Interfacing Hardware

There are several methods to measure temperature from a thermistor.

Voltage divider circuit- The simplest way to measure a resistance is using a voltage divider circuit,

also known as an electrical serial circuit. A voltage divider contains two resistors in series, with a voltage tap in between the two resistors (shown in figure 3.13). By using Kirchoff’s voltage law and Ohm’s law equations 3.7 (given below) can be obtained, which can then be used to calculate the resistance of the thermistor (Rt) (Recktenwald, 2013).

𝑅𝑡 = ( 𝑉_𝑖𝑛

𝑉_𝑜𝑢𝑡− 1) 𝑅1 (3.7)

Where Vin is the input voltage coming into the potential divider circuit, Vout is the voltage out and R1 is the constant load resistance.

Wheatstone bridge circuit- When a thermistor is connected to a Wheatstone bridge circuit as

shown in figure 3.14 it provides a thermometer with high accuracy and zero indication on the initial point of the scale. However a Wheatstone bridge has a narrower measuring range (Nawrocki, 2005).

If the Wheatstone bridge is balanced then

𝑅1

𝑅2=

𝑅3

𝑅𝑡 (3.8)

For measuring the temperature the out of balance voltage is measured and the resistance of the thermistor, Rt is determined by using the equation 3.9 below (AMETHERM, 2013).

𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛( 𝑅2

𝑅2+𝑅3−

𝑅𝑡

𝑅1+𝑅𝑡) (3.9)

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Figure 3-14: Wheatstone bridge circuit

The Wheatstone bridge has an advantage over the voltage divider circuit if the change in voltage is very small when compared to the offset voltage. Generally this can be difficult to observe when using a voltage divider circuit without using amplification. In the Wheatstone bridge circuit there is no offset at nominal offset (in practice there is a very small one) hence the saturation problem is greatly reduced or eliminated. Another disadvantage of using a voltage divider is that the initial accuracy without calibration is very limited. Since the output of the voltage divider circuit is a function of the actual values of the resistors it is important to obtain the actual values of resistors. In Wheatstone bridge circuits it depends on matching the resistances rather than absolute precision (Schweber, 2016).

Two-wire and four-wire resistant measurement-

Resistance measurements can be made using a two-wire or a four-wire configuration. It is often made using the two wire method (shown in figure 3.15a), where a test current is sent through the leads and the resistor being measured. Then the same set of leads is used to measure the voltage across the resistor, this is used to calculate the resistance. The main problem when using the two wire method to obtain the resistance occurs when obtaining small resistance measurements (resistances equal to or less than 1 kΩ). The resistance of the leads are added to the measurement, which for small resistance measurements would bring about a significant change in the measured resistance when compared the actual resistance (Janesch, 2013).

Since the two-wire method has its limitations when measuring small resistances a four-wire configuration shown in figure 3.15b is generally used for low resistance measurements. The four

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wire configuration uses two sets of leads to obtain the resistance measurement. The current is sent through one set of leads and the voltage is measured through another set of leads. Even though there might be a small current that flows through the leads where the voltage is being measured this is generally regarded as negligible. This ensures that the voltage measured by the volt meter is approximately equal to the voltage across the resistor. Thereafter the resistance is calculated using the measured voltage and the current sent (Kiethley, 2012).

Figure 3-15: Resistance measurement using a) a two-wire configuration, b) a four-wire configuration (Janesch, 2013)

3.6.1.1 Voltage divider circuit

A Voltage divider circuit (electrical serial circuit) shown in figure 3.13, was chosen as the hardware interphase for the thermistor. This was chosen mainly due to the simplicity of the circuit and fewer number of components, which would prove easier to integrate with a textile. Also the thermistor chosen has a very high resistance of 10 kΩ at 25 °C and the resistance of the copper wire (lead

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resistance) was measured at 15.95+/-0.33 Ω/m (data is given in Appendix 3). Therefore it can be seen that the wire resistance only brings about a minute change (of about 0.16 % if a meter of the copper wire is used as the lead) in the resistance of the thermistor. Hence it can be argued that a two-wire measurement is sufficient for obtaining the temperature off the thermistor. Another advantage of using a thermistor is its large temperature coefficient of about (-2 % to -6 %) therefore there is a large change in resistance when the temperature changes. Hence the change in voltage is large enough to be easily measured using a voltage divider circuit, and a Wheatstone bridge circuit is not required to obtain the measurements (Anonymous, 1995) (Kimball et al., 1993).

The thermistor resistance value was captured using the equation (3.7) given above. A 5 V was used as voltage in (Vin) for the potential divider circuit and a 10 kΩ resistor was used as R1.A 10 kΩ resistor was chosen as the load resistor due to the thermistor resistance at 25 °C being 10 kΩ. This ensured that the voltage was shared equally among the two resistors. The chosen voltage in (Vin) and resistance (R1) would bring about a maximum temperature rise of only 0.2 °C due to self- heating on the thermistor (as shown in figure 3.2d above). This was deemed expectable for this study since in pathologies like diabetic foot ulcer detections temperatures vary by about 4 °F (2.2 ⁰C) (Armstrong et al., 2007).

Once the thermistor resistance (Rt) was calculated, it was substituted in equation 3.1 given in the manufacturer’s data sheet to determine the temperature of the thermistor. The hardware setup shown in figure 3.16 was used to connect the voltage divider circuit to the computer.

Figure 3-16: Hardware setup

A multifunctional NI-DAQ USB 6008 unit from National Instruments was used to provide the analogue voltage Vin for the voltage divider circuit and the corresponding analogue Voltage (Vout)

was measured using the analogue input of the NI-DAQ.