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Resistors: A Resistor is an electrical device that resists the flow of electrical current. It is a A passive device used to control, or impede the flow of, electric current in an electric circuit by providing resistance, thereby developing a drop in voltage across the device.

We need some way to control the flow of current from a voltage source, like a battery, so we do not melt wires and blow up batteries. If you think of current, charge flow, in terms of water flow, a good electrical conductor is like big water pipe. Water mains and fire hoses have their uses, but you do not want to take a drink from one. Rather, we use small pipes, valves, and other devices to limit water flow to practical levels. Resistors do the same for current; they resist the flow of charge; they are poor conductors.

The value of a resistor is measured in ohms and represented by the Greek letter capital omega. There are many different ways to make a resistor. Some are just a coil of wire made of a material that is a poor conductor. The most common and inexpensive type is made from powdered carbon and a glue-like binder. Such carbon composition resistors usually have a brown cylindrical body with a wire lead on each end, and colored bands that indicate the value of the resistor.

Fig - 1: Resistor

There are other types of resistors. The potentiometer is a variable resistor. When the knob of a potentiometer is turned, a slider moves along the resistance element. Potentiometers generally have three terminals, a common slider terminal, and one that exhibits increasing resistance and one that has decreasing resistance relative to the slider as the  shaft is turned in one direction.

The resistance between the two stationary contacts is, of course, fixed, and is the value specified for the potentiometer. The photo resistor or photocell is composed of a light sensitive material. When the photocell is exposed to more light, the resistance decreases. This type of resistor makes an excellent light sensor.

The resistance value is specified in ohms, the standard symbol is "R" or Ω.  Resistor values are  often stated as "k" (kilo, or times 1,000) or "M", (meg, or times 1,000,000) for convenience.  There are a few conventions that are followed, and these can cause problems for the beginner.  To explain - a resistor has a value of 2,200 Ohms.  This may be shown as any of these:
  • 2,200 Ohms

  • 2,200 Ω

  • 2,200R

  • 2.2k

  • 2.2k Ω

  • 2k2

The use of the symbol for Ohms (Omega, Ω is optional, and is most commonly left off, since it is irksome to add from most keyboards.  The letter "R" and the "2k2" conventions are European, and not commonly seen in the US and other backward countries :-)  Other variants are 0R1, for example, which means 0.1 Ohm

The schematic symbols for resistors are either of those shown below.  I use the Euro version of the symbol exclusively.

Resistor Symbol
Fig - 2 :Resistor Symbols

The basic formula for resistance is Ohm's law, which states that-

1. R = V / I   Where V is voltage, I is current, and R is resistance

The other formula you need with resistance is Power (P)

2. P = V2 / R
3. P = I2 * R

The easiest way to transpose any formula is what I call the "Transposition Triangle" - which can (and will) be applied to other formulae.  The resistance and power forms are shown below - just cover the value you want, and the correct formula is shown.  In case anyone ever wondered why they had to do algebra at school, now you know - it is primarily for the manipulation of a formula - they just don't teach the simple ways.  A blank between two values means they are multiplied, and the line means divide.

Transposition Triangles for Resistance
Fig - 3 : Transposition Triangles for Resistance

Resistor Colour Code
Fig - 4 : Resistor Colour Code

Tolerance : The tolerance of resistors is mostly 1%, 2%, 5% and 10%.  In the old days, 20% was also common, but these are now rare.  Even 10% resistors are hard to get except in extremely high or low values (> 1M or < 1R), where they may be the only options available at a sensible price.

A 100R resistor with 5% tolerance may be anywhere between 95 and 105 ohms - in most circuits this is insignificant, but there will be occasions where very close tolerance is needed (e.g. 0.1% or better).  This is fairly rare for audio, but there are a few instances where you may see such close tolerance components.

Power Ratings : Resistors are available with power ratings of 1/8th W (or less for surface mount devices), up to hundreds of watts.  The most common are 1/4W (0.25W), 1/2W (0.5W), 1W, 5W and 10W.  Very few projects require higher powers, and it is often much cheaper to use multiple 10W resistors than a single (say) 50W unit.  They will also be very much easier to obtain.

Like all components, it is preferable to keep temperatures as low as possible, so no resistor should be operated at its full power rating for any extended time.  I recommend a maximum of 0.5 of the power rating wherever possible.  Wirewound resistors can tolerate severe overloads for a short period, but I prefer to keep the absolute maximum to somewhat less than 250% - even for very brief periods, since they may become open circuit from the stress, rather than temperature (this does happen, and I have experienced it during tests and repairs).

Resistance Materials : Resistors are made from a number of different materials.  I shall only concentrate on the most common varieties, and the attributes I have described for each are typical - there will be variations from different makers, and specialized types that don't follow these (very) basic characteristics.  All resistors are comparatively cheap.

  • Carbon Composition:  Low to medium power.  Comparatively poor tolerance and stability.  Noisier than most others.

  • Carbon Film:  Low power.  Reasonable tolerance and stability.  Reasonably quiet.

  • Metal Film:  Low to medium power.  Very good tolerance and stability.  Quiet.

  • Wirewound:  High to very high power.  Acceptable to very good tolerance, good stability.  Quiet.  May have inductance.

Resistors make noise.  Everything that is above 0K (zero Kelvin, absolute zero, or -273 degrees Celsius) makes noise, and resistors are no exception.  Noise is proportional to temperature and voltage.  Low noise circuits will always use low resistor values and low voltage wherever possible.

Resistors may also have inductance, and Wirewound types are the worst for this.  There are non-inductive Wirewound resistors, but are not readily available, and usually not cheap.

Two Resistors in Parallel
Two Resistors in Parallel

Two Resistors in Series
Two Resistors in Series

Learn More on Basics of Electronics:

Electronics Definitions: Electronics is the branch of science that deals with the study of flow and control of electrons (electricity) and the study of their behavior and effects in vacuums, gases, and semiconductors, and with devices using such electrons.

Rules of Electrical Circuits: * A voltage of 1V across a resistance of 1 Ohm will cause a current flow of 1 Amp, and the resistor will dissipate 1 Watt (all as heat).

What is an electronic circuit? A circuit is a structure that directs and controls electric currents, presumably to perform some useful function. The very name "circuit" implies that the structure is closed, something like a loop.

Current: Charge is mobile and can flow freely in certain materials, called conductors. Metals and a few other elements and compounds are conductors. Materials that charge cannot flow through are called insulators. Air, glass, most plastics, and rubber are insulators, for example. And then there are some materials called semiconductors, that seemed to be good conductors sometimes but much less so other times. Silicon and germanium are two such materials. The flow of charge is called electrical current. Current is measured in amperes (a), amps for short (named after another French scientist who worked mostly with magnetic effects).

Wiring Symbols: There are many different representations for basic wiring symbols, and these are the most common.  The conventions I use for wires crossing and joining are marked with a star (*) - the others are a small sample of those in common use, but are fairly representative.  Many can be worked out from their position in the circuit diagram (schematic).

Voltage: Voltage is something is a type of "pressure" that drives electrical charges through a circuit.
Bodies with
opposite charges attract, they exert a force on each other pulling them together. The magnitude of the force is proportional to the product of the charge on each mass.

What is charge?  Charge may be defined as the quantity of unbalanced electricity in a body (either positive or negative) and construed as an excess or deficiency of electrons. Charge comes in two forms, positive (+) , and  negative charge ( - ) .

Batteries: Charges can be separated by several means to produce a voltage. A battery uses a chemical reaction to produce energy and separate opposite sign charges onto its two terminals. As the charge is drawn off by an external circuit, doing work and finally returning to the opposite terminal, more chemicals in the battery react to restore the charge difference and the voltage. The particular type of chemical reaction used determines the voltage of the battery, but for most commercial batteries the voltage is about 1.5 V per chemical section or cell.

Ohm’s Law: Ohm's law describes the relationship between voltage, V , which is trying to force charge to flow, resistance, R , which is resisting that flow, and the actual resulting current I .

Power: Power is the Electric energy produced per unit time.

Capacitors: In simple words, we can say that a capacitor is a device used to store and release electricity, usually as the result of a chemical action. Also referred to as a storage cell, a secondary cell, a condenser or an accumulator. A Leyden Jar was an early example of a capacitor.

Inductors: An inductor is an electrical device (typically a conducting coil) that introduces inductance into a circuit. An inductor is a passive electrical component designed to provide inductance in a circuit. It is basically a coil of wire wrapped around an iron core. simplest form an inductor is made up of a coil of wire. The inductance measured in henrys, is proportional to the number of turns of wire, the wire loop diameter and the material or core the wire is wound around.

Semiconductor devices: A conductor made with semiconducting material. Semiconductors are made up of a substance with electrical properties intermediate between a good conductor and a good insulator. A semiconductor device conducts electricity poorly at room temperature, but has increasing conductivity at higher temperatures. Metalloids are usually good semiconductors.

Silicon: Silicon, atomic number 14 on the periodic table, is a semiconducting material from which integrated circuits (computer chips of all types--processors, memory chips, etc.; CCDs; transistors; etc.) are created.

Silicon is one of the most common elements. Silicon is also the semiconductor material out of which almost all modern transistors are made.

Diodes: A Diode is an electronic device that allows current to flow in one direction only. It is a semiconductor that consists of a p-n junction. They are used most commonly to convert AC to DC, because they pass the positive part of the wave, and block the negative part of the AC signal, or, if they are reversed, they pass only the negative part and not the positive part.

Electronic Component name abbreviations: Here is a list of Electronic Component name abbreviations widely used in the electronics industry.

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All about Semiconductor

Ball Grid Array (BGA) Package

SMD Surface Mount Electronic Components for SMT

How to Solder – Hand Soldering Tutorial

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How to Generate Electricity – How is Electricity Generated


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