The heat transfer constitutes an important physical unit for resistor design. The heat transfer coefficient a indicates how much power in Watt [W] per surface area unit A= cm², m², and at a temperature increase of 1 Kelvin [K] can be transferred to the surrounding medium. a depends on the structure of the surface (smooth, […]

The thermal resistance Rth [K/W] of a resistor indicates by how much the temperature rises when subjected to a load of 1 W. A resistor with Rth = 1,5 K/W is subjected to 200 W and heats by 300 K, i.e. at an ambient temperature of 35 °C it will assume a temperature of 335 […]

The thermal time constant T of a resistor indicates how fast its temperature may rise under certain operating conditions. After 1T, it has reached about 67% of the final temperature and after 5T it has more or less reached its final temperature. The time constant is dependent on the resistor type and mass, on the […]

The temperature coefficient TC (symbol [α] = K-1) of the electrical resistance indicates the change of electrical resistance of a (resistor) conductor under the influence of heat. The change of the resistance value per kelvin [1/K] is the temperature coefficient. It is often also indicated in ppm (parts per million). In the case of metals, […]

Sheet resistors use sheets that are punched in a wave pattern or provided with slots to increase the current distance and create a defined resistance value. With elements of the same size, different punching or slotting can create a large number of different resistance values that form a system and can be freely combined. Given […]

The specific heat, symbol [c] = J ∙ g-1 ∙ K-1 is a measure for the energy storing capacity of a substance. It indicates which amounts of energy applied in J (joule, 1 J = 1 Ws = 1 kgm) will raise the temperature of a substance with the mass 1 g (gram) by 1 […]

Depending on their design and application, resistors must be protected against contact with live components or be installed accordingly. Resistors with direct cooling are manufactured in the protection class IP00 for installation or as guarded designs with IP20 to IP23. In all cases, sufficient cooling air feed must be ensured thus requiring sufficiently dimensioned inlet […]

Slip designates the difference between synchronous rotating magnetic field und and the rotor in a non-synchronous induction motor: The rotor attempts to follow the stator rotating magnetic field but can never reach it due to its velocity of circulation, which would then be equivalent to a rotor frequency of fL = f∙s = 0 and […]

Like the squirrel cage motor, the slip ring motor resembles a three-phase transformer with rotatable winding. Unlike the former, the ends of the windings are brought out to a set of slip rings with current collectors = brushes. These external rotor connections allow introducing resistors into the rotor circuit. The resistors limit the starting current […]

Ohm’s Law and the SI unit ohm are named in honor of Georg Simon Ohm (German physicist, 1787-1854). Ohm’s Law describes the relationship between current, voltage and resistance in an electric circuit. A conductor exhibits a resistance R of 1 ohm if, when applying a current U of 1 Volt, a current I of 1 […]