Electricity is invisible, has no smell and can’t be heard yet it powers much of the world around us. Electricity can kill, burn or stun yet can help paralysed patients to move, save a life with a defibrillator and allow us to communicate rapidly around the world.
Electricity occurs in nature in thunderstorms, electric eels and static electricity. Numerous experiments and discoveries have framed our modern day understanding of electricity. but not everyone has the understanding they want of all things electrical and electronic.
This guide is designed to help directors and managers to better understand electricity and electronics.
1. A brief overview of electronics
These include the Ancient Greeks discovering static electricity by rubbing fur on amber; Benjamin Franklin flying a kite in a thunderstorm; Alessandro Volta’s discovery that certain chemical reactions could produce electricity leading to an early battery; Michael Faraday creating the electric dynamo; Swan and Edison both inventing the electric bulb; Nikola Tesla’s alternating current induction motors; Guglielmo Marconi pioneering long range radio transmissions; Gauss’s law for magnetism; and more. Electrical and Electronic Engineering today is truly standing on the shoulders of giants.
Multiple other scientists have contributed to the field of electronics. Many of which give their name to a measurement or theory including:
- André-Marie Ampère
- François-Marie Arouet, known by his nom de plume Voltaire
- Georg Simon Ohm
- Joseph Henry, and
- Ernst Werner von Siemens
for Amperes, Volts, Ohms, Henry’s and Siemens respectively.
Electricity is harnessed and put to work in games, toys, machines, communications, healthcare, vehicles, security, computing, lighting, heating, control and more but what is electricity?
Electricity is the flow of charge. Everything in the world is made up of tiny atoms. These atoms are made up of protons, neutrons and electrons. The electrons are in orbit around the atom and when sufficient force is applied these electrons can escape the orbit and become free. Within an atom Protons have a positive charge and electrons have a negative charge. Moving a negative charge electron from an atom leaves the atom with a positive charge
Charles-Augustin de Coulomb first published a law relating stationary electrically charged particles and the interaction force between them. Opposite charged particles attract and particles with the same charge repel with the amount of force between them depending on how far the particles are apart. The forces are known as electrostatic forces, these forces cause electrons to push other electrons away and allows charge to flow.
Materials can be broadly classified as conductors or insulators depending upon how tightly its electrons are bound to its atoms. Conductors have weakly bonded electrons, insulators have strongly bonded electrons. Metals including gold, silver and copper have weakly bound electrons and so are good conductors. Conversely rubber, plastic, glass and air have tightly bound electrons and so are good insulators. Both conductors and insulators are important to control the flow of charge. There are two other types of conductor namely semi-conductors and super conductors. Semi-conductors are used extensively used throughout modern electronics and will be covered in later sections.
2. Volts, current, amperes, watts, ohms, farads, henrys, ac and dc.
The field of electronics uses the same system of physical units as other branches of science and engineering, namely the International System of Units or SI units from the French Système International d’Unités.
For convenience the units utilise multiplier prefixes from the SI units system to cover very small to very large numbers. The most commonly used prefixes are:
Using the above system 1000000 ohms can be written 1MΩ, similarly 0.000001 Farads can be written 1 µF. You will see that 1nF can also be written as 1000pF.
AC and DC
Where electric charge (current) flows in one direction only it is known as direct current or DC. A battery is a good example of DC.
Where the direction of electric current changes periodically it is known as alternating current or AC. Mains power is a good example of AC. The periodicity of the current is known as its frequency and is measured in Hertz to describe the number of times per second it changes.
3. Passives: What are Rs, Cs and Ls
Electronic components can be broadly separated into active and passive categories.
Active devices are circuit elements that electrically control the flow of electrons, at least one active device is required to properly qualify as electronic. Passive devices cannot use an electronic signal to control the flow of electrons, charge or current.
Strictly speaking resistors, capacitors, inductors, transformers and diodes are all passive devices. However, diodes are frequently excluded from the passive category.
Resistors are devices used to impede (or resist) current flow. Ignoring parasitic effects, the characteristics of resistors are the same at all frequencies.
The key equation relating voltage, current and resistance is Ohms Law (V=IR) where the voltage, V, across a resistance, R, depends upon the current, I, flowing through it.
Resistors produce heat when an electric current flows through them, this is electrical power. The power rating of resistors can therefore be an important parameter for resistors.
Capacitors store electric charge between a pair of conductors separated by an insulator.
Capacitors are often used within power supplies to provide an energy store that helps smooth the voltage applied to electronic circuits. They are also used for energy storage for high demand devices such as camera flashes which operate infrequently, but need a significant amount of energy when they do operate.
The unit of capacitance is the Farad. In practise a farad of capacitance is huge, so many applications use micro, nano or picofarad capacitors.
The amount of capacitance within a device relates to the area of the conductors and their separation with larger plate areas as well as smaller separations giving the highest capacitance. Changing the insulation material or dielectric between the plates also impacts the achievable capacitance.
In addition to capacitors acting as charge storage, they also have a property similar to resistance which changes with frequency. This is known as reactance. The reactance of capacitors is greatest at DC or low frequencies and decreases as frequency increases.
Inductors store energy in a magnetic field produced when electric current flows in it. Inductors are also known as chokes or coils, and are typically constructed from an insulated wire wound around a core.
The reactance of inductors increases with frequency which makes them useful for filtering signals with unwanted noise as they allow DC currents to flow while resisting higher frequency signals.
When used in conjunction with capacitors they are useful in electronic filter circuits to separate different frequency signals. They are also used in radio receivers, loudspeaker crossovers and many other applications.
4. Not All About the Circuits
When designing the electronics for a new product, getting the electronics design right is important. Equally important however, is being able to purchase the components used, reliably place them on the printed circuit board and knowing that the manufactured product works before sending out to a customer. circuits
To ensure the above can be met there are three important design reviews in addition to the schematic and printed circuit board design. The collective name for these reviews is DFx reviews. These consist of Design for Purchasing (DFP), Design for Manufacture (DFM), and Design for Test (DFT). Let’s have a look at each of these in turn: circuits
Design for Purchasing
Electronic components come and go. One that are popular and current today may well be difficult to find in a few years time.
Every single component within a design has what is known as a product lifecycle status. This gives information on the likelihood of continued supply but not necessarily about its availability.
- Marketing & Sampling
- Not recommended for new designs / End of Life
Marketing & Sampling
Strictly speaking these are two separate stages but essentially they indicate that even if you can get hold of a few parts to develop with they are not yet available for volume manufacturing. These parts can be interesting if you operate right on the cutting edge of latest technology, but tread with caution. These parts have not been widely used and will likely have incomplete datasheets.
As the parts are being sampled, they are also being tested. Therefore, there is a much higher risk of the component not working as required with no established work arounds or errata. The dates for volume availability are indicative at best making production planning challenging.
Components with the “available” status can be purchased and used in production. It is worth checking the datasheet dates to get a sense of how long the part has been on the market for as it may have just been released or might be close to being obsoleted. Some component lines develop quicker than others, for instance those used in mobile phones tend to be available for a few short years. When the mobile phone demand shifts to the latest, greatest components, there could be price increases and obsolescence issues looming.
Stay in touch with manufacturers and distributers for key, single sourced components. Many have automated updates for impending lifecycle status changes.
Not recommended for new designs/End of Life
Components that are marked as “Not recommended for new designs” or NRND/NRD are exactly that. This is a warning that the manufacturer intends to stop supplying the component soon. Unless there is a very good reason for using that particular component it should be avoided and alternates sought.
At this stage manufactures usually provide the opportunity to secure inventory of the component. This is intended to allow manufacture to continue for a finite period while the product itself is being superseded by the next model or until a replacement part is identified and tested or a partial redesign can be made.
The component can no longer be purchased from the supplier nor from genuine distributors. There could be components available from other sources, but buyer beware. It’s not uncommon for these “gray-market” components to be poor quality, have been stored incorrectly or to even be counterfeit.
Design for Manufacture
Design for manufacture in this instance focuses mainly on the printed circuit board component population. Additionally, good designs should consider how the populated boards are fixed in place in the enclosure, access to clips, screws and connectors, but let’s keep that for another day.
The printed circuit board’s job is two-fold. It provides mechanical fixings for components by either plated holes for leaded components or surface pads for surface mount components. It also connects the components together to form the various circuits required for the design.
On the face of it, the requirements are fairly basic, but the manufacturing processes to manufacture printed circuit boards and to successfully and repeatedly place components means a good deal of care is needed. A good DFM review aims to capture potential manufacturing issues caused by the pcb design. There are many issues to be aware of here, some of the more common are listed below.
The bare PCB itself may have issues caused by acid traps, slivers and islands (although these are less common now due to pcb manufacturers moving to photo activated etching solutions instead of acid baths). Insufficient annular rings around drill holes and vias could cause failures if the drill hole takes up too much of the pad copper.
During component population if there are imbalances between the copper pads for two terminal surface mount components they can stand up on one pad due to unequal cooling. This causes open circuits and is known as “tombstoning”.
Dense designs may need to place vias within component pads. This can cause solder to wick down the via hole and leave insufficient solder to connect to the component properly. Via’s in pads may need to be filled to avoid this.
Design for Test
Before shipping product to your customer, some degree of testing, programming, or configuration may be needed. Testing is often performed on a risk basis depending on the type of product, its application and the consequences and costs of the product not working. Life safety equipment has a different degree of testing to a USB powered desk light.
The testing referred to here is for volume production testing and not for design debug, bring-up and verification.
The design for test review starts with checking how the unit will be programmed and configured and what testing is required. From there, the test coverage across the board is verified. This can vary from a test point on every single net if each component value must be verified with a full bed of nails test down to having no test points for simple boards with no test points for very simple units or those where access to the key interfaces is via a connector.