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ADN8831 evaluation board: Maintaining single set point temperature of 20°C using AN-695

Question asked by shaun_92 on Dec 29, 2016
Latest reply on Jan 9, 2017 by ytu

Hi,

I have recently purchased the ADN8831 evaluation board. My intention is to drive the TEC to cool to a single set point temperature of 20°C. The board was set with the following specifications:

Thigh = 25°C

Tmid = 20°C, Rmid = R1 = 12.49kΩ (obtained from thermistor table)

Tlow = 15°C, Rlow = 15.71kΩ

Therefore, R1 = 12.49kΩ, R2 = 0, R3 = 60.94kΩ, Vtempset = 1.25V (from equation 4), Vilimc = 2.25V (from equation 5), Vilimh = 0.5V (from equation 6) and Vlim = 0.8V.

 1) Using the single point temperature method on page 5 of AN-695, I could not get the green LED to illuminate constantly. This is because the temp is always fluctuating from 13°C - 24°C, while the ambience temperature is around 25°C. In addition, from the TEC current and voltage, we observed the TEC to be oscillating between the heating and cooling modes.

As we are trying to cool the TEC and obtain a lower temperature (than the ambience temperature) of 20°C, is there a reason why the TEC would heat and Vtempout does not lock on to Vtempset (20°C)?

 

2) Next, in order to stop the TEC from heating, Vilimh was decreased from default 0.5V to 0.025V (near 0V). However, the result is still the same and the TEC oscillates between heating and cooling modes.

How can we limit or prevent the TEC from heating?

3) We tried further by adjusting the PID for temp compensation and CD, RD RI, RP and CI was adjusted from the default to 10μF, 12.4kΩ, 61.9kΩ, 1MΩ and 47μF respectively. Even though the PID tuning resulted in a reduced temperature fluctuation (between 20°C-21.1°C), the TEC does not achieve a constant temperature.

We have attached the Vtempset and Vtempout voltage readings:

 Vtempset and Vtempout voltage readings

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