The resistor is a passive two-terminal electronic components. It function is to limit or reduce(resist) the flow of current or divide the voltage in a circuit. The unit of the resister is ohm(Ω). The resistance r of any material is proportional to its length and inversely proportional to its area of cross-section A.
R=ρl/A Ω
ρ=specific resistance or resistivity of the material.
Contents
Symbol
Type
- Fixed Type
- Wire wound resistor
- Carbon film resistor
- Metal film resistor
- Carbon composition resistor
- Carbon resistor
- Film type resistor
- Chip resistor
- Variable Type
- Rheostat
- Potentiometer
- Trimmer
- Preset
Color Code
| Color | Digit value | Multiplier | Multiplied Out | Tolerance |
| Black | 0 | 100 | 1 | |
| Brown | 1 | 101 | 10 | |
| Red | 2 | 102 | 100 | |
| Orange | 3 | 103 | 1,000 | |
| Yellow | 4 | 104 | 10000 | |
| Green | 5 | 105 | 100,000 | |
| Blue | 6 | 106 | 1,000,000 | |
| Violet | 7 | 107 | 10,000,000 | |
| Gray | 8 | 108 | 100,000,000 | |
| White | 9 | 109 | 1,000,000,000 | |
| Gold | ±5% | |||
| Silver | ±10% |
Power
Measuring power across a resistor (Ohm’s Power Law)
Ohms Law
Ohm’s law describes the fundamental behavior of
resistors.
LED Current Limiting
For example, assume you have a 9V battery to power an
LED. If your LED is red, it might have a forward voltage around 1.8V. If you
want to limit the current to 10mA, use a series resistor of about 720Ω.
Kirchhoff’s Law
Kirchhoff’s law can be used to analyses networks
of resistors.
Resistor networks
Voltage Dividers
Resistivity properties of materials
| Material | ρ (Ωm) at 20°C | σ (S/m) at 20°C | Temperature coefficient (1/°C) x10^-3 |
| Silver | 1.59×10−8 | 6.30×107 | 3.8 |
| Copper | 1.68×10−8 | 5.96×107 | 3.9 |
| Gold | 2.44×10−8 | 4.10×107 | 3.4 |
| Aluminum | 2.82×10−8 | 3.5×107 | 3.9 |
| Tungsten | 5.60×10−8 | 1.79×107 | 4.5 |
| Zinc | 5.90×10−8 | 1.69×107 | 3.7 |
| Nickel | 6.99×10−8 | 1.43×107 | 6 |
| Lithium | 9.28×10−8 | 1.08×107 | 6 |
| Iron | 1.0×10−7 | 1.00×107 | 5 |
| Platinum | 1.06×10−7 | 9.43×106 | 3.9 |
| Tin | 1.09×10−7 | 9.17×106 | 4.5 |
| Lead | 2.2×10−7 | 4.55×106 | 3.9 |
| Manganin | 4.82×10−7 | 2.07×106 | 0.002 |
| Constantan | 4.9×10−7 | 2.04×106 | 0.008 |
| Mercury | 9.8×10−7 | 1.02×106 | 0.9 |
| Nichrome | 1.10×10−6 | 9.09×105 | 0.4 |
| Carbon (amorphous) | 5×10−4 to 8×10−4 | 1.25 to 2×103 | -0.5 |
Application of Resistors
- variable resister is used in volume control in
the TV application, - LED Current Limiting
Examples
Example 1
find the resistance of a copper
wire of 0.75km long and having a cross sectional area of 0.01cm2.
(take ρ=1.72×10-8ohm-m).
ANS=12.9ohm
Example 2
find the cross sectional area of
an aluminium wire of 700m long and having a resistance of 0.24ohm (take ρ=2.83×10-8ohm-m)
ANS=8.254×10-5m2.
Example 3
if a R = 300 ohm is attached
across the terminals of a V = 12 volt battery, then a current of (V/R)12 / 300 = 0.04 amperes flows
through that.
Example 4 (Ohm’s law equations)
Consider
a 1-ohm resistor in a circuit with a voltage drop from 100V till 10V across its
terminals. What is the current through the resistor? The
triangle reminds us that:
Example 5 (Ohm’s law equations)
Consider
a 10-ohm resistor in a circuit subject to a current of 2 Ampere and a voltage
of 120V. What is the voltage drop across the resistor? Using
the triangle shows us that:
Answerer
Thus the voltage at the end terminal is 120-20
= 100 V.
Example 6 (Ohm’s Power Law)
What must be the minimal power rating of this resistor?
Answer:
According to the wheel, P=I^2*R= 0.100^2*50=0.5 W. So the minimal power
rating should be at least 0.5W, but recommended is to go high above this value
for extra reliability and lifetime.
Example 7 (Ohm’s Power Law)
What is the current in the circuit?
This is a basic example of Ohm’s law. Voltage and resistance are known,
so we can calculate current with the equation:
I=V/R=6/1.2=5 A.
Example 8 (Ohm’s Power Law)
An electric heater (resistor) with a consumption of 1kW
is connected in a circuit with 8A current. What is the voltage drop over the
heater?
Voltage can be expressed in current and power with the formula:
V=P/I= 1000/8=125 V
Reference Website
- Wikipedia – Resistor
- Resistorguide
- Sparkfun – resistors
- https://www.electricaltechnology.org/2015/01/resistor-types-resistors-fixed-variable-linear-non-linear.html
Video tutorial
Basic Electricity – Resistance and Ohm’s law