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A New Trilogy of LTSpice Circuits With NTC Thermistors, PART II: LTSpice Simulation of PID Temperature Control of a Peltier Thermoelectric Element With a NTC Thermistor

Although it is common knowledge that their efficiency is poor, Peltier thermoelectric elements are considered the most versatile cooling elements; they are used in a wide range of applications, such as laser diode cooling systems or portable fridge coolers, as well as automotive thermoelectric generators.

The relatively recent emergence of SPICE models realistically describing this element type1,2 makes it possible to integrate them into temperature control circuit SPICE simulations. This article describes a straightforward current driving circuit for the cooling of such a Peltier element, with the cold mass temperature measured by a NTC thermistor.

Before digging into the total application, I would like to focus a bit on the sensor device that makes the temperature control possible in the first place; in this case, it’s an NTCALUG03A ring tongue sensor, one of the smallest devices on the market. The picture in Figure 1 compares it to a more common larger device. The SPICE electro-thermal model for the NTCALUG was also made available very recently4 .

The first very simple simulation I will present is the transient behavior of the NTCALUG03 sensor when screwed / pressed onto a metal plate (as presented in Figure 1).

Figure 1

The circuit of the NTCALUG03 macro-model is presented in Figure 2a, where the NTC is placed in a classic voltage divider. Figure 2b shows the transient temperature evolution, where the plate on which the ring is screwed or pressed is going through a series of temperature changes. We see the short latency reaction time just after the changes, and at steady state see the gradient between the plate and thermistor element.

Figure 2a

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Figure 2b

In yellow: the temperature of the plate surface; in white: the response of the NTCALUG (pressed against the plate at time = 0 s)

In yellow: the temperature of the plate surface; in white: the response of the NTCALUG (pressed against the plate at time = 0 s)

Now that we have presented the temperature sensor, which will be screwed onto the cold mass of the Peltier, let’s present the control circuit. There are numerous IC micro controllers on the market that are able to drive thermoelectric elements, but for the purpose of this SPICE simulation, I’m going to re-use the full analog one that was used in a previous article3 for three reasons: it’s proven to be very efficient, you should never change a winning device, and I already have its full SPICE model available, with all the flexibility of possible modifications. The whole schematic is presented in Figure 3.

Figure 3

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Some remarks about this circuit:

  • The thermoelectric Peltier element sub-circuit (freely adapted from the one in Reference 1) has five pins: C or Tcold, which is the temperature of the thermal mass and will have to follow the set temperature profile as closely as possible (fixed by V3, node Vtset); H or Thot, which is the temperature of the heat sink; Tamb, which is the ambient temperature input, programmable with V7 (in this simulation it will remain constant at 20 o C); I is the current input; and GND is the ground
  • The current is provided by a transistor pair commanded by the PID
  • In the Wheatstone temperature sensing bridge there is an adjustable resistor Radj, which will be adapted to precisely correct the sensor temperature gradient between the ring tongue and the surface. This will keep the thermoelectric thermal mass at the set temperature going progressively from 20 o C) to 5 o C), which is the final steady-state operating temperature
  • A fail-safe opamp U11 detects the eventual short circuits on the sensor, which will be programmed for this simulation with the help of an “evil” switch MYSW. This U11 opamp, when activated, will then immediately switch off all current on the PID

The next simulation sweeps the value of Radj in order to reach nominality of the cold steady-state temperature. The results are shown in Figure 4.

Figure 4

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For different Radj values, the cold temperature (in yellow in the lower pane) will closely follow the set temperature profile (lower pane in white). The difference between the two is represented on the higher pane.

Extracting some measurements from the SPICE log file, we can derive the Radj value needed to reach a cold temperature exactly equal to 5 o C) (for the steady state between 1,500 s and 3,000 s).

The second simulation will use the intermittent short circuits of the temperature sensor visualized on the higher pane in Figure 5. The lower pane represents the thermal behavior following these short circuits. In the absence of a fail-safe, the short circuit is perceived as a hot temperature, and the Peltier element is going to end up freezing at -6 o C). With a fail-safe, however, as soon as the first short occurs, the current is removed and the Peltier element returns to the ambient temperature.

Figure 5

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Drawing a conclusion from this simulation of a full analog temperature control circuit of a Peltier element, we can state that the results make sense and they are likely to give practical orientation for design engineers. All the PID parameters, the Peltier characteristics, and NTC sensor design parameters can be tuned, including the thermal mass cold resistance and hot heat sink resistance, ambient temperature, PID constants, and the passive thermistor / resistor tolerances. Optimization is thus possible and our theoretical results present an ideal precursor for further real experiments.

To see a demo video of this simulation, please visit this site.

As usual, the simulations presented in this article are available on request at this e-mail address: edesign.ntc@vishay.com.

References:

  1. Improved SPICE Modeling and Analysis of a Thermoelectric Module
  2. Chakib Alaoui in International Journal of Engineering (IJE), Volume (5) : Issue (1) : 2011
  3. Old-School Analog Temperature Control Circuits Solved with Modern LT spice Thermistor Dynamic Models, Part 2
  4. LTSPICE

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