Thermal Indicator Circuit for Heatsinks

The post details how to build a simple 3 LED thermal indicator circuit for heatsinks and other similar applications to quickly understand the level of heat on the selected surface through a sequential 3 LED panel.

The temperature of a heatsink can be measured with a wet finger: if it sizzles, the temperature is too high. The circuit in figure 1 is an alternative method of checking that does not cause blisters. A green ‘safe’ LED lights as long as the temperature of the heat· sink does not exceed 50° Centigrade, an orange ’caution’ LED for temperatures of 500 . . . 750 Centigrade and a red ‘danger’ LED for temperatures above 75° Centigrade. The proposed thermal indicator circuit is simplicity itself, two special zener diodes, D1 and D2, are connected in series to ensure an accurate zener voltage of 5.96V at 25° Centigrade. The zener voltage will rise by 20 mV for each degree Centigrade rise in temperature. The voltage level corresponding to the temperature of the heat·sink is compared with two reference voltages by ICT and lC2. When the temperature reaches 50° Centigrade the output of lC2 goes high so T3 conducts and causes D4 to light and at the same time D5 is extinguished by T4. At or above 75° Centigrade the output of lC1 is high and T2 and T3 then conduct to make D3 light and D4 extinguish. Under normal conditions, that is, considering a heatsink of sufficient cooling area, a temperature of 75° Centigrade will never be reached. Figure 2 shows the Pd/PO (max) relation between the power dissipation of a class B amplifier and its effective power output PO/PO (max) under normal conditions. The drive signal is sine wave. The effect of the quiescent current on the dissipation has been ignored.

Parts List
Resistors:
R1 = 22 k
R2 = 5k6
R3,R12 = 820
R4 = 220
R5 = 180
R6 = 470
R7 = 4k7
R8,R9,R10,R11 = 15 k

Semiconductors:

D1 ,D2 = L|\/l335 (National Semiconductor)
D3 = LED red 5 mm
D4 = LED orange 5 mm
D5 = LED green 5 mm
T1,T2,T3,T4 = BC 54713
IC1,lC2= 3140

Thermal Indicator Circuit for Heatsinks