Heatsinks are a necessary inclusion in circuits which are rated to work with high wattage and high current supplies. The heatsink is installed on devices so that it draws the excess heat from the power devices, preventing their malfunction.
The convention is to use the maximum allowed temperature for the silicon chip as the reference for the rating of the heat sink. However, Tthis is not a very reliable or the recommended method.
The heatsink indicator circuit here monitors the heatsink temperature constantly. The green LED remains on when the temperature is below 50-60°C.
Above 50-60°C till 70-80°C, the yellow or orange LED stays lit and above that temperature, the red LED lights up. You can also add a relay to disconnect the load when it heats up.
The basic design of the heatsink temperature monitor employs a window comparator. The control voltage is provided by sensor D1 and it rises 10mV per 1°C rise in temperature.
When the sensor voltage is below that the wipers potentials of P1 and P2, op amps A1 and A2 have low outputs and D2 lights up.
If the voltage across D1 is greater than that at the wiper of P1 but less than that at the wiper of P2, A2 has a high output, causing D2 to turn off and D3 to light up.
If the sensor voltage goes above that at the wiper of P2 too, both op amps register high outputs, causing D5 and transistor T1 to be turned on. Zener diode D4 is used to make sure that D5 still lights brightly and T1 conducts hard enough.
How to Calibrate
Calibration of the heatsink monitor circuit is done by putting the sensor in a tray of water along with a thermometer. Then heat the tray. P1 is set to the minimum resistance and P2 to the maximum.
The crossover point from green to yellow is set between 50-60°C with P1 and that from yellow to red is set between 70-80°C with P2. Then install the sensor into the heat sink permanently.