Chapter 4: Resistance

Siemens (S)

how conductance is measured. It is a reciprocal obtained by dividing the number into one. R=1/G G=1/R

Potentiometer (pot)

A variable resistor used to control voltage

Rheostat

A variable resistor used to control current i

Three types of resistors: Carbon composition Wirewound Film Resistor

Carbon composition: Were the most widely used. Wirewound: Come in many different styles, more accurate than CC resistors Film Resistor: The same sizes as carbon composition but with the accuracy of wire round.

Resistors

Components maanufactured to possess a specific value of resistance to the flow of current.

Reading a resistor's language code

Example: RN60D5112F The resistor value is the primary concern. The value of the resistor may be indicated by three to five digits. In all cases, the last digit indi- cates the number of zeros to be added to the pre- ceding digits. . In the example given, the last digit (2) indicates the number of zeros to be added to the first three digits (511). So 5112 translates to 51,100 ohms.

Resistance (R)

Opposition to the flow of current. Is measured in ohms.

Conductance (G)

The ability of a material to pass electrons.Is measured in Siemens.

Mho ∪

The unit of conductance (ohm spelled backwords).

Reading a resistor's color code

There is a color code associated with resistors. The first band, closest to the end of the resistor, represents the first digit of the resistor value. The second band represents the second digit of the resistor value. The third band represents the number of zeros to be added to the first two digits. The fourth band represents the tolerance of the resistor. A resistor may have a fifth band (Figure 4-15). This band indicates the reliability of the resistor. It tells how many of the resistors (per thousand) will fail after 100 hours of operation.

Variable resistors

Variable resistors allow the resistance to vary.

Series- parallel

a combination of a series circuit and a parallel circuit. Most circuits can be broken down to a simple par- allel or series circuit. The procedure is as follows: 1. Calculate the parallel portion of the circuit first to determine the equivalent resistance. 2. If there are series components within the parallel portion of the circuit, determine the equivalent resistance for the series compo- nents first. 3. After the equivalent resistance is deter- mined, redraw the circuit, substituting the equivalent resistance for the parallel por- tion of the circuit. 4. Do final calculations.

Series circuit

provides a single path for current flow. The more resistors connected in series, the more opposition there is to current flow. The more opposition there is to current flow, the higher the resistance in the circuit. Total resistance in series circuit is R(f)=R(1)+R(2)...R(x)

Parallel circuit

provides two or more paths for current flow. Each current path in a parallel circuit is called a branch. The more resistors are connected in parallel, the less opposition there is to current flow. The less opposition there is to current flow, the lower the resistance in the circuit. The total resistance in a parallel circuit is given by the formula: 1/R(f)=1/R(1)+1/R(2)...1/R(x)

Three types of resistive circuits

series circuit, the parallel circuit, and the series-parallel circuit (

Tolerance

the amount that the resistor may vary and still be acceptable.

Ohm

the resistance of a circuit or circuit element that permits a steady current flow of one ampere (one coulomb per second) when one volt is applied to the circuit.