LED-current limiter accepts ac or dc

Roger Griswold, Micrel, San Jose, CA; Edited by Martin Rowe and Fran Granville -July 14, 2011

LED drivers have lots of features and require plenty of external components. When your application requires no PWM (pulse-width-modulated) dimming or controlled frequency operation, your primary concern may be that too much current could damage or destroy your LEDs. In this case, you can make a simple LED-current limiter from a common low-dropout linear regulator. The circuit in Figure 1 is an LED light bulb for a landscape-lighting system. Landscape lighting typically operates from 12V ac, and peak voltage is approximately 17V. Because the regulator is in series with the LED string, the LED-string current equals the regulator’s output current.

LED-current limiter accepts ac or dc figure 1The circuit uses reasonably priced, 150-mA, warm-white LEDs; low-cost rectifier diodes; and Micrel’s 2.5V MIC5209-2.5YS regulator (Figure 1). The regulator must source at least the required LED current and handle the peak input voltage minus the drop across two of the four rectifier diodes and the drop across the LEDs. Selecting a regulator with the lowest possible output and dropout voltages lets LED current flow for a larger portion of each ac cycle, and it reduces the power requirement of current-setting resistor R1. As output and dropout voltages decrease, cost increases. The regulator sees the peak voltage at approximately 5.1V and dissipates approximately 0.2W.

The MIC5209-2.5YS’ output voltage regulates to 2.5V between its output and ground. R1 sets the LED-string current using R1=(2.5/ ILED), where ILED is the LED-string current. With a value of 16.9Ω for R1, the string current is 148 mA. The circuit has slightly more than 2.5W peak dissipation. With an ac input, the current flows only about half the time, so the average power dissipation is approximately 1.26W.

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Design Ideas
You can easily modify the circuit to accept almost any input voltage. Simply change the number of LEDs and make sure that the rectifier diodes can handle the reverse voltage. Add or subtract one LED for each 3.33V increase or decrease in peak input voltage. Don’t use LEDs for the rectifier diodes to get more light output because LEDs don’t have sufficient reverse-breakdown voltage and will fail. The input bridge accepts either ac or dc and negates the need to worry about the polarity of a dc input.

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