Electrical connection network

Generally Polymer based heaters are powered by DC power source due to the use of batteries. The power supply will force a current to flow through the knitted heating elements resulting in generation of heat. The knitted heating elements are powered by two bus bars connected to the powers supply and for efficient heating, the voltage drop in the bus bars has to be very low. Hence, high conductive (very low resistance) yarn was utilised for knitting the bus bars. The selection of conductive yarn is important. Although there are different kinds of conductive yarns commercially available two different types of conductive yarns were used in the research to knit the bus bars:

1. Shieldex; a nylon yarn plaited with a thin layer of silver, produced by Statex GmbH; 2. Copper thread, which is produced by multiple strands of fine copper wires.

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Figure 5.6.5.1: Knitted heater elements produced with FabRoc, PE and conductive yarns

As stated earlier Statex yarn is produced by twisting 34 PA (nylon) filament fibres with a thin layer of silver together. The electrical resistance of Statex yarn would be influenced by the number PA-Ag filaments and the thickness of the silver layer. However, the thickness of the silver layer that is plaited onto the surface of a PA filament would impact on the overall flexibility of the Statex yarn. The Statex yarn used in the research could easily be processed in flat-bed knitting machine. However, it is important to use a yarn with a low electrical resistance as this would affect the voltage drop of the bus bars. The ambition of the research is to design a textile fabric with enhanced moisture management by creating a temperature gradient within the fabric with the help of localised heating elements. Once developed such a fabric could be used to produce outdoor garments, and this would require the design of heating elements of low overall resistance in order to use low voltage batteries. Connections from the power supply to the heater element, which is produced with FabRoc yarn, are achieved with two courses knitted from either Statex or copper yarn at either end of the heater element, named as bus bars in Figure 5.6.5.1. Care have to be taken to avoid contact between the two bus bars to prevent an electrical short circuit. As knitting more courses of Statex or copper yarn to create a bus bar should reduce the total resistance and, hence improve the

Knitted FabRoc yarn

Silver coated yarn busbars

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conductivity of current conductivity. Therefore, this was studied in detail by producing samples with 10cm long bus bars consisting of different number of courses (Figure 5.6.5.2). Resistance of each bus bar was measured individually by using an Agilent 34410A precision multimeter. The procedure is summarised as follows;

1. Sample preparation: this involved measuring the bus bar length to precisely 10cm by using a meter rule and then press studs were placed at these ends. Press studs also act the measuring points as they provide reliable hard part for the multimeter connectors.

2. Multimeter preparation: settings for measuring resistance were selected on the menu and connections for the measuring leads properly connected on the device. 3. Measuring: the two connecting leads were touched (with reasonable pressure) against the two press studs placed at 10cm apart on each bus bar and the corresponding reading was recorded. This step was repeated 5 times to ensure reliable recorded results. The same procedure was performed for the remaining bus bars for both copper and Statex samples.

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Figure 5.6.5.2: Statex yarn bus bar courses

The study showed (Figure 5.6.5.3) that in the case of Statex the resistance dropped significantly when the number of conductive courses were increased to three and four, and a significant change in resistance was not observed beyond four conductive courses.

1 knitted conductive course

2 knitted conductive courses

3 knitted conductive courses

4 knitted conductive courses

5 knitted conductive courses

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Figure 5.6.5.3: The influence of the number of Statex courses on the electrical resistance of a bus bar

The above experiment was repeated with bus bars knitted with copper wire see figure 5.6.5.4 below; 0 100 200 300 400 500 600 0 1 2 3 4 5 6 7 R esi st a n ce [ O h m s]

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Figure 5.6.5.4: Copper yarn bus bar courses

Copper is widely used in electrical and electronic industry due to its efficient and reliable conductivity and due low resistance. Knitting copper yarn is a challenge, especially due its low breaking strength and very little stretch (high modulus). The copper yarn used has been produced by twisting eight copper strands of 25µm diameter, see Table 5.6.2.1. For further details. The results for bus bars produced with copper yarn is given below in figure 5.6.5.5 below

1 knitted conductive course

2 knitted conductive courses

3 knitted conductive courses

4 knitted conductive courses

5 knitted conductive courses

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Figure 5.6.5.5: The influence of the number of copper courses on the electrical resistance of a bus bar

The study of the bus bars made from Statex yarn and copper produced interesting results; generally they all showed that the more bus bars used the less the electrical resistance becomes. This shows that more electrical channels are created by addition of knitted courses and this would reduce resultant resistance of the bus bar. Statex yarn had higher resistance than copper per unit length. From this copper yarn showed less resistance change after just 2 bus bars compared to 3 for Statex yarn.