Electrical Info

This site is great for students or hobbyists looking to expand their understanding of general electrical topics.

electrical

Introduction

Electricity is considered the driving force that creates almost all the power for the industrialized world. It is used to cook meals, light homes, heat and cool buildings, drive motors, and supply the ignition for most automobiles. The specialized person who understands electricity can easily obtain employment in almost any part of the world.

History of Electricity

Regardless of the simple fact that the practical application of electricity came to be common primarily during the past hundred years, it truly has been viewed as a force for much longer. The Greeks were the first ones to uncover electricity about 2500 years ago. They determined that when amber was rubbed with it became charged with an unknown force that had the ability to attract objects such as feathers, dried leaves, parts of cloth, or other lightweight materials. The Greeks called amber elektron. The term electric was obtained from it and meant, “to be like amber,” also to possess the capability to attract other objects.

This puzzling force remained just a curious phenomenon until about 2000 years later, when some individuals set out to conduct studies. Back in the early 1600s, William Gilbert discovered that amber was not the sole material that could be charged to attract other objects. He called materials that could be charged electriks and materials that could not be charged non-elektriks.

About 300 years back, a handful of men began to learn about the behavior of assorted charged objects. In 1733, a Frenchman named Charles DuFay discovered that a piece of charged glass would repel some charged objects and attract others. These men soon found that the force of repulsion was really just as important as the force of attraction. From these experiments it was eventually determined that certain materials repelled one another, and others attracted each other. Benjamin Franklin chose to call these attractions and repulsions, positive and negative.

Oscilloscope

The Oscilloscope

 Oscilloscope

Many of the electronic control systems in today’s industry produce voltage pulses that are meaningless to a VOM. In many instances, it is necessary to know not only the amount of voltage present at a particular point, but also the length or duration of the pulse and its frequency. Some pulses may be less than 1 volt and last for only a millisecond. A VOM would be useless for measuring such a pulse. It is therefore necessary to use an oscilloscope to learn what is actually happening in the circuit.

The oscilloscope is a powerful tool in the hands of a trained technician. The first thing to understand is that an oscilloscope is a voltmeter. It does not measure current, resistance, or watts. The oscilloscope measures an amount of voltage during a period of time and produces a two-dimensional image.

Voltage Range Selection

The oscilloscope is divided into two main sections. One section is the voltage section, and the other is the time base. The display of the oscilloscope is divided by vertical and horizontal lines. Voltage is measured on the vertical, or Y, axis of the display, and time is measured on the horizontal, or X, axis. When using a VOM, a range-selection switch is used to determine the full-scale value of the meter. Ranges of 600 volts, 300 volts, 60 volts, and 12 volts are common. The ability to change ranges permits more accurate measurements to be made.Oscilloscopes can be divided into two main types: analog and digital. Analog oscilloscopes have been used for years and many are still in use; however,

osc4 e1331080158602 300x209 Oscilloscope

(Voltage is measured on the vertical or Y axis and time is measured on the horizontal or X axis)

digital oscilloscopes are rapidly taking their place. Analog scopes generally employ some type of control knob to change their range of operation. The setting indicates the volts per division instead of volts full scale. The settings indicate that Channel 1 is set for 0.2 volts per division and Channel 2 is set for 0.5 volts per division.

(Voltage control of an analog oscilloscope.)

Digital oscilloscopes often indicate their setting on the display instead of marking them on the face of the oscilloscope.  In the lower left-hand corner of the display the notation CH1 200mV can be seen. This indicates that the voltage range has been set for 200 millivolts per division.

Oscilloscopes can display both positive and negative voltages.

 

 Oscilloscope

 

The Time Base

The next section of the oscilloscope to be discussed is the time base. The time base is calibrated in seconds per division and has ranges from seconds to microseconds. Analog type oscilloscopes use a range selection switch.

Measuring Frequency

oscc 300x207 Oscilloscope

Since the oscilloscope has the ability to display the voltage with respect to time, it is possible to compute the frequency of the waveform. The frequency (f) of an AC waveform can be found by dividing 1 by the time (t) it takes to complete one cycle (f = 1/t). Many oscilloscopes have the ability to measure frequency automatically and dis- play the value for you.

Attenuated Probes

Most oscilloscopes use a probe that acts as an attenuator. An attenuator is a device that divides or makes smaller the input signal. An attenuated probe is used to permit higher voltage readings than are normally possible. For example, most attenuated probes are 10 to 1. This means that if the voltage range switch is set for 5 volts per division, the display would actually indicate 50 volts per division. If the voltage range switch is set for 2 volts per division, each division on the display actually has a value of 20 volts per division.

Probe attenuators are made in different styles by different manufacturers. On some probes, the attenuator is located in the probe head itself, while on others the attenuator is located at the scope input. Regardless of the type of attenuated probe used, it may have to be compensated or adjusted. In fact, probe compensation should be checked frequently. Different manufacturers use different methods for compensating their probes, so it is generally necessary to follow the procedures given in the operator’s manual for the probe being used.

probe 300x231 Oscilloscope

Oscilloscope Controls

The following is a list of common controls found on the oscilloscope:

1. POWER. The power switch is used to turn the oscilloscope ON or OFF.

2. BEAM FINDER. This control is used to locate the position of the trace if it is off the display. The BEAM FINDER button will indicate the approximate location of the trace. The position controls are then used to move the trace back on the display.

3. PROBE ADJUST (sometimes called calibrate). This is a reference voltage point used when compensating the probe. Most probe adjust points produce a square wave signal of about 0.5 volts.

4. INTENSITY and FOCUS. The INTENSITY control adjusts the brightness of the trace. A bright spot should never be left on the display because it will burn a spot on the face of the cathode ray tube (CRT). This burned spot results in permanent damage to the CRT. The FOCUS control sharpens the image of the trace.

5. VERTICAL POSITION. This is used to adjust the trace up or down on the display. If a dual-trace oscilloscope is being used, there will be two vertical POSITION controls. (A dual-trace oscilloscope contains two separate traces that can be used separately or together.)

ossc 300x218 Oscilloscope

6. CH 1–BOTH–CH 2. This control determines which channel of a dual-trace oscilloscope is to be used, or if they are both to be used at the same time.

7.ADD –ALT.– CHOP. This control is active only when both traces are being displayed at the same time. The ADD adds the two waves together. ALT. stands for alternate. This alternates the sweep between Channel 1 and Channel 2. The CHOP mode alternates several times during one sweep. This generally makes the display appear more stable. The CHOP mode is generally used when displaying two traces at the same time.

8.AC–GND–DC.The AC is used to block any DC voltage when only the AC portion of the voltage is to be seen. For instance, assume an AC voltage of a few millivolts is riding on a DC voltage of several hundred volts. If the voltage range is set high enough so that 100 VDC can be seen on the display, the AC voltage cannot be seen. The AC section of this switch inserts a capacitor in series with the probe. The capacitor blocks the DC voltage and permits the AC voltage to pass. Since the 100 VDC has been blocked, the voltage range can be adjusted for millivolts per division, which will permit the AC signal to be seen.The GND section of the switch stands for ground. This section grounds the input so the sweep can be adjusted for 0 volt at any position on the display. The ground switch grounds at the scope and does not ground the probe. This permits the ground switch to be used when the probe is connected to a live circuit. The DC section permits the oscilloscope to display all of the voltage, both AC and DC, connected to the probe.

9.HORIZONTAL POSITION. This control adjusts the position of the trace from left to right.

10.AUTO–NORMAL. This determines whether the time base will be triggered automatically or operated in a free-running mode. If this control is operated in the NORM setting, the trigger signal is taken from the line to which the probe is connected. The scope is generally operated with the trigger set in the AUTO position.

11.LEVEL. The LEVEL control determines the amplitude the signal must be before the scope triggers.

12.SLOPE. The SLOPE permits selection as to whether the trace is triggered by a negative or positive waveform.

13.INT.–LINE–EXT. The INT. stands for internal. The scope is generally operated in this mode. In this setting, the trigger signal is provided by the scope. In the LINE mode, the trigger signal is provided from a sample of the line. The EXT, or external, mode permits the trigger pulse to be applied from an external source.

Interpreting Waveforms

When using the oscilloscope, one must keep in mind that the display shows the voltage with respect to time. Learning to interpret the waveforms seen on the display of an oscilloscope will take time and practice, but it is well worth the effort. The oscilloscope is the only means by which many of the waveforms and voltages found in electronic circuits can be understood. Consequently, the oscilloscope is the single most valuable piece of equipment a technician can use.

Basic Terminology

Electricity contains a standard collection of values. Before a person can operate with electricity, it is best to understand these values and realize exactly how to make use of them. Given that values of electrical measurement have been standardized, they will be understood by every person that makes use of them. An example is carpenters, who use a standard system for measuring length, such as the inch, foot, meter, or centimeter. Imagine exactly what a house would look like if it was constructed by multiple carpenters who used different lengths of measure for an inch or foot. Precisely the same holds true for anyone who electricity. The standards of measurement need to be exactly the same for everyone. Meters ought to be calibrated to indicate the same amount of current flow or voltage or resistance. A volt, an ampere, or an ohm is identical everywhere in the world.

Ampere

The ampere is named after André Ampère, a scientist who lived from the late 1700s to the early 1800s. Ampère is particularly known for his own work dealing with electromagnetism. The ampere (A) is equal to 1 coulomb per second. The definition of an ampere includes a quantity attribute, the coulomb, and a time attribute, the second. One ampere of current flows through a wire when 1 coulomb flows past a point in one second. The ampere is a measurement of the quantity of electricity that is flowing through a circuit. The letter I, which stands for intensity of current, and the letter A, which stands for ampere, are similarly used to represent current flow in algebraic formulas.

Coulomb

A coulomb is defined as the charge transferred by a current of 1 ampere in one second. A coulomb is a quantity measurement with regard to electrons. One coulomb consists of 6.25 X 1018, or 6,250,000,000,000,000,000 electrons. To help understand the quantity of electrons contained in a coulomb, think of comparing one second to 200 billion years. Considering that the coulomb is a quantity measurement, it is very similar to a quart, a gallon, or a liter. It takes a certain amount of liquid to equal a liter, similar to how it takes a certain number of electrons to equal a coulomb.
The coulomb is named for a French scientist who resided in the 1700s named Charles Augustin de Coulomb. Coulomb researched electrostatic charges and developed a law dealing with the attraction and repulsion of these forces. The law, often known as Coulomb’s law of electrostatic charges, states that the force of electrostatic attraction or repulsion is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them. The number of electrons contained in the coulomb was determined by the average charge of an electron. The symbol for coulomb is the letter C. It is the System Internationale (SI) unit of electric charge.

Flow theory

Electron flow theory

You will find actually two theories related to current flow. One theory is termed the electron flow theory and states that since electrons are negative particles, current flows from the most negative point in the circuit to the most positive. The electron flow theory is the more commonly accepted as being correct.

Conventional current flow

An additional theory, known as the conventional current flow theory, is older than the electron flow theory and states that current flows from the most positive point to the most negative. Even though it continues to be established almost to a certainty that the electron flow theory is correct, the conventional current flow theory is still widely used for several reasons. Many electronic circuits utilize the negative terminal as ground or common. When the negative terminal is used as ground, the positive terminal is considered to be above ground, or hot. It is easier for most people to think of something flowing down rather than up, or from a point above ground to ground.

Ohms law

Ohm’s law states that it takes 1 volt to push 1 ampere through 1 ohm. Ohm realized that all electric quantities will be proportional to each other and can as a result be expressed as mathematical formulas. He discovered that if the resistance of a circuit remained constant and the voltage increased, there was a corresponding proportional increase of current. Similarly, if the resistance remained constant and the voltage decreased, there’d be a proportional decrease of current. He also found that if the voltage remained consistent and the resistance increased, there would be a decrease of current; and any time the voltage remained constant and the resistance decreased, there would be an increase of current. This unique finding led Ohm to the conclusion that in a DC circuit, the current is directly proportional to the voltage and inversely proportional to the resistance. Ohms law

The three basic ohms law formulas are:

The first formula can be used to find volts, when the current and resistance are known. The second formula can be used to find current, when the volts and resistance are known. The third formula can be used to find resistance, when the volts and current are known.

Another chart can also be used when the power (watts) must be determined.

 

 Ohms law

Volt

Voltage is known as the potential difference within two points of a conducting wire containing a constant current of 1 ampere whenever the power dissipated between these points is 1 watt. Voltage can also be referred to as potential difference or electromotive force (EMF). It is the force that pushes the electrons through a wire and is usually referred to as electrical pressure. A volt is the amount of potential needed to cause 1 coulomb to produce 1 joule of work.

One other thing keep in mind is that voltage cannot flow. Voltage in an electrical circuit is like pressure in a water system. To say that voltage flows through a circuit is like saying that pressure flows through a pipe. Pressure can push water through a pipe, and it is correct to say that water flows through a pipe, but it is not correct to say that pressure flows through a pipe. The same is true for voltage. Voltage will push current through a wire, but voltage cannot flow through a wire. Voltage is often considered to be the potential to do something. For this reason it is frequently referred to as potential. Voltage needs to be present before current can flow, similar to how pressure must be present before water can flow. A voltage, or potential, of 120 volts is present at a common wall outlet, however there is no flow until some device is connected and a complete circuit exists. The same is true in a water system. Pressure is present, but water cannot flow until the valve is opened and a path is provided to a region of lower pressure. The letter E, which stands for EMF, or the letter V, which stands for volt, can be used to represent voltage in an algebraic formula.

Multiple Tapped Transformer Primary Winding