Alternating Current (AC)

• is a type of electrical current, in which the direction of the flow of electrons switches back and forth at regular intervals or cycles. Current flowing in power lines and normal household electricity that comes from a wall outlet is alternating current. The standard current used in the U.S. is 60 cycles per second (i.e. a frequency of 60 Hz); in Europe and most other parts of the world it is 50 cycles per second (i.e. a frequency of 50 Hz.).
• Alternating current describes the flow of charge that changes direction periodically. As a result, the voltage level also reverses along with the current. AC is used to deliver power to houses, office buildings, etc.

Generating AC

AC can be produced using a device called an alternator. This device is a special type of electrical generator designed to produce alternating current.

A loop of wire is spun inside of a magnetic field, which induces a current along the wire. The rotation of the wire can come from any number of means: a wind turbine, a steam turbine, flowing water, and so on. Because the wire spins and enters a different magnetic polarity periodically, the voltage and current alternates on the wire. Here is a short animation showing this principle:

Generating AC can be compared to our previous water analogy:

To generate AC in a set of water pipes, we connect a mechanical crank to a piston that moves water in the pipes back and forth (our “alternating” current). Notice that the pinched section of pipe still provides resistance to the flow of water regardless of the direction of flow.

Applications 

Home and office outlets are almost always AC. This is because generating and transporting AC across long distances is relatively easy. At high voltages (over 110kV), less energy is lost in electrical power transmission. Higher voltages mean lower currents, and lower currents mean less heat generated in the power line due to resistance. AC can be converted to and from high voltages easily using transformers.AC is also capable of powering electric motors. Motors and generators are the exact same device, but motors convert electrical energy into mechanical energy (if the shaft on a motor is spun, a voltage is generated at the terminals!). This is useful for many large appliances like dishwashers, refrigerators, and so on, which run on AC.

Direct Current (DC)

• electrical current which flows consistently in one direction. The current that flows in a flashlight or another appliance running on batteries is direct current.

• Direct current is a bit easier to understand than alternating current. Rather than oscillating back and forth, DC provides a constant voltage or current.

Generating DC

DC can be generated in a number of ways:

• An AC generator equipped with a device called a "commutator" can produce direct current
• Use of a device called a "rectifier" that converts AC to DC
• Batteries provide DC, which is generated from a chemical reaction inside of the battery

Using our water analogy again, DC is similar to a tank of water with a hose at the end.The tank can only push water one way: out the hose. Similar to our DC-producing battery, once the tank is empty, water no longer flows through the pipes.

Applications

Almost all electronics projects and parts for sale on SparkFun run on DC. Everything that runs off of a battery, plugs in to the wall with an AC adapter, or uses a USB cable for power relies on DC. Examples of DC electronics include:

• Cell phones
• The LilyPad-based D&D Dice Gaunlet
• Flat-screen TVs (AC goes into the TV, which is converted to DC)
• Flashlights

• Hybrid and electric vehicles

Steps in measuring AC 

  Step1: Take Precautions

You must take the following precautions:

1.NEVER touch the probe tips

2. Always try to put the probe in correct position

3. Put the regulator in the correct position before taking any reading

Step 2:Put the probe in correct position

• Turn the multimeter knob to the ACV or AC Voltage section
• Set the multimeter in the range Higher than the expected value

Step 3: Put the common(Black)probe

• Also make sure that the red probe is connected to the ‘V’ and the black probe to the COM as shown in the image given

Step 4:Insert the testing probes into the socket.

• Polarity is not an issue in AC.

 Step 5:Turn on the power supply
 Step 6:Power ON the Multimeter
 Step 7:Turn ON the AC supply.

The multimeter will display the AC voltage which is in the specified range of 220-240V, indicating the AC supply to be working normally.

Steps in measuring DC

Step1: Connect the testing leads to the multimeter.

Step2: Turn the knob to the DV voltage segment in the multimeter at 40DCV.

Step3:Connect the probes in the circuit according to the polarity.

Step4:Power on the Multimeter.
Step5: Note down the reading.