FAQ1007

Answer

A number of factors influence the total control range of a temperature controller, and all these must be considered when you design and specify a thermal control system.

1. The sensitivity and range of the sensor affects the total range and temperature control accuracy

• • Thermistors have higher noise rejection than other sensors and operate over a -60°C to 300°C range. They are the most sensitive temperature sensor with extremely fast response time. The close tolerances (0.1-0.2°C) guarantees interchangeability. Thermistors are specified by their resistance at 25°C: Wavelength offers 5 kΩ, 10 kΩ, 20 kΩ, 50 kΩ, and 100 kΩ thermistors. For the best temperature controller performance you should match the thermistor to your particular application requirements.

  • RTDs operate over a wider range than thermistors, usually between -200°C and 273°C, but are not as accurate. When compared to thermistors, they also have a factor of 10 less signal around ambient temperature.

  • Linear sensors (eg. LM335 and AD590) operate over a wide range -25°C to 125°C, but are not as accurate as thermistors.

2. The load characteristics affect the total temperature range

• • The amount of heat that needs to be dissipated if the load is active and generating heat may limit the lowest temperature achievable.

  • The total thermal mass of the load will limit how far its temperature can be changed from ambient temperature.

3. The thermoelectric cooler characteristics; typically a single-stage thermoelectric cooler can sustain about a 60ºC differential, and two stage TECs can sustain a ~75°C differential.

4. If the current limit of the temperature controller is set below the current capacity of the thermoelectric cooler, it can limit the temperature controller range.

5. The ambient temperature and heat-sink capacity:

• • Ambient temperature is the temperature of the air or environmental conditions surrounding the load. Ambient can and possibly will change when the load is activated, usually raising the temperature.

  • The heatsink capacity is the amount of heat that can be dissipated from a load using a heatsink. The heatsink must be sized to meet the heating and cooling requirements of the load, and its capacity is directly affected by the ambient temperature. If the ambient temperature is too high, the heatsink may be unable to cool the load sufficiently. A controlled decrease in temperature actually can result in a temperature increase, a condition called thermal runaway, which often leads to the destruction of the load.  More information on thermal runaway can be found on the Temperature Controller Basics page.

6. The sensor voltage at the desired operating temperature must be within the feedback range of the controller. For example, if the desired operating temperature is -10°C, a 10 kΩ thermistor is chosen (55340 Ω at -10°C), and the sensor bias current is 10 µA then, using the equation V=IR, the sensor voltage is 10 µA x 55340 = 0.553 V. This is usually within the feedback range of the controller. If a 100 µA bias current is used, however, then the sensor voltage will be 5.53 V and it will likely be out of range. A minimum voltage of 0.25 V guarantees our stability specifications.

7. The sensor voltage at the desired operating temperature must be within the setpoint range of the controller. For example, with a temperature controller powered by a +5 V power supply, the setpoint range is usually limited to 0-3 V. If the setpoint needs to be 4 V, then a higher voltage power supply may be required.

Designing and specifying a thermal management system is not necessarily difficult, but to make sure it will perform as you need it to you need to keep these things in mind. Wavelength Electronics has more information available in the reference documents TN-TC01: Optimizing Thermoelectric Control Systems and AN-TC09: Specifying Thermoelectric Coolers.