhere in the circuit. The voltage provided by the power supply need to be large enough. The formulas of the voltage and the current in the circuit are as below:
VTotal=VF1+VF2+VF3……+VFn
ITotal=IF1=IF2=IF3……

2. Parallel connection: In the parallel circuit, the negative leads of each LED are connected together in parallel, and the positive leads are connected together in parallel also. The voltage for each LED is the same in this circuit, but the total current is the sum of the current of all the LEDs. The forward voltage provided to each LED should be the same to make sure that the operating current for each LED (IF) is consistent. And the current provided by the power supply should be large enough, that is the disadvantage of this circuit. The formulas of the voltage and the current in this kind of circuit are as below:
VTotal=VF1=VF2=VF3……
ITotal=IF1+IF2+IF3……+IFn

3. Series-parallel connection: The greatest advantage for the series-parallel circuit is that the influence of inconsistent LED voltages can be reduced largely. When the connecting way of LED array or the amount of LEDs in the circuit changed, the current restricting resistances should be adjusted in time to control the current flow in the circuit effectively. Of course, in the series-parallel circuit, the output current will change markedly along with the change of input voltage and the environment temperature.