ELECTRONICS COMPONENT 1

RESISTANCE

Resistor and Resistor Circuits

Resistance is the degree at which a resistor can oppose the flow of current in a circuit. The unit of resistance is in ohms. The higher the resistance or resistors in a circuit, the lower the current flow and lower the resistance, the higher the current flow in a circuit according to ohms law (V = IR) provided the voltage remains constant.

For example, if we have a resistor of 10Ω and a voltage of 12v in a circuit, according to ohms law;

V=IR, the current flowing through the circuit will be;

I = V/R

I = 12/10 = 1.2amps                                         eqn (1)                                                

Assuming we increase the resistance from 10Ω to 20Ω;

We have our current to be;

I = 12/20 = 20amps                                          eqn (2)

Now comparing (1) & (2), we can confirm that the higher the resistance in a circuit, the lower the current flow and the lower the resistance, the higher the current flow along a circuit.

What is electrical current? Electrical current represented by the letter “I” in formulas, is the rate or flow of electric charge carried out by moving electrons in a metallic conductor or electronics components such as resistor, transistors etc. Voltage represented by the letter “V” is the pressure at which the charge is used that is why we also refer to it as electromotive force (EMF). Its unit is in volts.

The circuit symbols used for representing a resistor in a circuit are;

                            

Resistors can be connected in series (end to end) or parallel (across one another) or in a combination of series and parallel. 

Series Connected Resistor

Connecting resistors in a string or pigtail to another is called connecting them in series. When connected this way, the resistance of one resistor adds to the next in line. For example a 200Ω resistor in series with a 100Ω resistor is the same as having a 300Ω resistor.

 R1 + R2 + R3 + R4 = RT (Total resistance)

Parallel Connected Resistor

 When resistors are connected in parallel (parallel; meaning they are tied across one another) their combined resistance is less than any of the individual resistance.

                             

Resistor values are measured in ohms.  A thousand ohms is written as 1k to eliminate all the zeros.  The “K” represents three zeros.  A million ohms is represented by 1M. Therefore; 1000 ohms = 1K and 1000k ohms = 1M ohms. Since resistors are so small their value is marked by a color code.

Note that when two resistors of the same value are connected in parallel, the net resistance is half the value of an individual resistor e.g assume R1 = 100Ω and R2 = 100Ω, the net resistance RT becomes;

Series-Parallel Connected Resistor

 

Resistor Color Code

 

Resistors use color coded strips to indicate their value in ohms because of its small size. This color code however stands for digits that is usually used to indicate the value of a given resistor. Although resistors value can also be checked by using an ohmmeter or multimeter but this values varies because of the tolerances, temperature and so many factors that will hinder knowing the actual value of a given resistor.

DIGITS	DIGITS	MULTIPLIER	TOLERANCE
0	Black	1
1	Brown	10
2	Red	100
3	Orange	1000
4	Yellow	10000
5	Green	100000	0.05
6	Blue	1000000	0.25
7	Purple	10000000	0.1
8	Gray	100000000
9	White	1000000000
	Gold		5
	Silver		10
	None		20

Most resistors have four bands where the first band represents the digit, the second band also represent another digits, the third band becomes the multiplier, and the fourth band represent the tolerance.

Let us assume we have a resistor with a color code of brown, black, red and gold.

Brown = 1

Black = 0

Red = 2 = third band (multiplier) =100

Gold = 5% (tolerance)

So therefore the value of the resistor will be;

10×100 = 1000Ω

i.e. = 1KΩ

The major reason why using color code chart to indicate the value of  a given resistor is that color code chart gives the actual value of the reason while using meter will alter its actual value (resistor) . 

A given example is when trying to measure a 100ohms resistor using a multimeter set in ohms range. The meter will give a higher resistance of the order of 105ohms or above at lower temperature and might give a lower reading in the of 90 or below at higher temperature. So with this variation, the actual value can not be determined using a meter.

CAPACITORS

A capacitor is a device that stores electrical charge when a potential difference (voltage) exists between the two conductors which are usually two plates separated by a dielectric material (an insulating material like air, paper, or special chemicals between two sheets of aluminum foil). Capacitor blocks DC voltage and pass AC voltages. They are used as filters, AC coupling capacitors are used as by-pass capacitors. 

They are also used in conjunction with resistors and inductors to form tuned circuits and timing circuits. A capacitor value C (in farads) is dependent upon the ratio of the charge Q(in coulombs) divided by the V (in volts).

Common capacitors come in values of microfarads or picofarads. Often you will have to convert between pico farads and microfarads. You would have to look very hard to find a circuit which will not have a capacitor in it. They are one of the most fundamental passive components in a circuit. What makes capacitors special is their ability to store charge for a short period of time. Common application includes local energy storage, voltage spike suppression and complex signal filtering.

There are two special ways of representing a capacitor in a circuit. They always have two terminals which go to connect to the next of the circuit. The capacitor symbols consists of two parallel lines, which are either flat or curved; both lines should be parallel to each other, close, but not touching.

Charging and Discharging of a Capacitor

When positive and negative charges come together on the capacitor plates, the capacitor becomes charged. A capacitor can retain its electric field i.e. holds its charge because the positive and negative charges on each of the plates attract each other but never reach each other.

At some point the capacitor will be so full of charges that they just don’t accept anymore. There are enough negative charges on one plate that they can repel any others that try to join. This is where the capacitance (farads) of a capacitor comes into play which tells you the maximum amount of charge the capacitor can store. If a path in the circuit is created, which allows the charges to find another path to each other, they will leave the capacitor, and it will be discharged.

For example, in the circuit bellow, a battery can be used as the power supply source to induce potential (voltage) across the capacitor. This will cause equal but opposite charges to build up on each f the plates until they are so full that they repel any more current from flowing. An LED placed in series with the cap could provide a path for the current and the energy stored in the cap could be used to briefly illuminate the LED.