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))