Thermoelectric Cooling Solutions

## Application Tips

Thermoelectric cooler (TEC) is a semiconductor device which is characterized by unique heat pump performance with high efficiency, compactness and durability. The unique ability of TECs to heat and cool, permits them to lower temperature of an object below ambient as well as to stabilize temperature of objects in widely varying ambient conditions.

Here we mention some application tips that can help in optimal use of TECs.

## PID CONTROLLER Close loop DC control is the best solution for TEC operation. It provides most effective use of TECs.

In practice two control methods are in use:

• Proportional-Integral (PI) Control
• Proportional-Integral-Derivative (PID) Control

For most applications PI Control looks is enough. But RMT recommends to use most complex PID control loops.

Usually there is not so much difference in hardware of such two types (PI and PID), only one additional coefficient - a derivative circuit to the PI controller which improves the transient response of such control.

Effectiveness of PID/PI controls depends on correct adjustments of parameters of the Loop:

• Kp, the proportional constant, controls the loop gain.
• Ki, the integral constant, offsets errors.
• Kd, the derivative constant, affects the damping rate.

Calculation of the parameters requires knowing or to examining some specific properties of TEC. One of the methods - Ziegler-Nichols tuning approach.

Process gain dT/dU (or dT/dI) Response of the TEC to applied power
Time constant τ Time constant of controlled system
Dead time τD Delay of the TEC system response

By these parameters it is easy to calculate parameters of both PI or PID controls:

Control Method Kp Ki Kd
PI 0.9 K 3.3 τD
PID 1.2 K 2 τD 0.5 τD

where K = time constant / (process gain × deadtime) .

In many cases tuning of the PID parameters is done by trials and errors. It is quite long way and requires experience. There are high-end TEC Controllers which provide auto-tuning functions. Even with PID loop parameters obtained this way, final fine tuning is very useful.

Knowledge of object of the control (TEC) is very important for such working. Rough estimations on the basis of the knowledge make PID tuning easier and more effective:

Process gain

Roughly estimated value of process gain correlates with maximal performance parameters of a TEC. There are TEc maximal parameters DTmax and Imax or Umax. Thus   