Resistors Colour Code

Ignore the colour of the resistor body.

Most resistors have three coloured bands close together at one end and one single band at the other.

The three adjacent band give the resistor value.

The band nearest the wire lead gives the value of the first digit. e.g Brown = 1.

The next band gives the value of the next digit e.g. red = 2

The third band gives the number of zeros which follows the two digits.e.g. orange = 3 zeros = 000.

Therefore a resistor with brown, red, orange bands would have a value of 12000 ohms.

This resistor has a value of 2,700,000 ohms.

A green blue black resistor would be 56 ohms. (black indicates that there are no zeros).

Black = 0
Brown = 1
Red = 2
Orange = 3
Yellow = 4
Green = 5
Blue = 6
Violet = 7
Grey = 8
White = 9

If the third band is silver then divide the value of the first two digits by 100, if gold divide by 10.
e.g. red violet gold is 2.7 ohms.

The fourth band indicates the tolerance.
e.g. brown indicates plus or minus 1%.
a 100 ohm 1% resistor can be in value between 99 ohms and 101 ohms.

brown 1%
red 2%
gold 5%
silver 10%
none 20%

If there is a fifth pink band this indicates a high stability resistor.

Wires and Cables

Wires are mostly made from copper which is cheap and has a low resistance to the flow of electric current.

Wires come in standard wire gauges (S.W.G.) where the smaller the diameter of the wire the bigger the SWG. e.g 22 swg has a smaller diameter than 16 swg.
Copper oxidises (tarnishes) making it difficult to solder. It is therefore tinned giving us 22 swg tinned copper wire, for example.

If two bare wires touch it forms a short circuit. To avoid this wires are INSULATED using PVC etc.
When wire are used for coils such as transformers the insulation is varnish. Since this is very thin wires will take the minimum of space.
A broken wire is called an OPEN CIRCUIT and prevents current flowing.

A single strand of wire 0.6 mm in diameter is called 1/0.6; this is very rigid and snaps if bent too often. Flexible leads are made from several strands of wire. 7/0.2 is 7 strands of wire each 0.2 mm in diameter. Fine strands of wire can be woven into a mesh or braid which can be used to screen out unwanted interference. (The sunglass effect).
Television aerial lead uses screened cable called COAXIAL CABLE.
Leads which carry small signals such as audio are often screened to reject external interference.

Where several leads are needed they can be combined into a single cable. This can be a multicore cable, a cableform or a cable loom.
Cables are often terminated in plugs or sockets which may take some time to connect. A quicker technique is to use INSULATION DISPLACEMENT CONNECTORS (IDC) which take only seconds to fit.

Thicker wires can carry higher currents than thin ones as bigger pipes can carry more water than thin ones.

7/0.2 can carry 1 amp maximum
16/0.2 can carry 3 amp maximum
24/0.2 can carry 5 amp maximum
32/0.2 can carry 10 amp maximum

Use wire strippers to remove insulation. Avoid nicking the wires or breaking strands in flexes.
When soldering avoid whiskers, burning insulation and wicking (allowing solder to run up under the insulation of flex which makes it rigid and brittle).

FIBRE OPTICS is often used instead wires in some applications.

What is Phase

The generator at the power station which produces our AC mains rotates through 360 degrees to produce one cycle of the sine wave form which makes up the supply.

In the next diagram there are two sine waves.

They are out of phase because they do not start from zero at the same time.

To be in phase they must start at the same time.

The waveform A starts before B and is LEADING by 90 degrees.

Waveform B is LAGGING A by 90 degrees.

The last diagram, known as a PHASOR DIAGRAM, shows this in another way.

The phasors are rotating anticlockwise as indicated by the arrowed circle.
A is leading B by 90 degrees.

The length of the phasors is determined by the amplitude of the voltages A and B.

Since the voltages are of the same value then their phasors are of the same length.

If voltage A was half the voltage of B then its phasor would be half the length of B.

All this has nothing to do with "set your phasors on stun".

The Cathode Ray Tube

Cathode Ray Tube

The Cathode Ray Tube (CRT) is used in oscilloscopes, radar, monitors and television receivers.

It consists of a glass envelope made from a neck and cone.
All air has been extracted so that it contains a vacuum.

At the narrow end are pins which make connection with an internal ELECTRON GUN.
Voltages are applied to this gun to produce a beam of electrons.
This electron beam is projected towards the inside face of the screen.

The face is coated with a PHOSPHOR which PHOSPHORESCES (glows) when hit by the beam.
This produces a spot of light on the centre of the face of the CRT.
By varying the beam current, spot BRIGHTNESS can be controlled.
Controlling the diameter of the beam controls FOCUS.

Phosphors come in a range of colours.

On its way from the gun to the screen the beam passes between 2 sets of plates.
They are called the X and Y plates (as in graphs).
By applying voltages to these plates the beam can be deflected.
This causes the spot to move from the centre of the screen to another position on the screen.
The X plates plates deflect the spot horizontally, the Y plates vertically.
Thus the spot can be deflected to any position on the screen.
External deflection coils are often used instead of the internal deflection plates.

Note that dropping a CRT causes it to IMPLODE which is as dangerous as an explosion.