Resistance to Voltage

The basic element of an ohm meter is a circuit that can take a resistance and convert it into a voltage. This is most simply realized by using a 

Simulator File name (5 kHz sine wave) Overshoot (V) Run time (s) 

A simple modification to this circuit alters the range of resistance to be detected. Changing R3 alters the range of the resistance that can 

The zener diode used for this simulation is used in its soft region, which means that it is used below its recommended test current. This region is often not defined in data books, and may not be correctly 

TABLE 6.5 1N4728 Soft Zener Voltage Comparison 

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Figure 6.63 Resistance-to-voltage response using soft zener.

These results show that all three of the SPICE simulators were not equipped to handle simulations of this particular zener diode in its soft region. However, zener diodes are not well defined or tested in this region of operation, so it may just be that the circuit tested had a worse than average zener diode in it. To further explore this problem, the experiment was modified to put more current than the specified test current in the zener diode. The circuit with these modifications made is shown in Fig. 6.64. The measurements across the zener diode are shown in Table 6.6, and the the resistance-to-voltage response is contained in Fig. 6.65. 

Figure 6.65 Response resistance-to-voltage converter using hard zener.

Odling William (1829-1921) Brit, chem., did early research on problems of valence and bounding, proposed the table of elements ( Outlines of Chemistry 1869) Ohm Georg Simon (1789-1854) Ger. phys., found (Ohm s) Law relating resistance to voltage and current strength, studied temperature resistance of metals, unit of electrical resistance named after him... 

There are several methods to determine and compare the resistance to partial discharges. Some tests are done on finished cables, such as the U-bend test, and others are done on laboratory samples molded from the insulation, that are subjected to partial discharges created by sharp objects, such as needles under high voltages. The tests compare either the energy required or the length of time required to erode or fail (short circuit) samples of similar thickness. 

Tests on Cable Constructions. The Association of Edison Illumination Companies (AEIC) has approved an accelerated cable hfe test in which typical underground distribution power cables can be statistically compared based on their resistance to water treeing (number of days to fail). The comparison can be made by varying the type of insulation and/or other cable layers in an environment that contains hot water (90°C) under 8V/fi (200 V/mil) voltage stresses (four times the typical power cables operating voltages). 

Asbestos-Based Grade. Grade MM is more resistant to heat than Grade C but not recommended for primary insulation at any voltage. It exhibits small dimensional changes when exposed to moisture. 

The winding insulation resistance to be used for comparison with the recommended minimum value (/, ) is the observed insulation resistance obtained by applying a d.c. voltage to the entire windings for one minute, corrected to 40°C. In practice, motors having insulation resistance readings as high as ten to a hundred times the minimum recommended value R , are not uncommon. 

These transformers may be PVC taped, thermoplastic (polypropylene) moulded, fibreglass taped, polyester resin cast or epoxy resin cast depending upon the system voltage and the surroundings. HT indoor transformers, for instance, are generally polyester or epoxy resin cast, and are economical with good dielectric properties. They are resistant to humid, chemically contaminated and hazardous areas. Outdoor HT transformers, how-ever. may be epoxy... 

The safe step voltage, j, should not be more than the total resistance to ground through the body, f 2isb (Section 22.10.1) X safe body current, 4, as a function of time, where... 

The resistance to ground should be sufficiently small to prevent spark ignition at the maximum anticipated charging current to the system. This can be achieved by ensuring either that the energy stored is less than the MIE or that the minimum ignition voltage cannot be attained (A-4-1.3). The necessary resistance depends not only on the flammable mixture but also on the electrical circuit. 

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