CompTIA A+ Exam Cram: Understanding and Testing Power Devices and Supplies
Everything relies on power. Clean, well-planned power is imperative in a computer system. It's so important, that I almost made this the first chapter of the book. I can't tell you how many power-related issues I have troubleshot in the past. Many of the issues that you see concerning power are due to lack of protection and improper planning, and as such you will see several questions (if not more) on the A+ exams regarding this subject.
Imagine a scenario in which you work for a technical services division of a company. You are required to install a new, more powerful power supply in a computer that contains many devices and requires a lot of electricity. You need to install the computer in a new area of the company's building. This requires you to plug the computer into an AC receptacle that has never been used or tested.
What kind of power supply should you select? How can you verify that the AC outlet is properly wired? And how can you protect the computer? This chapter answers all those questions and furnishes you with the knowledge you need to install, test, and troubleshoot power supplies and test power that comes from the wall outlet.
Understanding and Testing Power
The power for your computer is derived from electricity, which is basically the flow of electric charge. Electricity is defined and measured in several ways, most commonly
- Voltage, a representation of potential energy; sometimes it's more simply referred to as pressure; its unit of measurement is volts (V).
- Wattage or electric power, the rate of electric energy in a circuit, measured in watts (W).
- Amperage or electric current, the movement of electric charge, measured in amperes or amps (A).
- Impedance, the amount of resistance to electricity, measured in ohms (W).
Each of these is covered in this chapter, but by far the most common of these that you will be testing is voltage. Here are two examples of voltages you are probably familiar with:
- 120 Volts AC (the voltage associated with many U.S. homes)
- 5 Volts DC (the voltage associated with some of the internal power connections in your PC)
The difference in these two examples (aside from the amount of volts) is that a house's outlets use alternating current (AC), in which the flow of electrons alternate, and your computer, again internally, uses direct current (DC), in which the flow of electrons is one way.
Back to our scenario; because you can't control who wired the AC outlet that you will be connecting the computer to, or how clean the power is that comes from your municipality, you should test the outlet prior to plugging the computer in. Two good tools to use when testing are a receptacle tester and a multimeter.
Testing an AC Outlet with a Receptacle Tester
Type B AC outlets are the most common, and might also be referred to as wall sockets, electric receptacles, or power points. It is type B that you need to be concerned with for the A+ exam. If any of the hot, neutral, or ground wires are connected improperly, the computer connected to the outlet is a sitting duck, just waiting for irreparable damage. To ensure that the AC outlet is wired properly, you can use a receptacle tester, like the one shown in Figure 5.1. These are inexpensive and are available at most home improvement stores and electrical supply shops. When you plug in the receptacle tester, it tells you if the receptacle is wired properly or indicates which wires are incorrect.
Figure 5.1 A common receptacle tester and labeled receptacle
In Figure 5.1 the test has passed. With this particular tester, two yellow lights tell you that the outlet is wired correctly. Any other combination of lights tells you that there is a wiring error. The different combinations are usually labeled on the tester itself; for example, an open ground error is displayed by one single, yellow light on this tester. Important: If you receive any erroneous readings or if there are no lights at all, do not use the outlet and contact your supervisor and/or building management so that they can bring in a licensed electrician to fix the problem.
Testing an AC Outlet with a Multimeter
Every PC technician should own a multimeter, and we use one throughout this chapter. A multimeter is a hand-held device that, among other things, can be used to measure amps and impedance, and to test voltage inside a computer and from AC outlets. It has two leads, a black and a red. Whenever using the multimeter, try to hold both of the multimeter leads with one hand, and hold them by the plastic handles; don't touch the metal ends. It will be like holding chopsticks but is a safer method, reducing the severity of electric shock in the uncommon chance that one occurs. To test an AC outlet with a multimeter, run through the following steps:
- Place the multimeter's black lead in the outlet's ground. (The parts of the outlet are labeled in Figure 5.1.)
- Place the red lead in the hot opening
- Turn on the multimeter to test for volts AC (sometimes labeled as VAC). Hold the leads steady and check for readings. Optimally, the reading will hover around 115 volts or 120 volts depending on where you are in the United States. Watch the readings for a minute or so. Remember the reading or range of readings that display. A common reading is shown in Figure 5.2.
Figure 5.2 A receptacle tested with a multimeter
- Turn off the multimeter.
- Remove the red lead.
- Remove the black lead.
What was your reading? A steady reading closest to 120 volts is desirable. It might be less in some areas, but the key is that it's steady at one voltage; this is also known as clean power. If the reading fluctuates a lot, say between 113 volts and 121 volts, for example, you have one of the varieties of dirty power. This could be because too many devices use the same circuit or because power coming from electrical panel or from the municipal grid fluctuates, maybe because the panel or the entire grid is under/overloaded. A quick call to your company's electrician can result in an answer and possibly a long-term fix. However, we are concerned with an immediate solution, which in this case will be to install an uninterruptible power supply (UPS) or other line-conditioning device between the computer and the AC outlet. This can regulate the output of AC to the computer.
You can also test the neutral and ground wires in this manner. You should be especially concerned with whether the ground wire is connected properly. Previously we showed how to test this with the receptacle tester, but to test this with the multimeter, connect the black lead to ground and the red lead to neutral. This should result in a reading of 0 volts. Any other reading means that the outlet is not grounded properly, which can result in damage to a computer that connects to it. You can also use a voltage detector, which is a pen-shaped device that beeps when it comes into contact with voltage. On a properly grounded outlet, the only part that should give audible beeps is the hot. Everything else including the screw and outlet plate should not register any sounds. If sounds do register by simply touching the outlet plate with the voltage detector, the outlet is not grounded properly. If this is the case, or if you got any other reading besides 0 volts on the multimeter, contact an electrician right away.
Cram QuizAnswer these questions. The answers follow the last question. If you cannot answer these questions correctly, consider reading this section again until you can.
What tool would you use to test the amount of voltage that is coming from an AC outlet?
B. Voltage detector
C. Receptacle tester
B. Impedance tester
Which of the following is a representation of potential energy?
Which wire when tested should display zero volts on a multimeter?
Cram Quiz Answers
A. The multimeter is the only testing tool that can display voltage numerically.
B. Voltage is a representation of potential energy; an analogy for voltage would be water pressure in a pipe.
C. When testing the ground wire with a multimeter, it should display a reading of zero volts.