The circuit above is used with high power leds from half watt to 1-2 watts. This circuit prevents an led from overheating when lighting up for a long period of time. This circuit has a thermistor which measure the temperature and the npn transistor would control the amount of current flow to the led. The thermistor would be placed behind the led or on the heat-sink of the led.This circuit works by limiting the current depending on the temperature. When the thermistor is cold, the base of the transistor is referenced to VCC, but as the thermistor heats up, the thermistor decreases in resistance and the base slowly connects to ground. The more the thermistor heats up, the transistor's base is connected more to ground causing the transistor to increase in resistance and limit more current. As the thermistor cools, the transistors becomes more conductive letting more current through to the led.
