Meeting up with the Resistance
Part 1 | Part 2 | Part 3 | Part 4
Resistance in electronics is a lot like friction in mechanical systems. It is the measure of an object’s opposition to the flow of electrons through it. This opposition is in fact very useful and it makes it possible to generate things like heat and lift or to ensure that correct flow of voltage and current in a circuit.
Factors that determine Resistance in a material
- Material — Conductors have lower resistivity than insulators, hence they provide less resistance in the same volume of material
- Cross Sectional Area — Think of water flowing through a pipe the larger the diameter the easier the water flows and so resistance varies inversely with cross sectional area
- Length — The longer the material the more resistance it has because more opportunities exist for electrons to bump into other particles
- Temperature — For most materials the higher the temperature, the more than particles in the material move around making collisions all the more common for electrons and hence the higher the resistance.
In electronics diagrams we use the letter R to denote resistance and it is measured in unite known as Ohms that are abbreviated to the greek letter omega Ω
Resistors
These are passive device that are designed to have controlled amounts of resistance e.g 1K Ohm (1K Ω), Resistors don’t amplify of shape current an any way. That said they can be powerful when we need to control the flow of current, lets consider for following scenarios…
Limiting Current
Too much current can destroy many sensitive electronic components such as LEDs, Transistors, and Integrated Circuits by putting a resistor at the input to a sensitive component, you limit the current that reaches that component
Reducing and Controlling Voltage
You can use resistors to reduce the amount of voltage that reaches a component or even different parts of a circuit. When this is done we call this a voltage divider
Timer Components
When used with a capacitor, which is a component that stores electrical energy. We can make a resistor-capacitor combo to build very concise and accurate time dependent circuits
Fixed Vs Variable Resistors
- Fixed Resistors supply a constant factory defined amount of resistance
- Variable Resistance — Also known as potentiometers and rheostats, these allow you continually adjust the resistance from 0 to a factory determined maximum value. We use these when we want to vary the amount of current that is supplied to parts of a circuit.
In circuit diagrams one uses a zigzag symbol to represent a fixed resistor
And the same symbol with an arrow through it to represent a rheostat, when the arrow is on top it represents a potentiometer
Identifying Fixed Resistors
Most fixed resistors come in a cylindrical package with 2 leads sticking out, This cylindrical package will usually have colour coded bands going around it.
Each of these colours represents a value and the position of the band indicates how to use that value when calculating the resistance of the resistor …
Standard Precision resistors use 4 colour bands, the first 3 are for the values and the fourth for the tolerance
High Precision resistors use 5 colour bands the first 4 to indicate the values and the fifth for the tolerance
Types of Variable Resistors
Potentiometers allow you to adjust resistance continuously and are available in various packages known as
Dial pots
Slide pots
Trim pots
Combining Resistors in a Circuit
It’s not always possible to find resistors that have the exact amount of resistance one is seeking. However its usually possible to combine existing options into what we want.
Resistors in Series
When we arrange resistors in series in a circuit we restrict current with first one and then again with the second one and so and so forth until we have gone through all the resistors.
In the type of arrangement the resistance is the sum of all the individual resistors resistances
R = R¹ + R² + R³
In a series arrangement the combined resistance in greater than any of the individual resistances
Resistors in Parallel
When resistors are in parallel in a circuit you have 2 or more paths for current to flow.
This cases the overall effect to be a decrease in the overall resistance it is calculated as
R = R¹ * R² /(R¹ + R²)
The combined resistance of two or more resistors in parallel is always lower than any of the individual resistances.
Combining Series and Parallel Resistors
Many circuits combine series resistor arrangement with parallel resistor arrangement in various ways so as to resist current in some parts of a circuit while splitting current in other parts of the circuit
In some of these circuits one can calculate the equivalent resistance by combining the equations for resistors in series and resistors in parallel. Some of these circuits might be so complicated as to need matrix mathematics to calculate the total resistance, which happens to be out of the scope of this series.
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