LEDs, or light emitting diodes, are low voltage, direct-current lights that require special power supplies to optimize their light output and life span. Standard low-voltage transformers don't meet the power supply needs of LEDs unless they are modified with additional rectifiers and control electronics. When standard low voltage transformers supply power to LEDs directly, the LEDs will flicker, vary in brightness and may rapidly lose luminosity. LED drivers include these special components, and their power output matches the requirements of the LEDs they supply.
DC Versus AC
Many LEDs light up when supplied with power from low voltage transformers. Since they are diodes in addition to emitting light, they rectify AC current, especially when connected in a bridge arrangement. As diodes, they conduct for half the wave of an AC supply, while an LED connected in the opposite direction in a bridge configuration conducts the other half wave. Each LED receives power for only half of an AC cycle and is dark for the other half, leading to flicker. LED drivers rectify the AC current and provide DC power that will not cause an LED to flicker.
LEDs are very sensitive to changes in voltage. When the voltage across the LED is too high, it draws too much current, reducing its lifespan. If the voltage is too low, the LED draws too little current and gives off less light.
Low voltage transformers have a fixed internal resistance. The resistance of the LEDs connected to the transformer changes with age and with the operating temperature of the LED. Different LEDs may have different resistances. When the transformer supplies these changing resistances from its fixed internal resistance, the voltage it delivers to the LEDs changes, and the LEDs no longer operate optimally. LED drivers supply a controlled voltage to ensure that the LEDs always operate in their optimum range.
Low voltage transformers supply different levels of current when the line voltage on the input side fluctuates or when there are power surges. LEDs are very sensitive to overcurrents and can rapidly overheat and burn out during current fluctuations. LED drivers regulate the current to make sure it stays below the danger values for the LED. Even when input voltage levels rise during a surge, the LED driver keeps the current through the LED constant.
Low voltage transformers can't dim LEDs. For normal lights, a transformer can supply a reduced voltage through a variable resistance, delivering less power to a light. For LEDs, even a slight reduction in the voltage leads to a large reduction in light output. The LED brightness can't be controlled accurately with low voltage transformers.
LED drivers dim LEDs by delivering the full, regulated current in short pulses. For full light, the pulses run together for continuous current. For dimming, the pulses become shorter to reduce light output. The LED driver generates the pulses at a frequency high enough so that the LEDs don't seem to flicker.