# Old-School Analog Temperature Control Circuits Solved with Modern LTSpice Thermistor Dynamic Models, Part one

Part one: NE555 Maintains Dynamically SPICE-Generated System Temperature Within the Limits

A few months back, a dynamic voltage-controlled thermistor SPICE model was presented on planet analog, A Multi-Simulator NTC Thermistor SPICE Model With Temperature Driven By a Voltage

With the help of this model, we intend to publish a series of technical notes about the LTSpice modelling of several old-school temperature control analog circuits. Now this might not seem that innovative, but the groundbreaking aspect is that the temperature generated by these circuits is going to be modelled in a live, time-dependent way. In these circuits, starting with the ON / OFF control and going to the more sophisticated PID temperature control, the temperature of a system (room / oven / fridge) is generated in the form of a simple circuit, and will then be sensed dynamically by a thermistor and regulated by analog devices like timers or analog PID controllers.

The first example in Figure 1 involves a classical heater based on a 555 timer, as found in Electro Schematics 1

Figure 1

A classical heater based on a 555 timer (Image courtesy of Electro Schematics)

Now we are going to complete this circuit with a heated system (Figure 2) behaving like a couple of capacitors / resistors, and whose temperature V(Tsystem) must be regulated. Thus we apply to this system the equivalent electrical power generated by the load of Figure 1 (an analog behavior V=F() source), partly dissipated to an ambient temperature (a pulse source or a piecewise linear, as we are going to evaluate the influence of variation of the ambient temperature on the temperature control of our system). And to add a bit more spice to the modelling, this ambient temperature will present some noise, modelled here with a PWL source with file (a piecewise linear text file where some noise can be generated).

Figure 2

A heated system whose temperature V(Tsystem) must be regulated. We apply to this system the equivalent electrical power generated by the load of Figure 1

We are thus ready to build up our LTSpice simulation, adding the 555 circuit and the voltage- / temperature-driven thermistor to our new system to control its temperature. The complete circuit and the LTSpice directives are presented in Figure 3.

Figure 3

The complete circuit and the LTSpice directives

Among the parameters declared, we have the heating element value Rheat, dT to induce variations in the ambient temperature, and the different times t1 to t4 representing the moment in time where these variations will be applied. This particular simulation intends to see if LTSpice describes the well-known circuit behavior realistically, and we will emphasize the effect of the ambient temperature (see the lower pane in Figure 5) on the heating capability of the circuit.

Note that the voltage at the Tsystem node is linked to the input of the temperature of the NTC thermistor U3. We have completed the loop of the temperature control and are ready to simulate.

Here is the complete result of the transient simulation with four panes.

Figure 4

The current flowing through the heating element (upper) and the difference V(Trig)-VCC/3 (lower) of the 555 timer

Figure 4 represents the current flowing through the heating element (upper) and the difference V(Trig)-VCC/3 (lower) of the 555 timer.

Figure 5

The variation in time of the system and NTC thermistor temperatures (upper) and the ambient temperature

Figure 5 represents the variation in time of the system and NTC thermistor temperatures (upper) and the ambient temperature. The results can be synthesized as follows.

The input of this simulation is the file describing the ambient temperature variation (with 1o C random noise) going from 18o C during the 90 s range, to 8o C, then -2o C, and at last going back to 18o C (lower pane of Figure 5).

The temperature regulation of the system is performed first at an ambient temperature of 18o C, and the heating element keeps the system temperature swinging between 21.4o C and 23o C (upper pane of Figure 5 upper) during the early 90s range. The current flows through the heating element during the periods where the triac is on (upper pane of Figure 4), which is during the time span where V(trig)-VCC/3 > 0 (lower pane of Figure 4). V(trig) is directly linked to the voltage on the thermistor, which follows the system temperature with some delay defined by the response time {tau} (R9).

As the ambient temperature is declining to 8o C, the system temperature cycle changes slightly, but we see that the duty cycle of the triac increases. More power is needed to keep the same low temperature as the ambient temperature.

Up to this point, everything is going as planned.

When the ambient temperature decreases to -2o C, the duty time of the triac grows to 100 %, meaning that the maximum power is delivered. However, this is not sufficient in this case to keep the system temperature within the normal range, and the system temperature drops, followed by that of the NTC thermistor. Only when ambient temperature goes back to 18o C at the end of the simulation do we see that temperature cycling takes places again.

We conclude that we need to decrease the value of the resistance of the heating element to give more power. The simulation with Rheat = 100 Ω is reprinted in Figure 6. We see now that the desired system temperature swing is preserved, whatever the ambient temperature.

Figure 6

We needed to decrease the value of the resistance of the heating element to give more power. The simulation with Rheat = 100 Ω

In conclusion, the LTSpice modelling of our voltage-driven thermistor has now been successfully integrated as a driver of a full circuit controlling the temperature control of a system. The simulations are clear with vivid colors, the results logical, and the visualization of the waves helps a lot to understand the circuit behavior.

In the coming months, a new article will be presented on the subject of LTSpice modelling a PID controller using a voltage-driven thermistor and providing precise temperature control. We will take this opportunity to emphasize the influence of the electrical parameter tolerances of the NTC thermistor on the accuracy of temperature regulation.

References:

1 Marian, P. “555 Temperature Controller Circuit.” Electro Schematics . AspenCore, (March 2013). Web. January 2017.

## 2 comments on “Old-School Analog Temperature Control Circuits Solved with Modern LTSpice Thermistor Dynamic Models, Part one”

1. rorybronx
March 30, 2017

## What are the odds that the temperature control won't become uncontrolable at the end? I believe that's the most crucial question when  it comes to protection.

2. Alain Stas
March 30, 2017

This control circuit can only heat with a defined power given by the power source and the heating element value. When the duty cycle is 100%, the “regulated” temperature can only be followed if the loss of heat is not too high . here this happens as the outside temperature begins to be  too low. In the opposite direction if the ambient outside temperature would be too high, the heating system would completely stop. And the system temperature would end up as the ambient temperature.

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