Chapter 14: Power Supply and Electrical Issues^313
Volts A volt measures the electrical pressure in a circuit. Most PC’s operate
on several different voltage levels: +3.3V (V is the abbreviation for volts), +5V,
–5V, +12V, and –12V.
Watts A watt measures the electrical power in a circuit. PC power supplies
are rated in the range of 200 to 600 watts.
Continuity Continuity is an indicator of the existence of a complete circuit
or a continuous connection. Electricity cannot flow if a complete circuit is
not present. For example, if you attempt to measure the volts in one pin of a
device’s connector, it won’t register anything until it is grounded to another
of the connector’s pins, completing a circuit.Measuring Current
ThemeasurementsyouaremostconcernedwithonaPCarevoltsandamps.Voltsmeasure
pressure andampsmeasure current. Although it may sound contradictory, you don’t need
current to have voltage. When a water faucet is turned off, there is no current but there is
definitely water pressure. When an electrical circuit is open (which means it can’t flow),
voltage (pressure) is still in the line despite the fact that no current is flowing. An open cir-
cuit, like two separated pieces of a hose, will not allow electricity to flow. A closed circuit,
like the two pieces of hose connected together, allows the electricity to flow.
As illustrated in the to p part of Figure 14-1, when two wires are both attached to a
terminal of a battery but are not connected together, the circuit is open and no current
flows through the wires. If the two wires are connected together, the circuit is closed and
the current begins to flow. If you were to hold one of the wires in each hand, you would
close the circuit and feel its pressure (volts) as a shock.
A variety of devices can be used to read the power and fury of an electrical current.
Such devices as ammeters, ohmmeters, and voltmeters can measure specific properties ofWater Measure Electrical Measure
Water pressure Voltage
Rate of flow Amps
Friction in the hose OhmsTable 14-1. Comparison of Water and Electricity Measures