Column: Circuit drill
The impact of a low voltage source on a simple temperature switch
By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions S
ince transistors are semiconductor devices, their performance varies with ambient temperature, which enables the design of temperature switches
using simple transistor circuits. The higher the temperature of the transformer, the more current passes between the emitter and collector pins of the transistor. Figure 1 shows a simple temperature
switch circuit designed to provide 9Vdc output voltage when the temperature of the Q1 transistor exceeds 116.7o
C.
This circuit’s operation depends on its transistor’s parameter changes with ambient temperature; see Figure 2 for output voltage vs. Q1 temperature. The circuit’s output voltage drops from 9Vdc to just 260mVdc when the temperature exceeds 116.7o
C. The question to consider is: What
happens if the voltage source drops below the nominal 9Vdc? This is a legitimate scenario, since most temperature switch circuits are battery- operated. Will this drop in supply voltage impact the performance of such a switch, perhaps rendering the circuit useless? And, what is the magnitude of the impact? These questions and more will be
answered with the following experiment.
The setup The method used in this experiment centres around reducing the supply voltage level, starting at 9Vdc down
case, the circuit can still detect the high temperature.
The question to consider is:
What happens if the voltage source drops below the nominal 9Vdc? This is a legitimate scenario, since most temperature switch circuits are battery- operated
to 5Vdc and measuring the circuit’s response. Since the circuit is designed to flag temperatures of 116.8o
C, its response
is closely monitored for temperatures between 100o
C and 130o C. The plotted
curves are then compared to determine the effects; see Figure 3. By superimposing all the circuit’s
output voltages at various power supply voltage levels, we notice that: 1. The circuit’s output voltage drops when the power supply voltage drops. However, the circuit detection of high temperatures is still reliable, and the circuit performs reasonably well. This impact is acceptable if the load connected to the circuit output can operate at lower voltage. In such a
10 September 2022
www.electronicsworld.co.uk
2. The trigger temperature changes at various power supply voltage levels. As shown in Figure 3, the circuit drops its output voltage at 116.7o
C,
when the supply voltage is 9Vdc. In comparison, trigger temperature is 117.2o
C, when the supply voltage is
8Vdc. This means that the circuit becomes slightly less sensitive to temperature with a drop in supply voltage. The same observation is made with
further drops in supply voltage. The relationship between the supply voltage and the trigger temperature is shown in Figure 4. We can therefore conclude that
despite the simplicity of the transistor temperature switch, the circuit shows reasonable stability against drops in power supply voltage. As the supply voltage drops from 9Vdc to 5Vdc, the circuit can still trigger output voltage at high-temperature points. The experiment also shows that the
circuit’s trigger temperature shifts slightly when the supply voltage drops below 9Vdc. The shift is noticed at a rate of 0.5o
C for every 1Vdc drop in
supply voltage. Such a circuit is therefore ideal
for battery operation since there the power supply’s voltage level is expected to drop below the design’s nominal voltage. The trigger temperature increments are small, so the circuit can be considered reasonably stable.
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