21-09-2013, 04:48 PM
Water level controller cum motor protector
Water level controller .doc (Size: 344.5 KB / Downloads: 50)
INTRODUCTION
Now a days , usage of overhead tank (OHT) with an electrically operated water pump is a common sight. The pump , being a costly item, should be protected against damage due to high and low voltages. People find it inconvenient to switch off the pump even when their OHT starts overflowing, specially when they are busy or it is raining. This circuit provides a soloution of all such problem, the main features of this circuit:
1. Low and high voltage cut off
2. automatic switching OFF of the motor when the OHT is full
3. Use of convenient push to on buttons for switching on and off of motor.
The heart of circuit is IC CD4011, which has four inverter gates. When the circuit gets +12V power supply, capacitor C1 pull input of N1 low, and this causes the output of N2 to go low. This state is latched by resistor R3 and transistor T1 is biased to cut off state, and hence the both relay RL1 and motor M are in off state.
When we push switch S1 momentarily, the input of inverter gate N1 becomes high and output of gate N2 also become high. As a result, transistor T1 turns ON and both relay RL1 and motor are activated (provided transistor T2 and T3 are forward biased). When water level in OHT touched the sensors input of N1 becomes low which turns relay RL1 off and motor stops. The motor can be turned off when only also by pushing switch S2 at any time. Transistor T2 and t3 are both forward biased if the line voltage is within certain low and high voltage limit, as explained below.
Theory of operation
Ohm's law
The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law:
Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) through it where the constant of proportionality is the resistance ®.
Equivalently, Ohm's law can be stated:
This formulation of Ohm's law states that, when a voltage (V) is maintained across a resistance ®, a current (I) will flow through the resistance.
This formulation is often used in practice. For example, if V is 12 volts and R is 400 ohms, a current of 12 / 400 = 0.03 amperes will flow through the resistance R.
Series and parallel resistors
Main article: Series and parallel circuits
Resistors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent resistance (Req):
The parallel property can be represented in equations by two vertical lines "|" (as in geometry) to simplify equations. For two resistors,
The current through resistors in series stays the same, but the voltage across each resistor can be different. The sum of the potential differences (voltage) is equal to the total voltage.
To find their total resistance:
A resistor network that is a combination of parallel and series can be broken up into smaller parts that are either one or the other. For instance,
However, many resistor networks cannot be split up in this way. Consider a cube, each edge of which has been replaced by a resistor. For example, determining the resistance between two opposite vertices requires additional transforms, such as the Y-Δ transform, or else matrix methods must be used for the general case. However, if all twelve resistors are equal, the corner-to-corner resistance is 5⁄6 of any one of them.
Resistor marking
Most axial resistors use a pattern of colored stripes to indicate resistance. Surface-mount resistors are marked numerically, if they are big enough to permit marking; more-recent small sizes are impractical to mark. Cases are usually tan, brown, blue, or green, though other colors are occasionally found such as dark red or dark gray.
Early 20th century resistors, essentially uninsulated, were dipped in paint to cover their entire body for color coding. A second color of paint was applied to one end of the element, and a color dot (or band) in the middle provided the third digit. The rule was "body, tip, dot", providing two significant digits for value and the decimal multiplier, in that sequence. Default tolerance was ±20%. Closer-tolerance resistors had silver (±10%) or gold-colored (±5%) paint on the other end.
Four-band resistors
Four-band identification is the most commonly used color-coding scheme on resistors. It consists of four colored bands that are painted around the body of the resistor. The first two bands encode the first two significant digits of the resistance value, the third is a power-of-ten multiplier or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. The first three bands are equally spaced along the resistor; the spacing to the fourth band is wider. Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the true 5-color system, with 3 significant digits.
CAPACITORS
A capacitor (formerly known as condenser) is a passive electronic component consisting of a pair of conductors separated by a dielectric (insulator). When there is a potential difference (voltage) across the conductors a static electric field develops in the dielectric that stores energy and produces a mechanical force between the conductors. An ideal capacitor is characterized by a single constant value, capacitance, measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them.
Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass, in filter networks, for smoothing the output of power supplies, in the resonant circuits that tune radios to particular frequencies and for many other purposes.
The effect is greatest when there is a narrow separation between large areas of conductor, hence capacitor conductors are often called "plates", referring to an early means of construction. In practice the dielectric between the plates passes a small amount of leakage current and also has an electric field strength limit, resulting in a breakdown voltage, while the conductors and leads introduce an equivalent series resistance
Capacitance instability
The capacitance of certain capacitors decreases as the component ages. In ceramic capacitors, this is caused by degradation of the dielectric. The type of dielectric and the ambient operating and storage temperatures are the most significant aging factors, while the operating voltage has a smaller effect. The aging process may be reversed by heating the component above the Curie point. Aging is fastest near the beginning of life of the component, and the device stabilizes over time.[20] Electrolytic capacitors age as the electrolyte evaporates. In contrast with ceramic capacitors, this occurs towards the end of life of the component.
Temperature dependence of capacitance is usually expressed in parts per million (ppm) per °C. It can usually be taken as a broadly linear function but can be noticeably non-linear at the temperature extremes. The temperature coefficient can be either positive or negative, sometimes even amongst different samples of the same type. In other words, the spread in the range of temperature coefficients can encompass zero. See the data sheet in the leakage current section above for an example.