This circuit allows the trip point and the rate of reduction to be controlled. A stable reference is set using either a Zener diode or a voltage reference. For higher supply voltages, a 6.2 V Zener with its near zero temperature coefficient is suitable. For lower voltages, use a 2.5 V reference such as the Zetex ZR285-2.5. The PNP transistor is connected as a voltage follower. This tracks the voltage of the voltage divider set up by the R2 and the NTC thermistor. The R2 is used to control the temperature at which the thermal control starts to operate. For voltages above 1.25 on the base of the transistor, the transistor simply does not turn on and negative temperatures have no affect on the current in the LED. When the set temperature is reached, the rate at which the current is reduced is set by the beta value of the thermistor. The thermistor manufacturer’s data sheet will show you how to calculate the resistance at different temperatures using the beta value. Any PNP transistor can be used, but for the best results a low saturation device such as the Zetex FMMTL717 are suitable. Also note the change in emitter voltage on the PNP. This is typically -2.4 mV per degree centigrade which may need to be taken into account if the control circuit is close to the LED.