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Glossary of terms you will come across during your Phase 2 training.

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This is the effective resistance that a capacitor puts in the way of an alternating voltage source, much like a resistor in the circuit.

The number '18650' describes a battery, similar in size to an AA battery.

An 18650 is a lithium ion rechargeable battery. The correct name is “18650 cell”.

The 18650 cell has voltage of 3.7v and a capacity of between 1800mAh and 3500mAh.

The cell name is based on its dimensions: the cell is 18mm in diameter and 65mm in length.

For comparison purposes, an AA cell is a 14500 cell - 14mm in diameter and 50mm in length


An astable square waveAn astable circuit is an electrical circuit that does not stay in one stable condition. If you were to monitor it with an oscilloscope, it produces a 'square wave', this is a digital waveform with sharp transitions between low (0V) and high (+Vs). Note that the durations of the low and high states may be different. The circuit is called an astable because it is not stable in any state: the output is continually changing between 'low' and 'high'.

Three possible states are:

  • Astable: Not stable in any state, output is constantly changing between on and off 
  • Monostable: Stable in one state, either on or off 
  • Bistable: Stable in either state, on or off 

Assorted inductors, image courtesy of WikipediaA component in an electric or electronic circuit which possesses inductance.

Note 1: inductors resist changes in the rate of flow in a circuit. Inductors resist current rising initially, then once the current is flowing, inductors resist if we try to stop the current flowing

Note 2: Most of the time, we have inductors in circuits because we want and need them there. However, there are times when components, or parts of components, act as inductors, and this effect is unwanted.
For example, the windings in a relay are just a coil of wire, which, of course, will act as an inductor when we pass current through it to energise the relay. When we open the relay circuit, the magnetic field around the energising coil will collapse and generate a high voltage spike which can damage the rest of our circuit.
You can read about an example of this effect, and how we counteract it in the time delay circuit exercise that is part of the electronics learning unit.


the SI unit of electrical capacitance, equal to the capacitance of a capacitor in which one coulomb of charge causes a potential difference of one volt.

The farad is named after Michael Faraday, and the unit of measurement is represented by the capital letter 'F'

The farad is a huge amount of capacitance, so more common measurements are:

  • the microfarad, where 1μF=10-6F or 0.000001F
  • the nanofarad, where 1nF=10-9F or 0.000000001F

Example of use on Phase 2:

During the glow plug time delay circuit exercise, you will us an electrolytic capacitor with a value of approximately 2000 microfarads, or 2000 μF or 0.002F 


Symbol 'L', refers to the ability to produce induced voltage when cut by magnetic flux,

or, equally:

Is a measure of the amount of magnetic flux that is produced for a given electrical current.

The SI unit of inductance is the henry.

Think of the operation of the rotor in an alternator. The more electricity we pass through it, the stronger the magnetic field becomes around it. The stronger that field is, the more electricity we can generate.

The unit of inductance quantifies the ratio between the rate of current build up and the voltage across an inductor.

An inductance 'L' of 1 H (one henry) represents a potential difference of 1 V across an inductor within which the current increases or decreases at the rate of 1 ampere per second, or 1 A/s.

Note: 1 H is a huge amount of inductance, so more common measurements are:

  • the millihenry, where 1mH=10-3H or 0.001H
  • the microhenry, where 1μH=10-6H or 0.000001H
  • the nanohenry, where 1nH=10-9H or 0.000000001H

This is the ratio between the force necessary to keep a body moving at a constant rate over a surface, and the downward force that body exerts on that surface.

The co-efficient of friction is calculated by dividing the horizontal force (H) necessary to keep an object moving on a surface, by the downward force (D) exerted by that object.

μ=H/D (Coefficient of friction = Horizontal force divided by Downward force)


The American Scientest, Joseph Henry.

This is the S.I. unit of inductance, and is named after the American scientist, Joseph Henry. The unit is called the henry and the abbreviation is 'H'.


Knee voltage: graph plotting voltage across a diode agains the current flow through the diodeRelated to diodes.

Every diode has some internal resistance. This is caused by the depletion layer present at the PN junction of a diode.

Because this is a physical barrier, we must do work to overcome it. This means that, even when we forward bias a diode, the voltage must rise above a certain value before current flows through.

Not only that, the voltage must rise above a certain amount before we can get significant flow through a diode.

No current will flow through a diode when we apply voltage to the anode until we exceed a certain minimum voltage, the 'forward voltage' of that diode.

Even when we exceed the forward voltage, we don't get significant current flow through the diode until we reach the 'knee voltage'. Once we reach the knee voltage, even small increases in voltage result in massive increases in current flowing through the diode.

Once we reach knee voltage, small increases in voltage result in large increases in current flow. This explains why we cannot feed high power LEDs directly, we must 'drive' them using a control circuit - we get to the stage that a tiny increase in supply voltage results in a massive flow of electricity through the LED, and this can destroy the unit.

If you want to see a little more about driving LEDs, check out the video below:

In the context of the study of DC electricity and electronics, bias describes when we deliberately apply a certain DC voltage to one electrode relative to another.
For example, if we apply a certain voltage to the anode of a diode that is higher than the voltage on the cathode, we have 'forward biased' the diode.
Once this voltage bias exceeds a certain value (the forward voltage of the diode), current will flow through the diode.

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