Resistance and Electrical Resistors: Part I

Electrical resistance is a measure of the electrical voltage required to cause a certain electrical current to flow through an electrical conductor (component, circuit). Uniform quantities should be used or instantaneous values for variables that change over time. If the voltage is counted from a connection point A to a connection point B, the amperage in the conductor is counted positively as it flows from A to B; the resistance cannot be negative. In this field Ohm’s law is the most well known law. A resistor is a bipolar passive electrical component. It realizes ohmic resistance in electrical and electronic circuits. We are mainly discussing around direct current (DC).

In the case of alternating current (AC), it must be noted that the instantaneous values of the voltage and current change periodically. At the ohmic resistor, the proportionality between voltage and current exists not only for equal quantities, but also for instantaneous values at the respective time under consideration. Various complications can arise if we can swap AC component and DC component, for example household AC switches. In the case of all other electrical components, even those grouped together as linear resistors, the relationships between the instantaneous values of voltage and current are time-dependent. The mathematical treatment with the equations for is very complex. For this reason, the complex alternating current calculation has been developed for calculations where 110v, 220v, 440v etc supply voltage are taken in to consideration.

An electrical resistor is an ohmic resistor if its value is independent of the voltage, the strength of the current and any parameters. Ohm’s law applies to such resistance. Approximately and with limitations, an ohmic resistance can be realized by means of a component, in the simplest case a metal wire. When a voltage drop occurs due to the current in the resistor, electrical energy is converted into thermal energy.

Resistors are used, for example, to:

1. to limit the electric current
2. to divide the electric current in a circuit
3. convert the electric current into a voltage in order to measure it (indirectly)
4. to divide the electrical voltage in a circuit
5. convert electrical energy into thermal energy

Main Applications of Resistors

• Setting or limiting an electric current at a given electrical voltage (series resistor)
• Setting a specific electrical voltage at a given electric current (shunt)
• Dividing an electrical voltage in a certain ratio (voltage divider). For this purpose, at least two or more resistors are connected in series (series connection).
• Dividing an electric current in a certain ratio (current divider). For this purpose, at least two or more resistors are connected in parallel (parallel connection).
• Generation of a defined level in the event that a high-impedance terminal of an Integrated Logic Circuit is not wired or connected to the rest of the circuit via an electrical idle (pull-up/pull-down resistor)
• Generation of a defined level for setting operating points of active components, e.g. transistor or operational amplifiers
• Conversion of electrical energy into thermal energy (heating resistance) such as in incandescent lamps, soldering irons, replacement loads, fan heaters or resistance brakes
• Creation of defined input and output impedances (impedance matching)

Resistors can be built in such a way that they serve as a backup in the event of overload. They must not ignite in the process. These include specially designed film resistors, but also PTC fuses.

Parameters of Resistors

A linear resistor (which includes all resistors whose resistance value, in contrast to nonlinear resistors, does not depend on any other parameter) sets an electric current proportional to the applied electrical voltage and vice versa. It thus serves as a current-voltage converter or as a voltage-current transformer and cannot merely limit the current like an electrical fuse. In addition to the resistance value, the following other values are characteristic of a resistance:

• Tolerance of resistance value (delivery tolerance)
• Maximum power dissipation
• Maximum surface or film temperature
• Temperature coefficient (TK value)
• Dielectric strength
• Long-term stability (long-term drift) at maximum power dissipation or nominal power over the service life
• Processing stability (soldering drift if the processing involves a soldering process)
• Parasitic inductance (lower with low-induction resistors)
• Parasitic Capacity
• Current noise (the current noise not only increases with the resistance value, but is also material and voltage dependent (μV/V))
• Impulse load capacity (short-term overloadability), maximum peak factor with periodically varying load from alternating current or periodic pulses
• Voltage dependence of the resistance value (important for high-impedance measuring resistors)

Continued to part 2 of this series.