Resistors in Electronics

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Questions and Answers

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?

  • Five terminals
  • Four terminals
  • Three terminals (correct)
  • Two terminals

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?

<p>Voltage tapping (A)</p> Signup and view all the answers

Which material can potentiometers and rheostats be constructed from?

<p>Wire or carbon (D)</p> Signup and view all the answers

What does the suffix letter 'K' indicate when coding for resistors?

<p>Multiplier of 1,000 or kiloohms (A)</p> Signup and view all the answers

Which tolerance corresponds to the letter 'C' in resistor coding?

<p>0.25% (D)</p> Signup and view all the answers

How are Surface Mount Resistors characterized in their design?

<p>They are designed for direct surface soldering. (B)</p> Signup and view all the answers

What does the letter 'M' represent in resistor coding for tolerance?

<p>Tolerance of 20% (B)</p> Signup and view all the answers

What are the metal terminals on Surface Mount Resistors used for?

<p>To allow soldering to circuit boards (C)</p> Signup and view all the answers

What is the highest tolerance percentage represented by the code letter 'G'?

<p>2% (C)</p> Signup and view all the answers

Which letter is used to represent a tolerance of 5% in resistor coding?

<p>J (C)</p> Signup and view all the answers

What is a common feature of both Surface Mount Resistors and axial type resistors?

<p>They are printed with a numerical code. (B)</p> Signup and view all the answers

What does resistance oppose in an electrical circuit?

<p>Current flow (D)</p> Signup and view all the answers

What is the formula for calculating heat dissipated by a resistor?

<p>$I^2Rt$ (D)</p> Signup and view all the answers

Which resistor can safely dissipate more heat?

<p>1 watt (D)</p> Signup and view all the answers

What does the maximum current through a resistor depend on?

<p>Its power rating and resistance value (B)</p> Signup and view all the answers

What is tolerance in regards to resistors?

<p>The deviation from the specified value (B)</p> Signup and view all the answers

For a 1000 ohm resistor with a tolerance of ±10%, what could its actual value be?

<p>Between 900 and 1100 ohms (D)</p> Signup and view all the answers

Where is the resistance value and power rating typically found on larger power resistors?

<p>Printed on the resistor body (D)</p> Signup and view all the answers

Which of the following indicates high precision in resistors?

<p>Lower tolerance percentage (D)</p> Signup and view all the answers

What is the primary function of resistors in a circuit?

<p>To limit the amount of current flowing (A)</p> Signup and view all the answers

Which of the following is NOT a type of resistor?

<p>Oscillating resistors (C)</p> Signup and view all the answers

How are carbon composite resistors primarily constructed?

<p>By moulding mixtures of powdered carbon and insulating materials (C)</p> Signup and view all the answers

What is a defining feature of metal film resistors compared to carbon film resistors?

<p>They have a lower power rating (B)</p> Signup and view all the answers

What determines the resistance value of wire wound resistors?

<p>The length of the wire wound around the ceramic body (A)</p> Signup and view all the answers

What does the power rating of a resistor indicate?

<p>The amount of power it can dissipate without burning out (B)</p> Signup and view all the answers

Which of the following resistors typically has a higher power rating?

<p>Carbon composite resistors (D)</p> Signup and view all the answers

What is a common characteristic of carbon film resistors?

<p>They are made by depositing a thin layer of resistive material on ceramic (A)</p> Signup and view all the answers

What does the third digit in a three-digit resistor code represent?

<p>The multiplier for the resistance value (C)</p> Signup and view all the answers

What does a resistor code of '390' indicate?

<p>39 × 1Ω (D)</p> Signup and view all the answers

Which of the following is an example of a temperature dependent resistor?

<p>Negative temperature coefficient (NTC) resistor (C)</p> Signup and view all the answers

When subjected to excess voltage, what happens to a voltage dependent resistor's resistance?

<p>It decreases (A)</p> Signup and view all the answers

What is the purpose of a positive temperature coefficient (PTC) resistor?

<p>To sense temperature in recording instruments (D)</p> Signup and view all the answers

What does a resistor with markings '000' signify?

<p>A zero-ohm resistor (C)</p> Signup and view all the answers

In complete darkness, a light dependent resistor (LDR) has a resistance in the region of how many ohms?

<p>100,000Ω (C)</p> Signup and view all the answers

Which type of resistor is commonly used to control street lights?

<p>Light dependent resistors (LDR) (A)</p> Signup and view all the answers

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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

  1. The physical size of a resistor indicates how much heat can be dissipated in the form of heat.
  2. The tolerance of a resistor tells you the amount the resistor could deviate from the specified value.
  3. The most common type of fixed linear resistor is the carbon composite resistor.
  4. A thermistor has a positive temperature coefficient.
  5. A rheostat is a two-terminal device.
  6. A potentiometer is a three-terminal device.

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