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# Heatsink temperature monitor circuit

2015-01-23 09:05
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This heatsink temperature monitor circuit signals via 3 LEDs when the temperature exceeds 2 boundary levels. When the heatsink temperature is below 50 … 60oC (122 … 140oF) the greed LED lights. The yellow one signals that the temperature is within 60 and 70oC (140 … 158oF). The red one signals that the temperature has reached beyond 70 … 80o(158 … 176oF). At this point a relay can be triggered to break the power supply or otherwise protect the electronic device by turning ON a cooling fan.

As shown in the circuit diagram the heatsink monitor is made of 2 opamps that are set as window comparators. The input voltage level is set by the thermosensor IC LM335. This voltage level increses linearly by a factor of 10mV/oC. If this voltage is below the voltage level set by P1 and P2, the opamp outputs are almost at ground potential (0V) and the green LED lights up.

To set the desired temperature levels: put the thermosensor LM355 together with a thermometer into a pot of water which is being heated slowly. Turn P1 to its lowest point and turn P2 to its maximum. The switching point from green LED to yellow (50 … 60oC) is set with P1. The switching point from yellow to red is set withh P2. Watch thermometer while the water is being heated and adjust the potentiometers accordingly. Once the desired values are set, attach the thermosensor to the heatsink.

## Heatsink Monitor circuit diagram

Sent by Popovici Irai, PL.

Heatsink temperature monitor components values
R1 = 6.8K
R2 = R3 = 8.2k
R4 = R5 = R8 = 680
R6 = R7 = 2.7k
R9 = 10k
P1 = P2 = 500
D1 = Green
D2 = Yellow
D3 = Red
D4 = 1.2 – 3.3 V / 0.25W
D5 = 1N4001
T1 = 2SC734
A1, A2 = Any OpAmp IC operating at 12V