Ch 3 - Resistance

wire table

standardization of wire sizing for consumers

ductility

the ability of a substance to be drawn, pulled, or extruded through a small opening to produce a wire

malleability

the ability of a substance to be hammered or beaten into thin sheets

ohms

units of measure for resistance

rheostat

variable resistor, two-terminal device

How are surface mount resistors coded?

- Color - standard electrical color coding - 3-symbol - 3 numbers, first two for mantissa, last one for power of 10 for the multiplier - 2-symbol - Letter/number, letter corresponds to a fixed value, and number is the power of 10 for the multiplier

What are the four factors for resistance?

- material - length - cross-sectional area - temperature

What is superconductivity used for?

- power transmission - MRIs - Maglev trains - computers - electromagnets - generators and motors

mil

1/1000th inch

What is the resistance between terminal a and terminal c of a potentiometer at any given time?

100% of the potential resistance of the device.

What is the PPM/Celcius specificiation?

A material's rating for change in resistance over temperature, in parts per million per degree Celcius; for resistors

variable resistor

A resistor whose value can be varied between its minimum and maximum values.

potentiometer

A switch that can provide variable motion control. It can vary the resistance within the switch, which affects both the current and voltage flowing out of the switch. Three-terminal device

thermistor

A two-terminal semiconductor resistor that changes its resistance with a change of temperature. Can be NTC or PTC

varistor

A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground. Does not limit current

How is max voltage rating used when selecting resistors?

Alongside the wattage rating; used to estimate the current that could flow through the resistor based on specifications

AWG

American Wire Gauge; a standardization based on wire diameter.

What can we infer about a material with a high α20 value?

The material is more sensitive to changes in temperature, and therefore its resistance is affected more greatly.

circular mils

a circular mil is a wire with diameter 1 mil

superconductor

a material that has almost zero resistance when it is cooled to low temperatures

photoconductive cell

a two-terminal semiconductor device whose terminal resistance is determined by the intensity of incident light, always with negative coefficient

temperature coefficient of resistivity

an empirical quantity, denoted by α, which describes the change in resistance or resistivity of a material with temperature, can be used to calculate resistance at a specific temperature

Cooper effect

electrons pair up when in a superconductive state, helps to maintain high velocity with minimal energy loss; like drafting in cycling.

inferred absolute temperature equation, given two points

(abs(Ti) + T1)/R1 = (abs(Ti) + T2)/R2 set up an equality between two points, solve for Ti

A (circular mils)

(d(mils))^2; diameter (mils) squared

What are ohmmeters used for?

1. Measure the resistance of individual or combined elements 2. Detect open circuit (high-resistance) and short circuit (low-resistance) situations 3. Check network connection continuity; if it's 0 ohms, you're good 4. Test some semiconductor (electronic) devices

square mil to circular mil

A = (pi(d)^2)/4 = pi/4(1 mil)^2 = pi/4 sq mil == 1 CM

How does temperature affect insulators in terms of resistance?

As temperature increases, resistance decreases, negative temperature coefficient.

How does temperature affect semiconductors in terms of resistance?

As temperature increases, resistance decreases, negative temperature coefficient.

How does temperature affect conductors in terms of resistance?

As temperature increases, resistance increases, positive temperature coefficient.

How is a resistor's PPM/deg C rating applied?

Change R = (R(nom)/10^6)(PPM)(T - 20 C); Change R = change in resistance R(nom) = nominal resistance at 20 C PPM = PPM/deg C rating of resistor

What is the conductance formula?

G = 1/R siemens (S)

What is the advanced conductance formula?

G = A/ρ*l; G = siemens (S) A = cross-sectional area, CM ρ = resistivity constant for material l = length of material

Resistor Schematic Symbol

Marked by an R sometimes, jagged line

General Resistance Equation

R = ρ(l/A) R = resistance ρ = resistivity l = length A = cross-sectional area

How do you calculate resistance (R1) at a temperature (T1) given standard resistance and the temperature coefficient of resistivity for the material?

R1 = R20(1 + α20(T1 - 20 C)), where R20 is the resistance at 20 C and α20 is the constant at 20 C.

What is the full resistance formula with all four factors?

Resistance = (ρ(1/A))(1 + α20(T - 20 C)); ρ: resistivity of material A: cross sectional area (CM) α20: temperature coefficient of resistivity for material T - 20 C: change in temperature from standard

fixed resistor

Resistors that have a value that cannot be changed.

What does resistor size indicate?

The amount of power (as heat) dissipation the resistor is capable of

absolute zero

The temperature at which no more energy can be removed from matter; 0 Kelvin

How does a potentiometer work?

When the sliding element of the potentiometer moves (b), the R(ab) and R(bc) values change, depending on which terminals are rigged up. As the b terminal moves towards the unused terminal, the resistance increases, and vice versa. R(ac) = R(ab) + R(bc) at all times

memristor

memory resistor - capable of increasing or decreasing resistance based on direction of current flow; change in resistance is maintained (remembered) after the excitation is removed. Possible use in RAM or other computer components.

resistance

opposition to the flow of charge

conductance

reciprocal of resistance, siemens (s)

Inferred Absolute Temperature (Ti)

the linear approximation on a resistance/temperature graph of a particular material; where the graph reaches the x-axis (0 <= x <= inf)

critical temperature

the temperature above which a conductor loses superconductivity and returns to normal conduction

What are the units for α, the temperature coefficient of resistivity?

α20 = ((R1 - R20)/T1 - 20 C))/R20 :: Change R/Change T/R20 :: Ohms/Celcius/Ohms

Conversion factor from standard to metric for Resistance

ρ(ohm-cm) = (1.662E-7)*(ρ(CM-ohm/ft))