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Component selection criteria for embedded systems

As an embedded system designer, you will always have to make a choice of components for your projects. This choice is always made on the basis of the project that you are working on. It’s costs, functionality, and application are important factors. In this post, we will take a look at the component selection criteria for embedded systems for four major components. Resistors, capacitors, inductors, and transistors.

Resistors

types of resistors
Types of resistors (Source)

Passive components are the electronic components that cannot process an electrical signal without external energy. A resistor is a passive component. Resistors are famously known for their feature of controlling the flow of electric current. Resistance is a resistor’s ability to oppose the flow of current. We use the unit ohms (Ω) to express it. Resistors don’t have any polarity and you can connect them in any direction. They act as a load and thus are heat-dissipating elements. They also divide the voltage across their nodes.

What are the different types of resistors?

  • Resistor
    • Linear
      • Fixed value
        • Carbon composition
        • Metal film
        • Wire wound
      • Variable value
        • Wire wound
        • Potentiometer
        • Trimmers
    • Non-linear
      • Thermistor
      • LDR (Light-dependent resistor)
      • Varistor
      • Photoresistor

Carbon composition

Carbon composition resistors are made up of carbon granules. Insulating materials are also added which determines resistance. Carbon composition resistors have a resistance value ranging from 2.7Ω to 22MΩ. They have a tolerance between +/-5% to +/-20%.

Metal film

Metal film resistors are formed by depositing a thin metal layer on a cylindrical insulating support. These resistors have a resistance value ranging from 10Ω to 10kΩ. They have a tolerance between +/-1% to +/-5%.

Wire wound

Wire wound resistors have a resistance 1Ω – 100kΩ. And a tolerance between +/-1% to +/-10%

Variable resistors

As the name states, these are resistors whose value can vary depending on some controlling mechanism. Some of these resistors have values that vary in relation parameters like temperature and incident light. They find applications in everyday items like fan controllers and light dimmers.

What is the selection criteria for selecting a resistor for embedded systems projects?

We should select resistors on the basis of the following parameters:

  1. Low noise. As current passes through a resistor, it heats up. Thermal noise generates due to the movement of electrons. This should be as low as possible.
  2. They should be stable under high-frequency operations. As circuits become fast, resistors must respond appropriately.
  3. They should be cost-efficient.
  4. Accuracy is another important aspect to consider.
  5. Resistors should have low tolerance values and low thermal coefficients.

Capacitors

component selection criteria for embedded systems - capacitors
Different types of capacitors

Capacitors are two-terminal devices that store electrical charge. We can calculate this charge using the formula Q=CV when the power source is DC. Here C is the capacitance and V is the potential difference across the two plates. Capacitance is the capacity of a capacitor to store charge per unit potential difference. The units for representing the capacitance of a capacitor are usually in μF or pF. Capacitors offer low impedance to alternating current (AC) but high impedance to direct current (DC).

What are the different types of capacitors?

  • Capacitor
    • Fixed
      • Polarized
        • Electrolytic capacitors
          • Aluminum capacitor
          • Tantalum capacitor
          • Niobium capacitor
        • Super capacitor
          • EDLC super capacitor
          • Pseudo super capacitor
      • Non-polarized
        • Film capacitor
          • Paper film capacitor
          • PET film capacitor
          • PC Film capacitor
          • PP film capacitor
          • PEN film capacitor
          • PPS film capacitor
        • Ceramic capacitor
        • Mica capacitor
        • Plastic capacitor
    • Variable
      • Mechanically controlled capacitors
        • Air gang/Tuning capacitors
        • Trimmers capacitor
      • Electrically controlled capacitors

A few points regarding some of the capacitors listed above:

  • We cannot employ polarized capacitors in a circuit where the source of power is alternating (AC).
  • Non-polarised capacitors can be connected in any direction.

Paper capacitors

  • Paper capacitors consist of two tin foil sheets separated by thin paper. All these layers are rolled on to a compact cylinder.
  • They are available in the capacitance range of 0.001 μF to 2 μF.
  • Paper capacitors have a tolerance of about +/-15%.
  • They find applications in audio technology and a wide variety of other embedded systems.

Mica capacitors

  • Mica capacitors are made using alternate thin sheets of metal and mica sheets.
  • The metal sheets are connected together as one terminal. Similarly, the mica sheets together form the other terminal.
  • They are available in the capacitance range of 50 pF to 500 pF.
  • Mica capacitors have a tolerance in the range of +/-2% to +/-20%.
  • One of their applications is in RF tuning circuits.

Ceramic capacitors

  • These capacitors have a metallic coating on plates. The ceramic material is the dielectric between the plates of the capacitor.
  • Ceramic capacitors are available in the capacitance range of 47 pF to 0.01 μF.
  • They have a tolerance in the range of +/-2% to +/-20%.
  • These capacitors are very small in size and quite economical.
  • Ceramic capacitors find applications in RF circuits and coupling and tuning circuits.

What is the selection criteria for selecting a capacitor for embedded systems projects?

We should select capacitors on the basis of the following parameters:

  • The working voltage of a capacitor should be 20% higher than the working voltage of the circuit.
  • For an AC circuit, the capacitor should not have any polarity.
  • For a precise circuit performance, the tolerance of the capacitor needs to be kept in mind.
  • Temperature coefficient can be either positive or negative.
  • Leakage current is another important parameter that needs to be kept in mind. Different capacitors have different values of leakage currents. You need to choose according to your application.
  • As is unanimous with all embedded project considerations, the capacitor should be economical.

Inductors

types of inductors
Types of inductors (Source)

An inductor is an electronic component which opposes the change in the current flowing through a circuit. A counter electromotive force (EMF) is produced in the inductor for every current change. This property of an inductor is known as its inductance. It’s measured in Henry (H). The reactance of an inductor is given by => X(L)=2ΠfL.

What are the different types of inductors?

We’re going to study the three main types of inductors that you need to know.

  • Iron core inductors
  • Air core inductors
  • Ferrite core inductors

Iron Core Inductors

  • In this inductor, a wire is wound over an iron core.
  • This construction causes a decrease in inductance.
  • The core of an iron core inductor is laminated to decrease eddy current losses.
  • The maximum amount of losses in an iron core occur at high frequencies.
  • The inductance of iron core inductors varies between 1-25H.

Air Core Inductors

  • These have air inside the coil.
  • The wires of an air-core inductor can either be stiff to envelope ‘nothing’ and form an air inductor.
  • Or they can be wound around non-ferrous items like plastic, ceramic, or cardboard.
  • Inductance ranges between 250 μH to 0.25 mH

Ferrite Core Inductors

  • As the name probably foreshadows its structure, the ferrite core inductor has wires wound on a highly ferromagnetic substance called ferrite.
  • These inductors have a high inductance with minimum hysteresis and eddy current losses.
  • Hence they find applications in RF circuits and antennas.
  • They have variable inductance.

What is the selection criteria for selecting an inductor for embedded systems projects?

We should select inductors on the basis of the following parameters:

  • They should have a high value of saturation current.
  • Faster di/dt. Rate of change of current.
  • They should have a decent transient response.
  • They should have low ripple current.
  • Inductors should have low core losses and low eddy current losses.

Transistors

Transistors are the building blocks of modern digital electronics. They are used to make all kinds of sequential and combinational digital circuits like flip-flops and latches and so on. Transistors are three-terminal devices that are made by sandwiching an N-type material between two P-type materials or vice versa. Transistors are current controlling devices. They transfer the resistances from input to output. They can amplify weak signals.

What are the different types of transistors?

We can broadly group transistors into two main types:

  • Bipolar Junction Transistor (BJT)
    • NPN type
    • PNP type
  • Field Effect Transistor (FET)
    • Junction Field Effect Transistor (JFET)
      • n-Channel
      • p-Channel
    • Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
      • Depletion type
        • p-Channel
        • n-Channel
      • Enhancement type
        • p-channel
        • n-channel

What is the selection criteria for selecting a transistor for embedded systems projects?

We should select transistors on the basis of the following parameters:

  • They should have maximum collector current and V(CE) collector to emitter.
  • The gain should be high
  • Noise should be less.
  • Cost should be low

 

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