## RESISTOR COMBINATIONS

• When we do not get specific resistor values we have to either use variable resistors such as potentiometers or presets to obtain such precise values. Pots are too expensive to use forevery case.
• Another scheme is to combine two or more resistors to obtain the necessary precise values.Such resistor combinations can cost as little as 50p or so only.
• Then the question arises as to how one should combine these resistors, because, they can be combined in two different ways.
• These are called “Series” and “Parallel” combinations.

## Series Combinations

R Total= R1+ R2

• Calculating values for two or more resistors in series is simple, add all the values up.
• The connection ensures that the SAME current flows through all resistors.
• In this type of connection RT will always be GREATER than any of the included resistors.

Even if we have more than two resistors the total resistance is the sum of all the resistors connected in series:

R Total= R1+ R2+ R3 +•••••

• Total Applied voltage is divided by two resistors
• Current in the circuit is  I = V/(R1+R2)
• Voltage across R1 and R2 are from OHMS law.

V1= I*R1

V2=I* R2

Total voltage V=V1+V2

For Example if V=12v and the 2 resistors are 1k each, then the current in the circuit is

12/2k=6mA

The voltage across each resistor is 6v

Thus the series combination is characterized by

• The same current flows through all the resistors connected in series.
• The resultant resistor is SUM of all the resistors in series
• Series resistors divide the total voltage proportional to their magnitude.

## Resistors in Parallel

In Parallel combination, 2 paths are available for current, hence the current divides but the voltage across the resistors is same.

1/R total =1/R1 + 1/R2   or

R total = (R1*R2) / R1 + R2

• If the two resistors are equal, the current will divide equally and total resistance will be exactly half.
• For example if voltage is 12v and there are 2 resistance for 1k each,

The current through each resistance will be 12v/1k= 12 mA. Hence the total current is 12 mA.

Effective resistance is 0.5k

Thus the parallel connection is characterized by

• The same voltage exists across all the resistors connected in parallel, and
• The reciprocal of resultant resistor is the sum of reciprocals of all resistors in parallel, and
• Parallel resistors divide the total current in an inverse proportion to their magnitude.

## Potential Divider

Since series resistors divide voltage, this idea can be used to get smaller voltage from a power supply output. For example, we have a power supply with 10V fixed output. But we want only 5V from it.

Vout= Vin(R2/(R+R2))

• The Current I=Vin/R1+R2
• Since the current I flows through R2, voltage developed across it from Ohm’s law is Vo=I*R2=( Vin/R1+R2) * R2                                                                                                   Vo = (R2/R1+R2) Vi
• If R1=R2, then Vo=Vi/2

R1 and R2 cab be 100k or 100 ohm. Which one to be used?

If we need more current through load then R1 must be small. But too small a value will cause energy drain on the power supply. So the value must be chosen very carefully.

### Note:

• When two resistors are in parallel then their overall power rating is increased.
• If both resistors are the same value and same power rating, then the total power rating is doubled. If parallel resistances are not equal, then the resistors with smaller values will be required to handle more power.
• Four identical 0.25W resistors can be wired in parallel to give a resistor with one fourth the value in ohms, but four times the power rating. (1.0W). This is most useful when we require higher power handling, but don’t want to go out and buy more expensive (and physically larger) resistors.
• We have already seen earlier, that the power (in watts) can be calculated by multiplying voltage by current. P=V * I
• By using ohms law, the parallel or series resistor formulas and the above formula, a minimum power rating for a certain resistor can be calculated. If this is exceeded the resistor is likely to get hot and hopefully quietly breakdown.

## 10 Stage LED Sequencer

### 10 Stage LED Sequencer

Components

• IC1- CD4017
• IC2- NE555
• C1 – 1μ
• C2- 0.01 μ
• R1 – 470 Ω
• R2 – 100 KΩ
• R3- 100 Ω
• LED1-10 – RED LED
• 9volt DC power supply.

For power supply you can use 9 volt battery or can design separate power supply using step down transformer and 1N4007 diodes.

## Variable DC Power Supply using LM317

Below is the circuit for variable dc power supply

R1- 240 Ω

R2- 5K VR

R2 can be replaced by fixed value resistor for fixed power supply. Following formula can be used to calculate output voltage.

Vo=1.25(1+R2/R1)

Output voltage should be2 voltage greater than input.

Parts for current setup

•  D1, D2 – 1N4001
• C1-0.1 μ
• C2 – 10 μ 50v
• voltage regulator – LM317

## 20 Watt Inverter

This circuit will drive a 40 watt fluorescent or two 20-watt tubes in series. The transformer is wound on a ferrite rod 10mm dia and 8cm long. The wire diameters is 0.61mm wire for the primary and 0.28mm wire for the secondary and feedback winding.

The circuit will take approx. 1.5amp on 12v, making it more efficient than running the tubes from the mains. A normal fluorescent takes 20 watts for the tube and about 15 watts for the ballast.

Note: Do not remove the tube when the circuit is operating as the spikes produced by the transformer will damage the transistor.

Parts list

• Transistor – BC338 and TIP 3055
• Resistance – 47 K, 47 R, 180 R, 2R2
• Variable Resistance – 100k
• Capacitors – 100u 16v, 100n
• On/Off Switch
•  1 ferrite rod 10mm in 8mm long
• 30 m winding wire .28mm dia
• 4 m winding wire .61mm dia
• 2* 20 watt tube or 1* 40 watt tube
• 12 v DC power supply

## Lamp Dimmer

### 12v LAMP Dimmer

Parts

• IC 1 – NE555
• Transistor – 2N2955 -1
• Resistance – 1k(2 no.), 100 Ω
• Variable Resistance – 50k
• Capacitor – 0.1 µF
• Diode – 1N4001 – 3
•  12v 2 amp Bulb

Input Voltage is 12v. To create your own bench top power supply use the circuit shown in

http://digitalab.org/2012/06/regulated-dc-power-supply-circuit/

## LED Basics

Today LED has become an integral part of consumer electronics.

LED TV, LED Display, LED Lights and so on. These are becoming very popular because of there low power consumption.

What is LED?
LED stands for Light emitting diode.

A light emitting diode is essentially a PN junction semiconductor diode that emits a monochromatic(single) colour light when operated in a forward biased direction.

For detail in technical evolution refer the following url

http://en.wikipedia.org/wiki/Light-emitting_diode

Early LEDs were only bright enough to be used as indicators, or in the displays of early calculators and digital watches. More recently they have been starting to appear in higher brightness applications.

## LED Basics – Characteristics voltage drop

When a LED is connected around the correct way in a circuit it develops a voltage across

it called the CHARACTERISTIC VOLTAGE DROP. A LED must be supplied with a voltage that is higher than its “CHARACTERISTIC VOLTAGE”  via a resistor – called a VOLTAGE DROPPING RESISTOR or CURRENT LIMITING RESISTOR

How LED works?

LED and resistor are placed in series and connected to a voltage.As the voltage rises from 0v, nothing happens until the voltage reaches about 1.7v. At this voltage a red LED just starts to glow. As the voltage increases, the voltage across the LED remains at 1.7v but the current through the LED increases and it gets brighter. As the current increases to 5mA, 10mA, 15mA, 20mA the brightness will increase and at 25mA, it will be a maximum.

This is just a simple example as each LED has a different CHARACTERISTIC VOLTAGE DROP and a different maximum current.

In the diagram below we see a LED on a 3v supply, 9v supply and 12v supply. The current-limiting resistors are different and the first circuit takes 6mA, the second takes 15mA and the third takes 31mA. But the voltage across the red LED is the same in all cases.

## LED Basics – Head Voltage

As the supply-voltage increases, the voltage across the LED will be constant at 1.7v (for a red LED) and the excess voltage will be dropped across the resistor. The supply can be any voltage from 2v to 12 or more. The resistor will drop 0.3v to 10.3v. This is called HEAD VOLTAGE.

The voltage dropped across this resistor, combined with the current, constitutes wasted energy and should be kept to a minimum.

Most supplies are derived from batteries and the voltage will drop as the cells are used.

Here is an example of a problem:
Supply voltage: 12v
7 red LEDs in series = 11.9v
Dropper resistor = 0.1v
As soon as the supply drops to 11.8v, no LEDs will be illuminated.

Example 2:
Supply voltage 12v
5 green LEDs in series @ 2.1v = 10.5v
Dropper resistor = 1.5v
The battery voltage can drop to 10.5v
Suppose the current @ 12v = 25mA.
As the voltage drops, the current will drop.
At 11.5v, the current will be 17mA
At 11v, the current will be 9mA
At 10.5v, the current will be zero

Many batteries drop 1v and still have over 80% of their energy remaining. That’s why you should design your circuit to have a large HEAD VOLTAGE.

Some Basic circuits using LED

1. Polarity Tester

2. Continuity Tester

## Regulated DC Power Supply Circuit

Below is the regulated DC power supply( 12, 8, 5 v) circuit. This circuit can be customized by adding 9v and 6v voltage regulator.

Also, the 4 diodes can be replaced by bridge rectifier.

If you are looking for a variable power supply then click here

## Basic Power Supply Circuits – Part 2

Simplest DC power supply circuit using 78XX is shown below.

Parts

D1, D2, D3, D4 – 1N4007

IC1 – 7812

IC2 – 7805

C1 – 1000 mf

C2 – 330 mf

C3 – 10 mf

T1 – 12 v Step down transformer, 1 A.

## Basic Power supply circuits Part 1

To understand power supply circuits, we need to understand rectifiers.

A rectifier converts AC or alternating current to DC direct current. Rectifiers are used in Power supply circuits which we will discuss in detail.

The process of converting AC to DC is called Rectification.

### Half wave rectification.

It requires only single diode. Only positive cycle of the current is passed through the diode i.e only half of the AC wave is passed and hence the name half wave.

### Full Wave rectification

It requires 2 diodes or 4 diodes. In this both positive and negative AC cycle is passed through the diode alternatively.

### Bridge Rectifier

Bridge rectifier also produces same output as full wave rectifier.

The four diodes D1-D4 are arranged in series pair with only 2 diodes conducting current during each positive half cycle.
During the positive half cycle of the supply, diode D1 and D2 conduct in series while diodes D3 and D4 are reverse biased.

During the negative half cycle of the supply, diode D3 and D4 conduct while D1 and D2 are reverse biased.

In the next article I will show how to use rectifiers to build DC power supply circuits.

## Basic Electronics – 2

Passive Components

### RESISTORS

To oppose the flow of electrons ( current). The symbols are shown below.

Resistance is measured in units called “Ohm”. 1000 ohms is shown as 1k ohm (103 ohm) and 1000 k ohm is shown as M.ohms (106ohm).

Resistors can be broadly of two types.

• Fixed Resistors and Variable Resistors.

### Fixed Resistors:

Carbon Film (5%, 10% tolerance) and Metal Film Resistors (1%,2% tolerances) and wire wound

resistors. A fixed resistor is one for which the value of its resistance is specified and cannot be varied in general.

### Resistance Value

The resistance value is displayed using the color code ( the colored bars/the colored stripes), because the average resistor is too small to have the value printed on it with numbers. The resistance value is a discrete value.

For example, the values [1], [2.2], [4.7] and [10] are used in a typical situation.

### CARBON FILM RESISTORS

This is the most general purpose, cheap resistor. Usually the tolerance of the resistance value is ±5%. Power ratings of 1/8W, 1/4W and 1/2W are frequently used. The disadvantage of using carbon film resistors is that they tend to be electrically noisy.

### METAL FILM RESISTORS

Metal film resistors are used when a higher tolerance (more accurate value) is needed. Nichrome(Ni-Cr) is generally used for the material of resistor. They are much more accurate in value than carbon

film resistors. They have about ±0.05% tolerance.

### OTHER RESISTORS

There is another type of resistor called the wire wound resistor. A wire wound resistor is made of metal

resistance wire, and because of this, they can be manufactured to precise values. Also, high-wattage resistors can be made by using a thick wire material. Wire wound resistors cannot be used for high-frequency circuits.

### Ceramic Resistor

Another type of resistor is the Ceramic resistor. These are wire wound resistors in a ceramic case, strengthened with a special cement. They have very high power ratings, from 1 or 2 watts to dozens of watts. These resistors can become extremely hot when used for high power applications, and this must be taken into account when designing the circuit.

### SINGLE-IN LINE NETWORK RESISTORS

It is made with many resistors of the same value, all in one package. One side of each resistor is connected with one side of all the other resistors inside. One example of its use would be to control the current in a circuit powering many light emitting diodes (LEDs). The face value of the resistance is printed.

### 4S-RESISTOR NETWORK

The 4S indicates that the package contains 4 independent resistors that are not wired together inside. The housing has eight leads instead of nine.

### VARIABLE RESISTORS

There are two general ways in which variable resistors are used. One is the variable resistor whose value is easily changed, like the volume adjustment of Radio. The other is semi-fixed resistor that is not meant to be adjusted by anyone but a technician. It is used to adjust the operating condition of the circuit by the technician.

Semi-fixed resistors are used to compensate for the inaccuracies of the resistors, and to fine-tune a circuit. The rotation angle of the variable resistor is usually about 300 degrees. Some variable resistors must be turned many times( multi-turn Pot) to use the whole range of resistance they offer.

This allows for very precise adjustments of their value. These are called “Potentiometers” or “Trimmer Potentiometers” or “presets”.

### LIGHT DEPENDENT RESISTANCE (LDR)

Some components can change resistance value by changes in the amount of light falling on them. One type is the Cadmium Sulfide Photocell. It is a kind of resistor, whose value depends on the amount of light falling on it. When in darkness its resistance if very large and as more and more light falls on it its resistance becomes smaller and smaller.

There are many types of these devices. They vary according to light sensitivity, size,  resistance value etc.

### THERMISTOR

They are thermally sensitive resistor. The resistance value of the thermistor changes according to temperature. They are used as a temperature sensor. There are generally two types of thermistors, with Negative Temperature Coefficient(NTC) Positive Temperature Coefficient(PTC). The resistance of NTC thermistors decreases on heating while that of PTC thermistors increases.

### ELECTRIC POWER RATING

For example, to power a 3V circuit using a 12V supply, using only a resistor, then we need to calculate the power rating of the resistor as well as the resistance value. The current consumed by the 5V circuit needs to be known.

Assume the current consumed is 250 mA (milliamps) in the above example. That means 9V (=12-3 V) must be dropped with the resistor. The resistance value of the resistor becomes 9V / 0.25A = 36(ohm).

The consumption of electric power for this resistor becomes 0.25A x 0.25A x 36ohm = 2.25W. Thus the selection of resistors depends on two factors namely tolerance and electric power ratings.

### OHM’S LAW

Important and useful law.The current(I) flowing through a conductor is proportional to the voltage (V) applied across its ends. This can be written in algebraic form as V ∝ I Or V = IR where R is the proportionality constant. R is called Resistance and is measured in ‘Ohms’ ( Ω ).

Usually resistors are also specified in circuits in kilo Ohms(kΩ) and Mega Ohms(MΩ). The other useful relationships are V = RI, and R=V/I.