Design of LED current limiter accepting AC or DC

LED drivers have many functions and require quite a few external components. When the application does not require PWM (pulse width modulation) dimming or controlled frequency operation, the main concern may be that too much current may damage or destroy the LED. In this case, a common low-dropout linear regulator can be used to make a simple LED current limiter. The circuit in Figure 1 is an LED bulb used in a landscape lighting system. Landscape lighting typically uses 12V AC with a peak voltage of about 17V. Since the regulator is in series with the LED string, the current in the LED string is equal to the output current of the regulator.

The circuit uses reasonably priced 150mA warm-white LEDs, inexpensive rectifier diodes, and a 2.5V MIC52092.5YS regulator from Micrel (Figure 1). The regulator must be able to provide at least the required LED current, and withstand a voltage equal to the peak input voltage minus the voltage drop across two of the four rectifiers, minus the voltage drop across the LED. Choosing a regulator with the lowest possible output voltage allows the LED current to flow over a larger portion of each AC cycle, reducing the power required by the current-setting resistor R1. As the output voltage and dropout voltage decrease, the cost increases. The peak voltage of the regulator is about 5.1V, and the power consumption is about 0.2W.

Figure 1. A low-cost voltage regulator is the core of this LED current limiter.

The output voltage of the MIC52092.5YS is regulated at 2.5V between the output and ground. R1 sets the current of the LED string, the formula is R1=(2.5/ILED), where ILED is the current of the LED string. When the value of R1 is 16.9Ω, the LED string current is 148mA. The power consumption of the circuit is slightly more than 2.5W. When using AC input, the current flows only half of the time, so the average power consumption is about 1.26W.

The circuit can be easily modified to accept almost any input voltage. Just change the number of LEDs and make sure the rectifier can withstand the reverse voltage. Adding or removing a 3.33V LED will increase or decrease the peak input voltage. Do not use LEDs as rectifiers to get more light output because the LEDs will not have enough reverse breakdown voltage and will fail. The input bridge accepts AC or DC input, no need to worry about the polarity of the DC input.