Resistors in Electronics

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.