Basic Human Cell Structure

Basic Cell Anatomy 

In human physiology, we are concerned with the specific characteristics and mechanisms of the human body that make it a living being, to understand the function of organs and other structures of the body, it is essential to know about the basic organization of the cell and its functions. The human body, composed of living tissues can be considered as a power station generating multiple electrical signals with two internal sources, namely muscles and nerves, most of the physiological process were accompanied with electrical changes. The discovery formed the basis of explanation of the action of living tissues in terms of bioelectrical potential. Bioelectric potentials are generated at a cellular level. All living matter is composed of cells of different types. The variation of Human cells may from 1 micron to 100 microns and having diameter from 1 mm to 1m in length. The membrane thickness is 0.1 micron. Generally muscular contraction is associated with the migration of ions generating potential differences measurable with suitably placed electrodes.
The basic living unit of the body is the cell. To understand the function of organs and other structures of the body, it is necessary to study the basic organization of the cell. Each organ of our body consists of an aggregate cells. The entire body contains 100 trillion cells, 25 trillion RBC, which transports oxygen from the lungs to the tissues. The oxygen combines with carbohydrate, fat or protein to release the energy required for cell function.

Typical human Cell

Each cell consists of centrally located nucleus surrounded by cytoplasm (cell body). The nucleus is separated from the cytoplasm by a nuclear membrane and the cytoplasm is separated from the surrounding fluids by a cell membrane.


The different substances that makeup the cell collectively called protoplasm - composed of water, electrolytes, proteins, lipids and carbohydrates. Water is the principal fluid medium of the cell and its concentration is between 70 and 85 percent. The electrolytes present in the cell are potassium, magnesium, phosphate, sulphate, bicarbonate and small quantities sodium, calcium and chloride. The electrolyte provide in organic chemicals for cellular functions. (reactins proteins which constitute 10 to 20% of the cell mass are divided into structural proteins and globular proteins (enzymes). Proteins are in the form of long thin filaments composed of many proteins molecular. The globular proteins are in global form. These are mainly enzymes which catalyst the chemical reactions which provide energy for cellular function. Lipids are composed of different types of substances. They are soluble in fat solvents and insoluble in water. Phospholipids are cholester of important lipids. Carbohydrates play a major role in nutrition of the cell. They are stored in the form of glycogen. These are used to supply the cells energy.

Organelles present in the cell contain cell's chemical constituents. The cytoplasm is filled with cytosol, in which the minute and large particles and organelles are dispersed. Ribosomes are minute granular particles in the cytosol and composed of RNA (Ribonucleic Acid). Lysosomes are vericular granular and provide intracellular digestive systems that allow the cell to digest and remove bacteria. The Mitochondria organelles are called ‘Power house' of the cell. The mitochondria contain DNA (Deoxyribonucleic acid). DNA is the basic substance of the nucleus, So it is called as control centre of the cell. It controls replication. Nucleus contain large amount of DNA called genes. The gene first reproduce themselves and the cell splits by a special process called mitosis to form two daughter cells. The nucleus is surrounded by nuclear membrane. Inside the nucleus-nucleolus is present and contains large amount of RNA and Proteins. The cell size is determined by the amount of functioning DNA in the nucleus and 5-10 µm. For large amount of DNA, the cell size is larger. DNA grows due to the increased production of RNA and cell proteins.

Types of Resistors - Advantages and Disadvantages


Basically resistors are divided into two types namely fixed resistors and variable resistors. These two types are further classified into different types as shown below.
Types of Resistors

I. FIXED RESISTORS:


Resistors, whose ohmic values remain fixed at a constant value, are known as fixed resistors.

1. Carbon Resistors:


Carbon resistors are made of carbon. These are of two types:

(i) Carbon-composition resistors
(ii) Carbon film or cracked carbon or pyrolytic resistors.

(i) Carbon-Composition Resistors:

The carbon-composition resistors are made of finely divided carbon or graphite mixed with a resin binder in suitable proportion needed for the desired resistance value.
These resistors are available in resistance values of 1Ω to 20 MΩ and power ratings of 1/10, 1/8 , ¼, ½, 1, 2 watts.

Advantages:

• Carbon-composition resistors are small in size compared with wire-wound resistors.
• A very wide resistance range is available.
• These are the cheapest resistors.
• These have good RF performance.

Disadvantages:

• Carbon-composition resistors have no precision, and have very high tolerance.
• These resistors get easily heated and crack down on soldering.
• The resistance values of these resistors vary with aging.
• They are not useful for applications involving power levels above 5 watts.

Applications:

Carbon composition resistors are used for all general purpose circuits including entertainment (Radio, T.V).



(ii) Carbon Film Resistors:

Carbon film resistors are manufactured by depositing very thin film of carbon on to a substrate of ceramic or glass tube. Depending on the thickness of the film we have thin film (< 5 μm) resistors and thick-film (> 5 μm) resistors.

Advantages:

• Carbon film resistors are available in all resistor values, from very low values (< 1Ω) to many MΩ.
• They are available in very miniature size
• They may also be used as resistors in ICs.
• They can replace wire wound resistors in high-voltage applications.
• Carbon-film resistors have good high-frequency properties.
• They cost low.

Disadvantages:

• Carbon film resistors cannot withstand high temperatures.
• They cannot with stand to mechanical shocks.
• They cannot with stand to atmospheric moisture and humidity.
• They are chemically reactive and hence unstable.

Applications:

These are used in good high frequency performance and stability circuits such as computers, telephone circuits and high fidelity amplifiers.

2. Wire Wound Resistors :


Wire wound resistors are constructed from a long fine wire (usually nickel-chromium wire) wound on a ceramic core. Wire wound resistors are manufactured in two types

(i) power wire wound resistors
(ii) precision wire wound resistors.

Power wire wound resistors are available in low, medium and high power types as : 3, 5, 10 Watts (low); 10 to 60 W (medium), 60-1000 watts (high power).

Precision wire wound resistors are wound on ceramic tubes and sometimes epoxy moulded tubes are used. The winding is so done so as to minimize inductive and capacitive effects at high frequencies (beyond 200 kHz). These precision wire wound resistors are available up to 5 W with ½ % and 1% tolerances, and are useful upto 5 to 10 MHz.

Advantages:

• Wire wound resistors can be designed to produce very accurate resistance values, with very low tolerance (± 0.01%).
• They can with stand large power dissipation.
• They can be used in high-temperature situations.
• They are capable of carrying extremely large currents.
• Wire-wound resistors can with stand mechanical shock and vibration.
• They can be used in high-voltage circuits.
• Wire-wound resistors have very stable resistance values which do not change much with aging.

Disadvantages:

• Wire wound resistors are very large in size and weight.
• They are very costly.
• Power type wire wound resistors are not suitable beyond 200 kHz because of interwinding inductance and the capacitance.
• In many situations, wires can break, leading to the breakdown of the circuit in which these resistors are used.

Applications:

Ordinary type wire wound resistors are used in power supplies control circuits, as loads in Television receivers. Precision type wire wound resistors are used in bridges, voltmeters and other instruments.

Comparison of Carbon and Wire Wound Resistors:

S.No
Feature
Carbon Resistor
Wire wound Resistor
1.
Size
Small
Big
2.
Power rating
Low
High
3.
Tolerance
High
Low
4.
Applications
Carbon composition resistors are used for all general purpose circuits including entertainment (Radio, T.V). Carbon film resistors are used in computers, telephone circuits and high fidelity amplifiers.
Ordinary types are used in power supplies Control circuits, as loads in T.V. Receivers. Precision types are used in bridges, voltmeters and other instruments.


II. VARIABLE RESISTORS:


Variable resistors are the resistors whose resistance value can be varied. In some electrical/electronic circuits sometimes it is necessary to change the values of currents and voltages. For example it is often necessary to change the volume of sound and brightness in T.V, volume of sound and tone in radios and to regulate the speed of a fan. Such adjustments can be done by using variable resistors. Different types of variable resistors were mentioned in the classification of resistors.

Potentiometer:

The smaller variable resistors commonly used in electronic circuits are called potentiometers simply called as pots.

As shown in Figure, it is a three terminal variable resistor. The arrow indicates the movable contact on the resistive element. The position of the movable contact determines the resistance value in the circuit. Based on the material used for construction, potentiometers are classified into two types,

1. Carbon potentiometers and
2. Wire wound potentiometers.


Colour Coding of Resistors

Resistors are made in different sizes and shapes. Some resistors are large enough in size to have their value printed on the body. However there are some resistors that are too small in size to have numbers printed on them. Therefore colour coding is used.

Colour coding is the process in which colours are used to indicate numerical values. The use of bands or stripes is the most common system for colour coding the fixed resistors.

The colour coding for carbon resistors with axial leads is shown in Figure. Moulded carbon resistors have four colour bands printed on one end of the outer casing.
Moulded Carbon Composition Resistor
The colour bands are read from left to right from the end which has the bands closest to it as shown in Figure. The first band indicates the first significant digit and second band indicates second digit while the third band indicates decimal multiplier or number of zeros followed by first two digits. The fourth band indicates the tolerance – the amount by which the actual resistance R can be different from the printed value. Absence of fourth band means the tolerance is ±20%. The table gives the colour code for resisters.

Table - Colour Code:

Colour
Digit
Multiplier
Tolerance
Black
0
100 = 1

Brown
1
101 = 10
±1%
Red
2
102
±2%
Orange
3
103
-
Yellow
4
104
-
Green
5
105

Blue
6
106

Violet
7
107

Gray
8
108

White
9
109

Gold
-
10-1
±5%
Silver
-
10-2
±10%
No colour
-
-
±20%
Pink colour
-
-
High Stability

From the table it can be noted that the darkest colours indicate lowest numbers and lighter colours indicate highest numbers.

The colour coding for wire wound resistors with axial leads is sown in Figure. Note that the first band is double the width for wire wound resistors compared to the rest of the bands. Colour coding is same as that of carbon composition resistors.

Film resistors will have five strips instead of four. In this case the first three bands give first three digits, fourth band gives the multiplier and the fifth band gives the tolerance. These resistors have more precise values with lower tolerance values of ± 0.1 to ± 2 percent.

Resistors below 10Ω will have a third strip of gold or silver which have 0.1 or 0.01 multiplier as shown in Table.

Body End Dot System:

This system is discontinued standard. The numerical values associated with each colour are the same as that of the axial lead resistors.

In these resistors body colour indicates the first digit, one tip colour indicates the second digit, dot colour indicates the number of zeroes or the multiplier and the other tip indicates the tolerance if any.

Resistors - Specifications and Properties

A resistor is the most basic electronic component invariably found almost every electronic circuit. It is a passive element. The most important function of resistor is to resist the flow of current. The property of a resistor is known as resistance.

A resistor can be used as load, as potential divider and also as a biasing element in different circuits. It also acts as filter and timer in combination with capacitor. Resistors are basically available in two types viz., fixed resistors and variable resistors. Now a days resistors are available in a large variety of types and sizes, each type has specific advantages, disadvantages and applications.

Such variety types of resistors are shown in figure.




SPECIFICATIONS OF RESISTORS:


All resistors will have three main specifications that are to be considered. They are

1. Resistance value
2. Tolerance
3. Power rating

1. Resistance Value : It gives the value of resistor R in ohms. It's value is either printed or colour coded over the body depending upon the type of resistor. In general resistors from 1Ω to many MΩ are available.

2. Tolerance : It gives the variation of resistance value from the indicated value. It is generally expressed in percentage. It's typical values are ranging from ±1% to ±20%. Resistors with low tolerance values are preferred.

3. Power Rating : The power rating of a resistor is given by the maximum wattage. The resistor can dissipate without excessive heat. It is expressed in watts. The resistors with power ratings ranging from 0.1 watts to hundreds of watts are available. The power rating depends on the size of the resistor. Since it is current which produces heat, power rating also gives some indication of the maximum current a resistor can safely carry. However there are some other specifications that are to be considered while selecting a resistor. They are



4. Temperature Coefficient of Resistance : It gives the variation of resistance with a change in temperature. It is usually measured with reference to resistance at 25°C.

5. Voltage Coefficient : It is measured as the change of resistance of a resistor with a change in the applied voltage.

6. High Frequency Performance : Even though resistors are insensitive to frequency but at higher frequencies some factors like distributed capacitances in carbon resistors and inductive reactances in wire wound resistors are become dominant causing a change in resistance. Generally the resistors with lower resistance values will have better high frequency performance.

7. Noise Figure : When a d.c current is passed through a resistor R the voltage drop across the resistor is not only Ids. R, but it is superimposed by fluctuations called noise. Noise in resistors is of two types (1) Thermal agitation noise and (2) Current noise. Wire wound resistors exhibit little current noise when compared with carbon composition resistor.

8. Stability: A resistor under test is said to be more stable, if it is used for a long period under atmospheric condition, and its value measured at room temperature is nearer to it's initial measured value. carbon film, metal film and wire wound resistors are more stable than carbon composition resistors.

9. Size, Shape and Leads : Resistors are available in different sizes (small, big) and shapes (rod, disc, washer) with different types (axial, radial, lug) of leads or terminals. The selection of particular type of resistor depends on the requirement.

PROPERTIES OF RESISTANCE:


A resistor is a passive electronic component that offers a specific amount of electrical resistance to the flow of current when connected in a circuit. The property of a resistor is "resistance", it is defined as the "opposition to the flow of current". This opposition comes from the electrons present in the atom of material. In materials like copper, the electrons are more free to move compared to the electrons in a material like plastic. In other word copper has less resistance than plastic. Resistance is denoted by the letter 'R'.

The unit of resistance is the 'Ohm', its symbol is '01. It is defined as the resistance between the two ends of a conductor.

According to Ohm's law the resistance of a conductor is said to be one Ohm; when a constant current of one ampere flows through it for an applied potential difference of one volt.
In other words the resistance 'R' can be defined as the ratio of voltage applied to the current flowing through the conductor.

Mathematically it can be given as R = V / I

Where R = Resistance of the conductor in Ohms
V = Voltage applied in Volts.
I = Current in amperes.

The typical values of resistance can be varied from 0.1 ohm to several hundreds of ohms.

Dual Slope Converter Type of DVM

DUAL SLOPE CONVERTER TYPE OF DVM :


The block diagram of dual slope Digital Voltmeter (DVM) is shown in Figure

Description and Working of the Block Diagram:

The input chopper applies the input signal voltage to the integrator for a fixed time t1 to t2. During this time the integrator's output raises linearly as the capacitor charges. At the end of the charge period the integrator is disconnected from the input voltage and is connected to the discharging circuit.

The integrator capacitor discharges linearly during the period t2 - t3. During the total period of t1 to t3, the output of the zero crossing detector is positive. When the voltage on the integrator capacitor reaches zero the output of the zero crossing detector falls to zero. This is used to control the clock pulses to the counter.
The operation is effected by the control logic block shown in the block diagram. Before each measuring period the counter and control logic are reset by a pulse from the reset oscillator. Negative voltage is also measured in the same way. The difference is that the output of zero crossing detector is negative. This is sensed by the polarity detector and it is used to reverse the polarity of the constant current from the discharge circuit. The drift compensator circuit nulls out the effect of zero drift in the integrator and zero crossing detector. 
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CHOOSING AN ELECTRONIC VOLT-METER 


The following factors are to be considered in choosing an electronic voltmeter.

1. Input impedance: The input impedance must be as large as possible. This prevents the loading effect.

2. The ranges: The ranges on the meter scale can be in 2-3-10 sequence with 10 dB separation or in the 1.5-5-15 sequence. The readings on the scale must be compatible with the accuracy of the meter. A mirror below the dial of the meter is preferred when the accuracy is 1% or less.

3. The meter must be provided with a decibel scale using the standard reference level. A portion of the dial of the meter must indicate the additions required on the other scales while measuring dB.

4. The sensitivity: The sensitivity and band width are inversely proportional. High sensitivity gives low band width. Low sensitivity gives large bandwidth. Typically a voltmeter with 1 mV sensitivity may have its bandwidth of 10 MHz.
A voltmeter with a sensitivity of 100 mV may have its bandwidth as 5 MHz only. Selection is to be done with a compromise between sensitivity and bandwidth.

5. Power supply requirements: Battery operation is preferred for field work. However when power is available at site, and in laboratories mains operation is economical. Hence provision is to be made to have both mains and battery operations. A quick change over facility from mains to battery and vice versa is essential.


Single Slope (RAMP) Converter


(a) Description of the Block Diagram:


The block diagram of a single slope converter type voltmeter is shown in Figure. We find the capacitor, constant current source, and the comparator in the block diagram. An oscillator, gate, counter, decoder and driver for the display are arranged as shown in the block diagram. Two switches S1 and S2 are used.
Block Diagram of Single Slope Digital Voltmeter


(b) Working:


The voltage comparator derives its input from the charge developed across the capacitor. The capacitor is charged from a constant current source. The other input to voltage comparator is from the unknown voltage. In the block diagram they are shown as Ec (voltage across the capacitor) and Ex (unknown voltage).

The voltage comparator converts the analog information to digital signal, at its output. If the voltage at non-inverting input of the comparator is greater than the input at inverting input, the output will be high i.e. ' 1 '. If the voltage at non-inverting input is lower than inverting input the output of the comparator will be low i.e. ‘0’.

The output of the comparator is given to one of the three inputs of the AND gate. The output of a known frequency oscillator is applied to the second input of the gate. To the third input of the gate a switch is connected S2. S2 is an SPDT switch and can apply either a high level signal i.e. a logic 1 or a low level signal i.e. a logic '0' to the third input of the gate.



When the output of the comparator is ‘1’ and switch S2 is at high level, the oscillator signal is allowed to the counter and associated circuits. If the output of the comparator is '0', and S2 is applying a low level signal the signal from the oscillator cannot reach the counter.

Initially we require the counter to be reset to zero. To effect resetting of the counter the switch S1, is to be closed shorting the capacitor and keeping S2, at zero.

The measurement cycle can be started by opening the switch S1, and keeping switch S2 to logic ‘1' position. Now the situation at the input of the comparator is that the unknown voltage is greater than the voltage across the capacitor. As unknown voltage is at the non-inverting input and the capacitor voltage is at inverting input of the comparator the output of the comparator will be logic ‘1'. As S2, is at logic ‘1’ and the comparator's output is logic ‘1', the gate allows the oscillator output to reach the counter.

Hence the counter accumulates the counts. While count is going on, the voltage across the capacitor goes on increasing. This will be presenting an increasing voltage at the inverting input of the comparator. When this voltage of the capacitor increases and equals the value of the unknown voltage the comparator changes its output at the instant the capacitor voltage is just more than the input voltage. That is the output of the comparator changes from ‘1' to '0'. This prevents the signal flow from the oscillator through the gate to the counter circuit.

The counts accumulated in the counter represent the time the gate was open. This time is directly and linearly proportional to the unknown voltage. Finally we have to modify the charge rate of the constant current source or to adjust the oscillator frequency such that the accumulated value in the counter is not only proportional to the unknown voltage, but also a voltage of 1000 V produces exactly 1000 counts. If this is done the display gives the input voltage.

(c) Limitations of the Single Slope Converter Type Digital Volt Meter:


1. It measures only single polarity voltages.
2. Additional circuits are needed to detect input voltages greater than maximum charging voltage of the capacitor. That is called over ranging circuit.
3. It suffers from long term errors.
4. It is effected by the frequency drift of the oscillator.
5. It is effected by the drift of the output current of constant current source.
6. The accuracy is dependent on the stability of the capacitor.
7. The accuracy is dependent on the stability of differential voltage required to trip the comparator.
8. The converter is effected by the noise pulses of the input voltage.
9. The accuracy depends on the linearity of charging of the capacitor.

(d) Applications of Single Slope Converter Type Digital Volt Meters:


In spite of the limitations single slope converter type of digital voltmeters are used in low cost Digital Volt Meters.