Home > Articles > Certification > CompTIA

This chapter is from the book

Electronics Terms

Voltage, current, power, and resistance are terms commonly used in the computer industry. Voltage, which is a measure of the pressure pushing electrons through a circuit, is measured in volts. A power supply’s output is measured in volts. Power supplies typically put out +3.3 volts, +5 volts, +12 volts, and –12 volts. You will commonly see these voltages shown in power supply documentation as +5V or +12V. Another designation is +5VSB. This is for the computer’s standby power. This power is always provided, even when the computer is powered off. This supplied voltage is why you have to unplug a computer when working inside it.

The term volts is also used to describe voltage from a wall outlet. Wall outlet voltage is normally 120VAC (120 volts AC). Exercises at the end of the chapter explain how to take both AC and DC voltage readings. Figure 4.16 shows a photograph of a multimeter being used to take a DC voltage reading on the power connectors coming from a power supply. When the meter leads are inserted correctly, the voltage level shown is of the correct polarity.

Figure 4.16

Figure 4.16. DC voltage reading

The reading on the meter could be the opposite of what it should be if the meter’s leads are reversed. Since electrons flow from one area where there are many of them (negative polarity) to an area where there are few electrons (positive polarity), polarity shows which way an electric current will flow. Polarity is the condition of being positive or negative with respect to some reference point. Polarity is not important when measuring AC. Figure 4.17 shows rules to observe when working with meters.

Figure 4.17

Figure 4.17. Meter rules

Monitors and power supplies can have dangerous voltage levels. Monitors can have up to 35,000 volts going to the back of the CRT. Note that flat-panel displays and mobile device displays use low DC voltage and AC voltage, but not at the voltage levels of CRTs. 120 volts AC is present inside the power supply. Power supplies and monitors have capacitors inside them. A capacitor is a component that holds a charge even after the computer is turned off. Capacitors inside a monitor can hold a charge for several hours after the monitor has been powered off.

Current is measured in amps (amperes), which is the number of electrons going through a circuit every second. In the water pipe analogy, voltage is the amount of pressure applied to force the water through the pipe, and current is the amount of water flowing. Every device needs a certain amount of current to operate. A power supply is rated for the amount of total current (in amps) it can supply at each voltage level. For example, a power supply could be rated at 20 amps for the 5-volt level and 8 amps for the 12-volt level.

Power is measured in watts, which is a measurement of how much work is being done. It is determined by multiplying volts by amps. Power supplies are described as providing a maximum number of watts. This is the sum of all outputs: For example, 5 volts × 20 amps (100 watts) plus 12V 8 amps (96 watts) equals 196 watts. An exercise at the end of the chapter explains how current and power relate to a technician’s job.

Resistance is measured in ohms, which is the amount of opposition to current in an electronic circuit. The resistance range on a meter can be used to check continuity or check whether a fuse is good. A continuity check is used to determine whether a wire has a break in it. A conductor (wire) in a cable or a good fuse will have very low resistance to electricity (close to zero ohms). A broken wire or a bad fuse will have a very high resistance (millions of ohms, sometimes shown as infinite ohms, or OL). For example, a cable is normally made up of several wires that go from one connector to another. If you measure the continuity from one end of a wire to the other, it should show no resistance. If the wire has a break in it, the meter shows infinite resistance. Figure 4.18 shows examples of a good wire reading and a broken wire reading.

Figure 4.18

Figure 4.18. Sample resistance meter readings

Digital meters have different ways of displaying infinity. Always refer to the meter manual for this reading. When checking continuity, the meter is placed on the ohms setting, as shown in Figure 4.18. The ohms setting is usually illustrated by an omega symbol (Ω).

Polarity is not important when performing a continuity check. Either meter lead (red or black) can be placed at either end of the wire. However, you do need a pin-out diagram (wiring list) for the cable before you can check continuity because pin 1 at one end could connect to a different pin number at the other end. An exercise at the end of the chapter steps through this process.

The same concept of continuity applies to fuses. A fuse has a tiny wire inside it that extends from end to end. The fuse is designed so that the wire melts (breaks) if too much current flows through it. The fuse keeps excessive current from damaging electronic circuits or starting a fire. A fuse is rated for a particular amount of current. For example, a 5-amp fuse protects a circuit if the amount of current exceeds 5 amps.

Take a fuse out of the circuit before testing it. A good fuse has a meter reading of 0 ohms (or close to that reading). A blown fuse shows a meter reading of infinite ohms. Refer to the section on resistance and Figure 4.18. An exercise at the end of this chapter demonstrates how to check a fuse.

A technician needs to be familiar with basic electronics terms and checks. Table 4.1 consolidates this information.

Table 4.1. Basic electronics terms






Checking AC voltage on a wall outlet (typically 120VAC). Checking the DC output voltage from a power supply (typically +/– 12, +3.3, and +/– 5 VDC).


Amps (amperes)

Each device needs a certain amount of current to operate. A power supply is rated for total current in amps for each voltage level (such as 24 amps for 5-volt power and 50 amps for 12-volt power).



Resistance is the amount of opposition to electric current. Resistance is used to check continuity on cables and fuses. A cable that shows little or no resistance has no breaks in it. A good fuse shows no resistance. If a cable has a break in it or if a fuse is bad, the resistance is infinite.

Wattage (power)


Watts is a measure of power and is derived by multiplying amps by volts. Power supply output is measured in watts. Also, A UPS (uninterruptible power supply) is rated in volt-amps. The size of UPS to purchase depends on how many devices will plug in to it.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020