One of our products in development includes a heat sink, where a temperature sensor operates a fan and a protective shutdown circuit to control the heat sink temperature. Testing the temperature sensor operation requires measuring the temperature of the heat sink. The temperature measuring probe that is included with the Syscomp DVM-101 multimeter has been indispensable in tracking the temperature of the heat sink while monitoring the operation of the fan and limit protection circuits.
In the process, we discovered something interesting. The circuit temperature sensor is in a TO-92 plastic package, attached to the heat sink. It’s a small mass, so we expected it to track the heat sink temperature fairly closely. But when we measured the temperature of the heat sink with the DVM-101 temperature probe, and compared that to the temperature measured by the circuit sensor, we found the circuit sensor reading is always some fraction (like 80%) of the correct value.
If you think about the thermal conduction model for this arrangement, that result makes sense. The figure shows the thermal equivalent circuit. The thermal resistance of the plastic between the sensor chip and the heat sink, together with the sensor leads to the PC board and the plastic that is exposed to air, all act as a thermal voltage divider. Part of the design process is to determine this ‘divider factor’ and allow for it in the controller operation.
This divider effect is going to appear wherever the temperature sensor is encased in plastic. It might be less evident for something like a thermocouple, where there is very little thermal resistance between the sensor and the source of the heat.
You can find more on thermal modeling in Section 28 on Cooling in our Ebook Analog Circuit Design.