Basic Electronics - Resistors

BASIC ELECTRONICS
Electronic Components - Resistors
www.sf-innovations.co.uk

Resistors
This presentation is an introduction resistors and will help the reader
understand how and why these are used when we discuss controlling
external devices using the GPIO port of the Raspberry Pi.
These are used to limit the flow of current in a circuit and their resistance is
measured in ohms. Resistors are a fundamental building block in any
electronic circuit. They are used widely to set the current, voltage, gain and
a number of other important parameters.
Resistors come in various shapes and sizes and are mainly defined by their
Resistance, Tolerance, Power rating and Packaging

Resistance: This can vary from milli ohms up-to Mega ohms. The value of
a typical axial resistor is marked on it using an internationally recognised
colour coding system. There are some slight variations as one can find a 4,
5 or 6 rings as shown below.

Colour code examples: Below are examples of how a 22 ohm and 6200
ohm resistors are marked.
Note:
6200 ohm is the same as 6.2 kilo ohm
1000 ohms = 1 kilo ohm
1,000,000 ohms = 1 Mega ohm

Resistance values: This can be any value in practice. However resistors
are normally available in preferred values in ranges known as E12, E24 and
E96 ranges.
The E12 range is so called as there are 12 values in every decade.
Between 10 and 100 ohms:
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82 ohms
Between 100 ohms and 100 ohms:
100, 120, 150, 180, 270, 330, 390, 470, 560, 680, 820
Between 1000 and 10,000 ohms:
1000, 1200 etc right up to
Greater than 1 Mohm:
1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 M ohms.
The E24 range has 24 values and the E96 has 24 values per decade.

When resistors are
connected in series the
combined value is arrived at
by adding the value of the
individual resistors.
In the above example, this
would be:
Rnew = R1 + R2 + ... + Rn
Example 1:
If a 220 ohm resistor and a 330 ohm
resistor are in series, then the
combined resistance is (220 + 330) =
550 ohms
Example 2:
If three 150 ohm resistors are
connected in series, then the
combined resistance is (150 + 150 +
150) = 450 ohms
Combining Resistors: The examples below show how to work out the
value of the new resistance when resistors are combined in series or
parallel.

Example 1:
If two 560 ohm resistors are
connected in parallel, the
combined resistance would
be 1/[(1/560) + (1/560)] = 280
ohms.
Example 2:
If three 1000 ohm resistors
are connected in parallel,
then using the equation
shown here one would work
out that the combined
resistance is = 330 ohms.
When resistors are connected in
parallel, the combined value is arrived
at by adding the reciprocal of the
value of the individual resistors. The
reciprocal of this is the new value.
In the above example, this would be:
1/R new = 1/R1 + 1/R2 + ... + 1/Rn
Combining Resistors: The examples below show how to work out the
value of the new resistance when resistors are combined in series or
parallel.

Tolerance: The accuracy of resistors can be specified when they are
purchased. This can be 1%, 2%, 5% or higher depending on the
application.
Power rating: A resistor may be used in a small signal application where a
power rating of 125 mW or 250 mW is adequate. However in some
applications a higher power rating like 1W or 5W may be required.
Packaging: Resistors come in different packages. The main ones are axial
through hole or Surface Mount.
Power resistorsSelection of surface mount resistors
(note: the resistor values are printed on
SMT components with the last digit is a
multiplier.)

Experiments with resistors
You will need the following:
Selection of resistors – 1000 ohm (1 kilo ohm) & 330 ohm
9V PP3 battery with a battery clip
An LED
Digital Multi-meter

Experiment 1 Resistors in series
Use the Digital multi-meter on its
resistance range to check the value of a 1
kilo ohm resistor by connecting the test
probes on either end of the resistor.
Make sure that the meter is set to measure
resistance and a suitable range is
selected. Here it is set to the 2000 ohm
range as this is more than the values that
we are measuring.
Connect a 1 kilo ohm and 330 ohm
resistor in series. Check that the value of
the 2 resistors combined is 1.33 kilo ohms.

Experiment 2 Resistors in parallel
Connect a 1 kilo ohm and 330 ohm resistor in parallel. The combined value
should be given by the following equation.
1/Rnew = 1/1000 + 1/330
Rnew = 249 ohms
Using the multi-meter check that the value of the 2 resistors combined is
around 249 ohms.

Experiment 3 Controlling the current through an LED
Connect a 1 kilo ohm (1000 ohm) resistor in series with an LED and then
connect this to the 9V PP3 battery. Note: The long leg of the LED needs to
go to the positive of the battery.
The voltage across the resistor should be (9 – 2) where 9V is the voltage
across the battery and 2V is the voltage across the LED. Using Ohms law we
see that the current through the 1 kilo ohm resistor should be:
9V/1 kilo ohm which is 9 milli amps.

Experiment 4 Making the LED brighter
To make the LED brighter, you would need to reduce the value of the reistor.
For example if you use a 330 ohm resistor instead of the 1 kilo ohm resistor,
the current would be:
9V/330 ohm which is nearly 30 milli amps. This is more than 3 times the
current compared to when a 1 kilo ohm resistor is used and the LED is much
brighter.
Note: A way to quickly double the current, is to halve the resistor from 1000
ohm (1 k ohm) down to 500 ohms. This can be done easily by putting
another 1000 ohm resistor in parallel to the original 1000 ohm resistor.

Summary - Resistors
This presentation covered the following facts about resistors:
Colour coding
Ohms, kilo ohms and Mega ohms
Values – E12, E24 range
Resistors in series
Resistors in parallel
Experiments with resistors in series, parallel and LEDs.

Basic Electronics - Resistors

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