thermal indicator for heat sinks

elektor july/august 1983 thermal indicator for heat-sinks The temperature of a heat-sink can be measured with a wet finger: if it sizzles, the temperature is too high. The circuit in figure 1 is an alterna- tive method of checking that does not cause blisters: a thermal traffic light not unlike the audio traffic light featured in our March1983 issue. A green "safe" LED lights as long as the temperature of the heat- sink does not exceed500Centigrade, an orange "caution"LEDfor tempera tures of 50°...75°Centigrade and a red "danger"LEDfor temperatures above 75°Centigrade. The circuit is simplicity itself, two special zener diodes,DlandD2,are connected in series to ensure an accurate zener voltage of 5.96V at25°Centigrade. The zener voltage will rise by 20mV for each degree Centigrade rise in temperature. The voltage level corresponding to the temperature of the heat-sink is 1 04 Po1 2x LM 335 D2 7-96 21 Pd Po m x 4 = 0.4 z 0.27 78% compared with two reference volt- agesbyICl and1...