EN6000 Electrical Fundamentals PDF

EN6000 Electrical Fundamentals Electrical components - Resistors 1 EN6000 Electrical Fundamentals 2  EN6000 Electrical Fundamentals Resistors. In electrical and electronics it is often necessary to limit the amou...

EN6000 Electrical Fundamentals Electrical components - Resistors 1 EN6000 Electrical Fundamentals 2  EN6000 Electrical Fundamentals Resistors. In electrical and electronics it is often necessary to limit the amount of current flowing to a part of a circuit or to drop voltage a certain points, this can be done with the aid of resistors. Resistors can be divided into three different groups :- Fixed linear resistors Non linear resistors Variable resistors Fixed Linear Resistors. Fixed linear resistors are constructed differently Carbon composite. The carbon composite resistor is constructed by moulding mixtures of powdered carbon and insulating materials into a cylindrical shape( normally plastic or a paint case ). The outer sheath of insulating material gives the resistor a measure of mechanical an electrical protection, while copper connecting wires are provided at each end for connection. By changing the mixture of carbon powder and insulating material we can vary the resistance value for a given size of resistor. 3  EN6000 Electrical Fundamentals Carbon film resistors. The carbon film resistors are constructed by depositing a thin layer of resistive material on a ceramic body which is then cut to form a helix or spiral. Carbon film resistors have a lower tolerance than carbon composite resistors they are, though, a little more expensive. Metal film resistors. Metal film resistors are constructed by spraying a thin film of metal on to a ceramic body until the desired resistance has been obtained. Metal contacts or leads are attached to both ends for connections. The power rating of this resistor is less than the carbon composite resistor. Wire wound resistors. Wire wound resistors are constructed, as the name suggests, by wrapping a length of wire around a ceramic body, with terminals for connection at each end. The resistor is then encased in either a paint or epoxy resin coating. The resistance value is varied by the length of wire wound and is normally between 1Ω and 150 kΩ. Because they are low value resistors they tend to be high current and power thus are quite large in size. 4  EN6000 Electrical Fundamentals Power rating of resistors. The power rating of a resistor is defined as the amount of power in the form of heat that the resistor can dissipate without burning out. As we know, resistance is the opposition to electron flow or electron movement, and this opposition causes heat to be generated when current flows. The amount of heat dissipated can be found by : I2 * R * t or (V2 / R) * t The physical size is an indication as to how much heat the resistor can safely give off A large resistor can give off more heat than a smaller one. 1/8 watt 1/4 watt 1/2 watt 1 watt 2 watt 𝑃 The maximum current that can flow through the resistor without over heating it, I = 𝑅 Where P = Power rating of the resistor in watts and R = Resistance Value of the resistor in Ohms 5 EN6000 Electrical Fundamentals 6  EN6000 Electrical Fundamentals Tolerance. Tolerance is the amount of deviation or error from the specified value. For example a resistor of 1000 ohms marked with a tolerance of ±10% could have an actual value of between....................... and........................... ohms. It can be seen from this that two marked resistors of the same value could have actual values differing by as much as 200 ohms. This may be acceptable in some applications, but, not in others. When a higher precision is required a more expensive resistor with a lower tolerance should be used. 7  EN6000 Electrical Fundamentals Resistor values. Generally on larger power resistors, the resistance value, tolerance, and even the power (wattage) rating are printed onto the actual body of the resistor. Because it is very easy to "misread" the position of a decimal point or comma especially when the component is dirty, an easier system for writing and printing the resistance values of the individual resistance was developed. This system conforms to the British Standard BS 1852 Standard coding method where the decimal point position is replaced Tolerance by the suffix letters "K" for thousands or kilohms, the letter "M" for millions or Codes for megaohms both of which denotes the multiplier value with the letter "R" used where Resistors (±) the multiplier is equal to, or less than 1, with any number coming after these letters B = 0.1% meaning it's equivalent to a decimal point. C = 0.25% D = 0.5% F = 1% G = 2% J = 5% K = 10% M = 20% 8  EN6000 Electrical Fundamentals Tolerance Letter Coding for Resistors Also, when reading these written codes be careful not to confuse the resistance letter k for kiloohms' with the tolerance letter K for 10% tolerance or the resistance letter M for Megaohms with the tolerance letter M for 20% tolerance. Surface Mount Resistors or SMD Resistors are very small rectangular shaped metal oxide film resistors designed to be soldered directly onto the surface, hence their name, of a circuit board. Surface mount resistors generally have a ceramic substrate body onto which is deposited a thick layer of metal oxide resistance. The resistive value of the resistor is controlled by increasing the desired thickness, length or type of deposited film being used and highly accurate low tolerance resistors, down to 0.1% can be produced. They also have metal terminals or caps at either end of the body which allows them to be soldered directly onto printed circuit boards. Surface Mount Resistors are printed with either a 3 or 4-digit numerical code which is similar to that used on the more common axial type resistors to denote their resistive value. Standard SMD resistors are marked with a three-digit code, in which the first two digits represent the first two numbers of the resistance value with the third digit being the multiplier, either x1, x10, x100 etc. For example: “103” = 10 × 1,000 ohms = 10 kΩ “392” = 39 × 100 ohms = 3.9 kΩ “563” = 56 × 1,000 ohms = 56 kΩ “105” = 10 × 100,000 ohms = 1 MΩ Surface mount resistors that have a value of less than 100Ω’s are usually written as: “390”, “470”, “560” with the final zero representing a 10 xo multiplier, which is equivalent to 1. For example: “390” = 39 × 1Ω = 39Ω´s or 39RΩ “470” = 47 × 1Ω = 47Ω´s or 47RΩ Resistance values below ten have a letter “R” to denote the position of the decimal point as seen previously in the BS1852 form, so that 4R7 = 4.7Ω. Surface mount resistors that have a “000” or “0000” markings are zero-Ohm (0Ω) resistors or in other words shorting links, since these components have zero resistance. 9  EN6000 Electrical Fundamentals Non linear resistors. Non-linear resistors have their resistance determined by an outside influence such as light, voltage or heat, and their resistance is proportional to those factors. They can be put into three different categories:- Temperature dependant resistors ( TDR ). The resistance of these resistors varies as they are subjected to heat. There are two types : NTC, and PTC The positive temperature coefficient resistor is used extensively in the windings of electric motors as a temperature sensing device known as a thermistor, but are also used, when calibrated, to measure increase in temperature in recording instruments. Negative temperature coefficient resistors are sometimes used in series with projector lamps as a current sensing device to allow the lamp to heat progressively rather than have the full voltage applied directly. 10  EN6000 Electrical Fundamentals Voltage dependant resistors. Voltage dependent resistors(VDR) are made from a material that, when an excessive voltage occurs, the resistance falls. 11  EN6000 Electrical Fundamentals Light dependant resistors. The amount of light falling on the light dependent resistor governs its resistance. The greater the amount of light the......................... the resistance. The resistance of an LDR in complete darkness would be in the region of........................... ohms while in bright light it would reduce to................... ohms. They are used extensively in the control of street lights and alarm systems. 12  EN6000 Electrical Fundamentals Variable resistors. In some circumstances it may be necessary to have a resistor that is variable by the user, there are two main devices capable of this, they are :- A potentiometer is a device with three terminals, one connection to the rotating arm and the other two to each end of the resistor. They may be constructed from wire or carbon and are generally small with a variable resistance up to many k ohms. There are many different types of potentiometer some of which are shown below. The method of varying the resistance can be seen below. By moving the wiper along (A)-(C) the resistance at (B) can be changed and hence the current limited. Uses for the potentiometer are voltage tapping, current limiting and volume control 13  EN6000 Electrical Fundamentals Rheostats. Rheostats are a two terminal device, one terminal via the rotating arm and the other to one end of the resistor, and like potentiometers can be constructed from wire or carbon. Typical rheostats can be seen below. The operation is similar to that of the potentiometer in as much as its usage for limiting current. Its rating is similar to potentiometers. 14  EN6000 Electrical Fundamentals Review questions. 1) The physical size of a resistor lets you know how much.......................... can be dissipated in the form of........................... 2) The tolerance of a resistor tells you the amount the resistor could................ from the specified value. 3) The most common type of fixed linear resistor is the............................ 4) A thermistor has a................................. temperature coefficient. 5) A rheostat is a........................... terminal device. 6) A potentiometer is a........................... terminal device. 15