Electrical principles - Measurement
Meters

There is a range of meters used for measuring various electrical quantities. These meters are often expensive and delicate pieces of equipment, so they should always be handled with care and used according to the manufacturer's instructions.

Meters are available with two different forms of readout:
analogue – readout on a graduated scale with a pointer operated by a magnetic field
digital – readout on a digital screen.

Analogue display

An analogue meter must be positioned correctly. If placed on its side or an angle, the pointer could be affected by gravity and give an incorrect reading.

When viewing the scale, position yourself directly in front of the scale. If viewed from an angle, you will get an incorrect reading – the greater the angle the more inaccurate it will be. This is because the pointer is separated from the graduation scale by a short distance. This is called the parallax effect.

An analogue meter must be checked for zero before being used. There is usually a zero set screw adjustment to ensure that the pointer starts on the zero calibration before measurement is taken. Any time you change scales when measuring resistance, you must always set the zero again.

Always connect meters with the correct polarity – black to negative and red to positive. Failure to do this could damage the meter, particularly an analogue meter.

Digital meters are difficult to read in direct sunlight. Most have a battery to supply power. Make sure that the battery is not flat and that you have a spare available.

The four most common meters are:
voltmeter
ammeter
ohmmeter
multimeter.
Voltmeter

voltmeter
A voltmeter is used to determine the voltage (electrical pressure) present at any point in a circuit.
Connecting the voltmeter across the battery measures the voltage present in the battery.

Measuring battery voltage

Connecting one lead of the voltmeter to the input side of a globe and the other lead to earth results in a reading of the voltage present at the globe. Voltmeters are mostly used to measure voltages across components. They are connected in parallel with the component.

Measuring voltage at the globe

Ammeter
An ammeter measures the amount of current flowing through a circuit.
An ammeter must be connected in series with the circuit. An ammedter has a very low internal resistance and must never be connected directly to a power supply without being in series with a load. The ammeter used must be capable of carrying the current that you expect to read.
To connect an ammeter, the circuit needs to be disconnected (broken) and the ammeter placed in series with the circuit.

Measuring current flow

Ohmmeter
An ohmmeter is used to measure the amount of resistance in a component or section of circuit.
An ohmmeter has its own power supply. The circuit being tested must always be de-energised (turned off) when using the ohmmeter.

Measuring resistance of a globe

When using an ohmmeter:
isolate the component or circuit section that requires measurement to ensure a correct reading occurs
avoid touching the metal probes of the ohmmeter as this may lead to a false reading.
Warning
Failure to disconnect or isolate the component being measured from the power supply will cause damage to the ohmmeter.
Multimeter
A multimeter, as its name suggests, has multiple uses. Some of the quantities it can measure include voltage, current and resistance. Multimeters are available with analogue or digital readouts. Digital multimeters are commonly used as they give accurate results and are easy to read, except in direct sunlight.

Multimeters

The two different multimeters shown can measure a range of electrical quantities including voltage (AC or DC), ohms, continuity, capacitance, frequency and current (AC or DC).
The rotary switch is used to select the quantity (and range) you wish to measure. The left side meter is auto ranging and the user needs only to select the type of quantity to be measured. For the right hand side meter, the user needs to select the electrical quantity as well as an appropriate range based on the value expected.  It is best to start with a high range and then progressively rotate the switch to a lower range.

The test leads of the multimeter need to be connected into the meter in the appropriate positions. The black lead connects into the COM (common) position and the red lead into either V, A or mA position, depending on whether you are measuring voltage (V), resistance (Ω) or current (A or mA).

When testing a circuit with a multimeter, you need to connect the leads to the circuit appropriately depending on whether you are measuring voltage (V), resistance (Ω) or, current (A or mA). The connections are the same as for the voltmeter, ammeter and ohmmeter described above.

Connection of camera and scanner.

Storage device

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HOW  A LASER PRINTER FUNCTION

A laser printer is a high-quality, fast printer that uses a laser beam to create an image. The central part of the laser printer is its electrophotographic drum. The drum is a metal cylinder that is coated with a light-sensitive insulating material. When a beam of laser light strikes the drum, it becomes a conductor at the point where the light hits it. As the drum rotates, the laser beam draws an electrostatic image upon the drum, called the image. The undeveloped or latent image is passed by a supply of dry ink or toner that is attracted to it. The drum turns and brings this image in contact with the paper, which attracts the toner from the drum. The paper is passed through a fuser that is made up of hot rollers, which melts the toner into the paper.

Printing Process
The laser printer process involves six steps to print information onto a single sheet of paper.

Step 1: Cleaning
When an image has been deposited on the paper and the drum has separated from the paper, any remaining toner must be removed from the drum. A printer may have a blade that scrapes all excess toner from the drum. Some printers use an AC voltage on a wire that removes the charge from the drum surface and allows the excess toner to fall away from the drum. The excess toner is stored in a used toner container that is either emptied or discarded.

Step 2: Conditioning
This step involves removing the old latent image from the drum and conditioning the drum for a new latent image. Conditioning is done by placing a special wire, grid, or roller that receives a negative charge of approximately –600 volts DC uniformly across the surface of the drum. The charged wire or grid is called the primary corona. The roller is called a conditioning roller.

Step 3: Writing
The writing process involves scanning the photosensitive drum with the laser beam. Every portion of the drum that is exposed to the light has the surface charge reduced to about –100 volts DC. This electrical charge has a lower negative charge than the remainder of the drum. As the drum turns, an invisible latent image is created on the drum.

Step 4: Developing
In the developing phase, the toner is applied to the latent image on the drum. The toner is a negatively-charged combination of plastic and metal particles. A control blade holds the toner at a microscopic distance from the drum. The toner then moves from the control blade to the more positively-charged latent image on the drum.

Step 5: Transferring
In this step, the toner attached to the latent image is transferred to the paper. The transfer, or secondary corona, places a positive charge on the paper. Because the drum was charged negatively, the toner on the drum is attracted to the paper. The image is now on the paper and is held in place by the positive charge.

Step 6: Fusing
In this step, the toner is permanently fused to the paper. The printing paper is rolled between a heated roller and a pressure roller. As the paper moves through the heated roller and the pressure roller, the loose toner is melted and fused with the fibers in the paper. The paper is then moved to the output tray as a printed page.

The following mnemonic can help you memorize the order of the steps of the laser printing process: Continuous Care Will Delay Trouble Forever (Cleaning, Conditioning, Writing, Developing, Transferring, Fusing).

WARNING: The primary corona wire or grid, or the conditioning roller, can be very dangerous. The voltage runs as high as –6000 volts. Only certified technicians should work on the unit. Before working inside a laser printer, you should make sure that voltage is properly discharged.

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