Resistors Flashcards
Resistor
Definition:
A resistor is an electrical component that limits or regulates the flow of electrical current in a circuit. It is made of material that shows resistance to the motion of charges
Important Attributes:
Resistance Value: Measured in ohms (Ω).
Power Rating: The maximum amount of power it can dissipate, measured in watts (W).
Types: Can be fixed or variable.
Material: Typically made from carbon, metal, or metal oxide.
Examples:
Carbon composition resistor
Metal film resistor
Non-Examples:
Capacitor
Inductor
Fixed Resistor
Definition:
A fixed resistor has a resistance value that does not change.
Important Attributes:
Constant Resistance: Does not vary with voltage or current.
Stability: Provides stable resistance under varying conditions.
Common Use: Used in circuits where a specific
resistance is needed.
Examples:
Carbon film resistor
Metal oxide resistor
Non-Examples:
Variable resistor
Potentiometer
Variable Resistor
Definition:
A variable resistor is an electrical component whose resistance can be adjusted manually.
Important Attributes:
Adjustable Resistance: Resistance value can be changed.
Types: Includes potentiometers and rheostats.
Common Use: Used for tuning and calibration.
Examples:
Volume control knob (potentiometer)
Light dimmer switch (rheostat)
Non-Examples:
Fixed resistor
Diode
Rheostat
Definition:
A rheostat is a type of variable resistor used to control current by manually adjusting the resistance. AKA a current controlling device
Important Attributes:
Current Control: Primarily used to adjust current in a circuit.
Two Terminals: Typically has two terminals.
Construction: Often nichrome wire-wound for higher power ratings, around a ceramic core (Solenoid), and a sliding contact, aka wiper, that adjusts the resistance
Examples:
Light dimmer
Motor speed controller
Non-Examples:
Fixed resistor
Capacitor
Potentiometer
Definition:
A potentiometer is a three-terminal variable resistor that can be used to adjust voltage levels.
Important Attributes:
Voltage Divider: Often used to vary voltage in a circuit.
Three Terminals: One for the input voltage, one for the output voltage, and a third for the ground.
Applications: Used in audio equipment, sensors, and control devices.
Examples:
Volume control on a radio
Adjustable power supply
Non-Examples:
Fixed resistor
Inductor
How to calculate the resistance value of a resistor?
R = (p l)/A
R = resistance
p = specific resistivity of material
l = length of material
A = area of cross section of material
What happens when you exceed a resistor’s power rating?
If the power rating is exceeded for too long a period of time, the resistor will reach a point where it is no longer able to efficiently dissipate the heat and it will burn out.
What are carbon composite resistors made of?
A mixture of conductive carbon and insulating clay (ceramic).
What are carbon film resistors made of?
the consist of a ceramic core, which is coated in a thin layer of carbon. The resistance value is controlled by cutting a helical groove into the thin layer of carbon
What are metal film resistors made of?
A ceramic core coated with a thin layer of metal film, the resistance value is controlled by cutting a helical groove into the thin metal film layer
What are wire wound resistors made of?
Nichrome wire wrapped around a ceramic core. The resistance value is controlled by the amount of coils.
Where are wire-wound resistors most often used?
In high heat environments. The casing is often made form material that protects the core from external heat.
In situations where a high power rating is required. The casing of some wire-wound resistors can efficiently dissipate heat
What are SMD (surface mounted device) resistors made of?
A thin piece of resistive material on top of a ceramic base. The resistive value is controlled by cutting a groove into the resistive material to reduce it’s surface size
Pre-set resistor
Definition:
A preset resistor is a small adjustable resistor used to set precise values in a circuit, typically adjusted only once or infrequently.
Important Attributes:
Precision Adjustment: Used for fine-tuning circuit parameters.
Compact Size: Typically small and mounted on circuit boards.
Application: Used in calibration and trimming.
Examples:
Trimmer potentiometer
Adjustable resistor on a PCB
Non-Examples:
Standard potentiometer
Fixed resistor
Ohm’s law
Describes the relationship between voltage, current, and resistance
Ohm’s Law states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them.
Formula: V= IR
Definition:
A joule is the SI unit of energy, defined as the amount of work done when a force of one newton displaces an object by one meter.
Important Attributes:
Energy Measurement: Measures work, energy, or heat.
Equivalence: 1 Joule = 1 Newton-meter.
Applications: Used in physics, engineering, and everyday energy measurements.
Examples:
Energy consumed by a 1-watt device in one second
Heat produced by a resistor in a circuit
Non-Examples:
Volt (unit of electric potential)
Ampere (unit of electric current)
Power
Definition:
Power is the rate at which work is done or energy is transferred in a unit of time, measured in watts (W).
Important Attributes:
Formula: P = VI (Power = Voltage x Current) or P = I^2R (Power = Current squared x Resistance).
Measurement: Indicates how quickly energy is used or transferred.
Applications: Used to specify the power rating of devices and systems.
Examples:
The power rating of a light bulb (e.g., 60 watts)
Power output of an electric motor
Non-Examples:
Energy (which is the capacity to do work)
Resistance (which opposes the flow of electric current)
How to calculate the total resistance in a series circuit?
RT = R1 + R2 + R3
How to calculate the total current in series circuit?
It = Vt / Rt
How to calculate the total voltage in a series circuit?
Vt = It x Rt
Vt = V1 + V2 + V3 (total voltage is equal to the sum of the voltage drops across resistors)
How to calculate the voltage drop across a single resistor in a series circuit
V1 = It * R1
(It because the current stays the same in a series circuit)
How to calculate resistance of resistors connected in parallel?
1/Rt = 1/R1 + 1/R2
What happens to voltage across resistors connected in parallel?
The voltage stays the same for each resistor.
How to calculate the current for resistors in parallel circuit?
The voltage remains the same for each resistor so we divide the total voltage by each resistor
I1 = Vt / R1
I2 = Vt / R2
It = I1 + I2
It = Vt / Rt
Temperature coefficient of resistance
The temperature coefficient of resistance (TCR) is a parameter that describes how the electrical resistance of a material changes with temperature. It is usually denoted by the Greek letter alpha (α) and is expressed in units of per degree Celsius (°C⁻¹).
Important Attributes:
Positive or Negative:
Positive TCR: For most conductors, resistance increases with an increase in temperature (e.g., metals).
Negative TCR: For some materials like semiconductors, resistance decreases with an increase in temperature.
Linear Approximation: Over a limited temperature range, the change in resistance can often be approximated linearly.
Material Specific: The TCR value is different for different materials, indicating their sensitivity to temperature changes.
Formula: The resistance R(T) at temperature
𝑇 can be calculated using the formula:
R(T)=Ro^ [1+α(T−To)]
Applications: Important in designing circuits where temperature changes are expected, and precise resistance is crucial (e.g., in precision resistors, temperature sensors).
Examples:
Copper (Positive TCR): Copper has a positive temperature coefficient, so its resistance increases with temperature. It has a TCR of about +0.00393 °C⁻¹.
Silicon (Negative TCR): Silicon, a semiconductor, has a negative temperature coefficient at higher temperatures, meaning its resistance decreases as temperature increases.
Platinum RTD (Resistance Temperature Detector): Used in temperature sensing with a known positive TCR.
Non-Examples:
Materials with negligible TCR: Some materials like certain alloys (e.g., manganin, constantan) have a very low TCR, meaning their resistance is relatively stable over a range of temperatures.
Capacitors: Do not primarily depend on resistance changes with temperature.
Inductors: While they might have temperature effects, their primary function is not defined by resistance changes with temperature.
