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Further Details
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 PLC
PROGRAMMING & TECHNOLOGY
The Resistor
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We begin by looking in detail
at the resistor, used in PLC'S, Drives, Temperature Controllers, Computers
- Televisions - Radio - Walkman - Playstation - Automobiles etc etc.
In fact virtually every electronic device in the world today uses
this component, without the resistor all current control, biasing
networks and feedback in amplifiers would be impossible to achieve.
It is interesting to note that the current flow through a resistor
is proportional to the voltage across it, this will be shown later
R = V/I. Any conductive material has its resistive content dependent
upon its length. the longer a piece of wire - the higher its resistance,
the shorter its length and/or the greater its diameter - the lower
the resistance value will be. However not all resistors are made from
wire.
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There are several types
of resistor, these differences effect the components Stability,
Tolerance and Power rating. The figures below are general and depend
upon the manufacturer.
| TYPE |
TOLERANCE |
POWER
(watts) |
STABILITY |
OHMIC
RANGE |
Carbon film |
5 – 20% |
0.25 - 2 |
Poor above 150ppm |
1-10M |
Metal film |
0.5 – 5% |
0.125 - 0.6 |
Good 50-100ppm |
1-1M |
Precision metal film |
0.1 – 0.5% |
0.25 - 1 |
Excellent 15-50ppm |
20 - 1M |
Power oxide |
+/-5% |
up to 3 |
Reasonable 300ppm |
1 - 1M |
Wire wound |
+/-5% |
2 - 1000 |
Reasonable 100ppm |
0.1 - 100K |
Carbon resistors are the most generally used resistor type in the
world today. They are cheap, have a good range of values available
and their power ratings are ideal for Radios and Televisions.
Metal film are used in measurement instruments where stability is
paramount, more expensive than carbon but again have a good range
of values.
Wirewound are probably the next most widely used resistor. Wherever
current control in high voltage situations eg: 110volts and above
is required, the wirewound is more suited. The power rating can
support up to 1000watts and their stability at this rating is quite
reasonable considering that power generates heat which effects stability.
If you look at any resistor specification it will tell you how stable
it is over various temperature ranges, you then have to decide which
type is necessary for the application and whether some additional
heat dissipating device may be required eg: heatsink, fan or just
extra ventilation.
Before we move on we should be aware of the terminology used in
electronics when discussing resistors:
Values
- m or milli = milli Ohms (tenths or hundreths of an Ohm)
- R = Ohms used when discussing value 0.5 - 999 Ohms
- K = Kilo Ohms
- Meg or M = Mega Ohms (mega meaning millions)
Power
- w = watts therefore Kw means Kilo Watts, mw = milli watts
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Construction:
Usually the resistor is made by coating or winding wire onto a ceramic
rod. The coating process is done by passing the rod through a vapour
which deposits the film oxide onto the rod surface.
 < film
on surface of the rod 
The rod is then machined so that the conductive material resembles
a resistive length of wire.
The colored bands on the left side, refer to its value (this
value is 1000 ohms or 1Kohm) and the band on the right refers
to the components tolerance (Gold is 5%), this means that
the value of this component will be within +/- 5% of 1Kohm.
(Silver is 10%) for the values study the color code table
below. |
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Colour
Code:
Band1 - 1st number
Band2 - 2nd number
Band3 - multiplier
some resistors have 4 bands, the 4th being the multiplier.
Brown = 1 Black = 0 Red = 2
Therefore value = 1 0 00 = 1000
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COLOUR |
CODE |
| Brown |
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1 |
| Red |
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2 |
| Orange |
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3 |
| Yellow |
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4 |
| Green |
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5 |
| Blue |
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6 |
| Purple |
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7 |
| Grey |
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8 |
| White |
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9 |
| Black |
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0 |
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Therefore a marking of Red-Green-Orange would be 25000 or 25Kohms.
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LEARN
MORE ABOUT THIS ON OUR ONLINE COURSE
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INTRODUCTION TO PLC'S
Includes FREE! PLC Programming
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You
will gain experience in using a Support Tool. Learn to identify
the different types of PLC and understanding how the PLC works as
opposed to a PC. Learn how to write small program routines in Ladder
language, understand how and when to use Timers, create latches
and manipulate data values, which are all fundamental in any manufacturing
process. Plus you will gain a basic understanding of the PLC numbering
systems and how to convert numbers into the different formats that
PLC's utilize.
Then
move onto a PLC Programming Course. Scantime Engineers are PLC Programmers,
we teach you how to write and structure PLC programs also how to
write PLC Programs to Industrial Standards to maintain the safety
integrity of the Manufacturing Process and Plant.
FURTHER
INFORMATION -
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