Everything inside or attached to the PC system unit runs on electricity. Electricity is both the lifeblood and the mysterious evil of the personal computer. It’s a flowing entity, measured in amps, ohms, and
volts that should be approached with respect, if not outright fear.
Electricity flowing through a circuit is very much like water running through a hose. When you open a water faucet, the pressure in the water line forces the water to flow at some gallons-per-minute rate into
the hose. Friction reduces the force and rate of the water before it exits the hose. When electricity flows
into a wire from a source such as a battery, some of its pressure is lost to resistance in the wire.
The “electricity is like water in a hose” analogy points out the forces of electricity that can be measured. These forces are measured in volts, amps, and ohms. At the risk of running the analogy into the ground, Table 1 below lists the water hose analogous element against its electrical equivalent.
The electrical measures are listed with a description and an example of how they’re applied on the PC.
Table 2-2: Common Electrical Measurements
Amps: Measures a current’s strength or rate of flow
The amount of current needed to operate a device; for example, a hard disk drive needs 2.0 amps to start up, but only 0.35 amps for typical operation.
Ohms: Measures a conductor’s resistance (Resistance of less than 20 ohms to electricity)
Volts: Measures the electrical pressure in a circuit
Watts: Measures the electrical power in a circuit
Continuity: Indicates the existence of a complete circuit
means that current can flow through a computer system.
A PC power supply generates 4 levels of voltage: +5 volts (V), –
5V, +12V, and –12V.
A PC power supply is rated in a range of 200 to 600 watts.
A pin in a DIN connector registers 5V on a digital voltage meter (DVM or multimeter) when grounded to another pin.
Measuring the current
The primary measurements of electricity are volts and amps. Volts measure pressure, and amps measure current. Current isn’t needed to have voltage. When a water faucet is off, water pressure still exists.
There is just no current. Likewise, when an electrical circuit is open, voltage (pressure) is still in the line although no current is flowing. If you touch the wire and close the circuit, the current begins to flow,
and you can feel all of its pressure as a shock .
You can use a variety of devices to read the power and fury of an electrical current. Ammeters, ohmmeters, and voltmeters measure specific properties, but using a multimeter or DVM is more efficient for you to use because it combines these instruments into one tool.
Switching from AC to DC
Current is the flow of electrons in a wire. Electricity has two current types: AC (alternating current) and
DC (direct current). AC is what you get from the outlets in your house or office, and DC is the type used
inside the computer.
In alternating current, the current changes directions about 60 times per second, moving first one way,
and then the other. The voltage changing rapidly from a positive charge to a negative charge causes the
current to also switch the direction of its flow in the wire. AC power exists because it has advantages for
the power company and for your household electrical appliances, but these advantages have little value
on a low-voltage system like a PC.
When the flow of the electricity is in one direction only, it is direct current. What happens in direct current is that negatively charged particles seek out and flow toward positively charged particles, creating a direct electrical current flow. DC power maintains a constant level and flows in only one direction–always, predictably, and measurably, from a negative charge to a positive charge.
For example, wire a light bulb to a battery, and the current flows from the negative terminal to the positive terminal through the light bulb. Because the current of electricity causes heat and light in the right materials, the lighter materials in the light bulb glow.
The PC uses DC power. The PC’s power supply converts power from the AC wall outlet into DC power for the computer. Peripheral devices, such as printers, external modems, and storage drives, including CD-ROM and Zip drives, use an AC power converter to convert AC power into DC power.
Focus on the following electrical terms or concepts:
- Voltage and volts
- Amperes or amps
- Electrical resistance
- Alternating current
- Direct current
- 5 or 12 volts DC current
Reviewing Elementary Electronics
We now need to review basic electronics principles. What follows is a series of definitions and concepts. (not very many) This stuff is pretty basic, so if you’re an electronics whiz, you may already know it.
Remember A digital circuit is an electronic circuit that accepts and processes binary data using the
rules of Boolean algebra–the logic of AND, OR, NOT, and so on. Digital circuits are made up of one or
more electronic components placed in a series to work cooperatively to achieve the logical objective of
Conductors, insulators, semiconductors
A conductor , such as copper, carries an electrical current. An insulator , such as rubber, doesn’t carry an electrical current, which is why a copper wire conductor is usually wrapped with a rubber insulator.
In the conducting electricity scheme of things, halfway between a conductor and an insulator is a
semiconductor . Although its name technically means “half-conductor,” it’s really neither a conductor
nor an insulator. When a semiconductor is zapped with electricity, it toggles to either a conductor or an insulator, depending on what it was at the time it was zapped.
Semiconductors are the building blocks of the computer. Electricity can hold the properties of plus or minus polarity. The electronics in the PC are designed to take advantage of this by storing electricity in one polarity or the other and assigning a numerical value to each. In the PC, these numerical values are the ones and zeroes of binary data.
By using a semiconductor, which can be toggled between two electrical values, the result is a perfect place to store all of the binary values that course around inside of the PC.
Confused? Don’t be. It’s actually very simple: A semiconductor is simply an extremely simple on/off switch. Zap it once, it’s on; zap it again, it’s off. Zap it, on; zap it, off–and so on.
Resistors, capacitors, transistors, and diodes
These four electronic components are the building blocks on which virtually every electronic circuit in
the computer is built. Each plays a distinctly different and valuable role in a circuit, as I explain in the
A resistor acts like a funnel to slow down the flow of current in a circuit.
A capacitor is like a storage bin to hold a charge. The PC has a few large capacitors that can
literally kill you if you make contact with them, such as the capacitors in the monitor and in the power supply.
A diode is a one-way valve that allows the current to flow in only one direction.
A transistor is a semiconductor that stores one binary value.
Transistors, resistors, capacitors, and diodes produce logic gates. Logic gates create circuits, and circuits make up electronic systems.
Don’t Give Me Any Static
The term static has a variety of meanings in computer technology. To the computer technician, static means static electricity (electrostatic charge) and its evil twin– electrostatic discharge (ESD), also called electrical static discharge, that have the most importance.
ESD, as it is infamously called, is the evil demon that lies in wait for the unsuspecting service technician who fails to don the sacred wrist strap before kneeling at the PC altar.
ESD; something that can’t be emphasized too much. Focus on preventing ESD damage.
Always wear a grounded wrist strap that’s connected to either a grounding mat
or the PC chassis when you work on any part of the computer.
Static electricity is what makes your hair stand on end when you rub a balloon against your head. Of course, this assumes that you have hair–and that you’d have occasion to rub a balloon against your head. Static electricity also occurs when you walk across a carpet. Static electricity is not by itself a problem; the danger is in the discharge of the static electricity. You know, when you reach for the doorknob and zap! A blue spark as big as a towrope jumps from your finger to the metal. Although this may seem harmless (other than the pain), the potential for a lot of damage to a PC exists in that seemingly harmless spark. Remember that lightning is ESD in its most dreaded form.
Just because you can’t feel an electrostatic discharge doesn’t mean that it can’t do harm to an electronic component. A human feels ESD at around 3,000 volts, but a mere 30 volts will do damage to electronic components. ESD is a far greater threat to the PC than anything else the PC service technician might do accidentally.
Looking at the dark side of ESD
Most PCs are designed to have some ESD protection as long as their cases are intact and closed properly. Cases are chemically treated or have copper fittings designed to channel electrostatic discharge away from the sensitive components inside.
The danger from ESD damage begins when the case is opened and the fragile components on the motherboard are exposed. When a human with a static electrical charge touches anything inside the case, the charge can travel along the wires interconnecting the various electronic components. One of the wires may lead inside a component, and when the charge gets close enough to a metal part with an opposing charge, the internal wires and elements of components can explode or weld together.
Take a look at some ESD facts:
- Most of the computer’s electronic components use from three to five volts of electricity.
- An ESD shock of 30 volts can destroy a computer circuit.
- An ESD shock you can feel, such as on a doorknob, has around 3,000 volts.
- An ESD shock you can see carries about 20,000 volts.
The real problem with ESD damage is that not all of it is obvious. If an entire
component is destroyed, you know it, and you replace the piece. When a component
has been damaged but continues to work, though,days, weeks, or even months may
pass before the component fails completely. More frustrating is intermittent partial
failures that can’t be isolated.
Eliminating static electricity
You can avoid static electricity. Good environmental preventive measures that help to eliminate, or at least reduce, static electricity are as follows:
- Always wear a ESD grounding strap on your wrist or ankle that is connected to either the chassis
of the PC or to a grounding mat when working inside the PC.
- Treat carpeting inexpensively with antistatic chemicals to reduce static buildup. Aerosol cans of these chemicals are available in most computer or carpet stores. l Store all electrical components in antistatic bags when not in use.
- Install a grounded pad under the PC. Before you touch the computer, touch the pad, and discharge any built-up static electricity.
- If all else fails, install humidifiers to replace moisture in the air. Keep the humidity above 50 percent. Dry air can cause static electricity.
- However, when working on the monitor, do not, I repeat, do not wear a grounding strap. The monitor has a very large capacitor in it and a grounding strap invites all of its stored charge to run through your body–not always a pleasant experience.
- For all my ranting and raving on the threat of ESD, it does have some good uses. For example, it’s used to apply toner to paper in copy machines and laser printers and is used to clean the air of unwanted pollen, dust, and other debris.