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Questions and Answers
What is the primary function of a potentiometer?
What is the primary function of a potentiometer?
- Changing the resistance to adjust current (correct)
- Voltage regulation only
- Enhancing electrical power
- Acting as a fixed resistor
How many terminals does a potentiometer have?
How many terminals does a potentiometer have?
- Five terminals
- Four terminals
- Three terminals (correct)
- Two terminals
What type of device is a rheostat?
What type of device is a rheostat?
- A digital resistor
- An amplifier
- A variable resistor with two terminals (correct)
- A three-terminal device
What is one common use of a potentiometer?
What is one common use of a potentiometer?
Which material can potentiometers and rheostats be constructed from?
Which material can potentiometers and rheostats be constructed from?
What does the suffix letter 'K' indicate when coding for resistors?
What does the suffix letter 'K' indicate when coding for resistors?
Which tolerance corresponds to the letter 'C' in resistor coding?
Which tolerance corresponds to the letter 'C' in resistor coding?
How are Surface Mount Resistors characterized in their design?
How are Surface Mount Resistors characterized in their design?
What does the letter 'M' represent in resistor coding for tolerance?
What does the letter 'M' represent in resistor coding for tolerance?
What are the metal terminals on Surface Mount Resistors used for?
What are the metal terminals on Surface Mount Resistors used for?
What is the highest tolerance percentage represented by the code letter 'G'?
What is the highest tolerance percentage represented by the code letter 'G'?
Which letter is used to represent a tolerance of 5% in resistor coding?
Which letter is used to represent a tolerance of 5% in resistor coding?
What is a common feature of both Surface Mount Resistors and axial type resistors?
What is a common feature of both Surface Mount Resistors and axial type resistors?
What does resistance oppose in an electrical circuit?
What does resistance oppose in an electrical circuit?
What is the formula for calculating heat dissipated by a resistor?
What is the formula for calculating heat dissipated by a resistor?
Which resistor can safely dissipate more heat?
Which resistor can safely dissipate more heat?
What does the maximum current through a resistor depend on?
What does the maximum current through a resistor depend on?
What is tolerance in regards to resistors?
What is tolerance in regards to resistors?
For a 1000 ohm resistor with a tolerance of ±10%, what could its actual value be?
For a 1000 ohm resistor with a tolerance of ±10%, what could its actual value be?
Where is the resistance value and power rating typically found on larger power resistors?
Where is the resistance value and power rating typically found on larger power resistors?
Which of the following indicates high precision in resistors?
Which of the following indicates high precision in resistors?
What is the primary function of resistors in a circuit?
What is the primary function of resistors in a circuit?
Which of the following is NOT a type of resistor?
Which of the following is NOT a type of resistor?
How are carbon composite resistors primarily constructed?
How are carbon composite resistors primarily constructed?
What is a defining feature of metal film resistors compared to carbon film resistors?
What is a defining feature of metal film resistors compared to carbon film resistors?
What determines the resistance value of wire wound resistors?
What determines the resistance value of wire wound resistors?
What does the power rating of a resistor indicate?
What does the power rating of a resistor indicate?
Which of the following resistors typically has a higher power rating?
Which of the following resistors typically has a higher power rating?
What is a common characteristic of carbon film resistors?
What is a common characteristic of carbon film resistors?
What does the third digit in a three-digit resistor code represent?
What does the third digit in a three-digit resistor code represent?
What does a resistor code of '390' indicate?
What does a resistor code of '390' indicate?
Which of the following is an example of a temperature dependent resistor?
Which of the following is an example of a temperature dependent resistor?
When subjected to excess voltage, what happens to a voltage dependent resistor's resistance?
When subjected to excess voltage, what happens to a voltage dependent resistor's resistance?
What is the purpose of a positive temperature coefficient (PTC) resistor?
What is the purpose of a positive temperature coefficient (PTC) resistor?
What does a resistor with markings '000' signify?
What does a resistor with markings '000' signify?
In complete darkness, a light dependent resistor (LDR) has a resistance in the region of how many ohms?
In complete darkness, a light dependent resistor (LDR) has a resistance in the region of how many ohms?
Which type of resistor is commonly used to control street lights?
Which type of resistor is commonly used to control street lights?
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Study Notes
Resistors
- Resistors limit the amount of current flowing or drop voltage in a circuit.
- Resistors fall into three groups: fixed linear resistors, non-linear resistors, and variable resistors.
Fixed Linear Resistors
- Carbon composite resistors are made by molding carbon and insulating materials into a cylindrical shape.
- They offer mechanical and electrical protection.
- Resistance is varied by changing the mixture of carbon and insulating material.
- Carbon film resistors are made by depositing a thin layer of resistive material on a ceramic body and cutting it to form a helix or spiral.
- Carbon film resistors have a lower tolerance than carbon composite resistors but cost more.
- Metal film resistors are made by spraying a thin film of metal onto a ceramic body until the desired resistance is achieved.
- Metal film resistors have a lower power rating than carbon composite resistors.
- Wire wound resistors are made by wrapping a length of wire around a ceramic body and adding terminals for connection.
- They are low value resistors and tend to be high current and power resistors.
- They are quite large in size.
Power Rating of Resistors
- The power rating is the amount of power a resistor can dissipate as heat before burning out.
- You can calculate the heat dissipated using: I² * R * t or (V² / R) * t
- Larger resistors can dissipate more heat than smaller resistors.
- The maximum current a resistor can handle without overheating is: I = P / R where:
- P = Power rating of the resistor in watts.
- R = Resistance value of the resistor in ohms.
Tolerance
- Tolerance is the deviation or error from the specified value.
- For example, a 1000-ohm resistor with a tolerance of ±10% could have an actual value between 900 and 1100 ohms.
- Higher precision requires resistors with a lower tolerance.
Resistor Values
- The resistance value, tolerance, and power rating are printed on the resistor body.
- British Standard BS 1852:
- "K" = thousands or kilohms.
- "M" = millions or megaohms.
- "R" = multiplier is equal to or less than 1.
- Be careful not to confuse the resistance letter k for kilohms with the tolerance letter K for 10% tolerance.
- Be careful not to confuse the resistance letter M for Megaohms with the tolerance letter M for 20% tolerance.
Surface Mount Resistors (SMD Resistors)
- Surface mount resistors are small, rectangular shaped metal oxide film resistors that are soldered directly onto a circuit board.
- SMD resistors are highly accurate, up to 0.1% tolerance.
- SMD resistors are marked with 3 or 4 digit numerical codes:
- 3-digit code: The first two digits are the first two numbers of the resistive value, and the third digit is the multiplier (x1, x10, x100, etc.).
- For example: "103" = 10 × 1,000 ohms = 10 kΩ
- Surface mount resistors with values less than 100Ω’s are written as: “390”, “470”, “560” with the final zero representing a 10 xo multiplier, which is equivalent to 1.
- For example: "390" = 39 × 1Ω = 39Ω´s or 39RΩ
- "000" or "0000" markings indicate zero-ohm (0Ω) resistors or shorting links.
Non-Linear Resistors
- The resistance of non-linear resistors is determined by an outside influence, such as light, voltage, or heat.
- There are three types: temperature dependent resistors (TDR), voltage dependent resistors (VDR), and light dependent resistors (LDR).
Temperature Dependent Resistors (TDR)
- NTC = Negative temperature coefficient. Resistance decreases with temperature.
- PTC = Positive temperature coefficient. Resistance increases with temperature.
- Thermistors are PTC resistors used for temperature sensing in electric motors and recording instruments.
Voltage Dependent Resistors (VDR)
- These resisters have falling resistance when an excessive voltage occurs.
Light Dependent Resistors (LDR)
- The amount of light falling on an LDR governs its resistance.
- Greater the light, the lower the resistance.
- Resistance in complete darkness is around megaohms, and in bright light reduces to ohms.
- They are used in street light control and alarm systems.
Variable Resistors
- Variable resistors are user-adjustable resistors.
- There are two main types: potentiometers and rheostats.
Potentiometers
- Have three terminals: one connection to the rotating arm and the other two to each end of the resistor.
- Can be constructed from wire or carbon.
- Resistance is varied by moving the wiper along the resistor.
- Uses: voltage tapping, current limiting, and volume control.
Rheostats
- Have two terminals: one terminal via the rotating arm and the other to one end of the resistor.
- Can be constructed from wire or carbon.
- Operation is similar to potentiometers but with a focus on current limiting.
Review Questions
- The physical size of a resistor indicates how much heat can be dissipated in the form of heat.
- The tolerance of a resistor tells you the amount the resistor could deviate from the specified value.
- The most common type of fixed linear resistor is the carbon composite resistor.
- A thermistor has a positive temperature coefficient.
- A rheostat is a two-terminal device.
- A potentiometer is a three-terminal device.
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