Hardware Reference
In-Depth Information
Alternating Current
Let's take a look at what AC is. Why exactly is it called alternating current? If you recall, in the topic's introduction
I talked about DC. DC is when the flow of the electric charge flows in only one direction. So your flow of current will
always remain the same. For example, your current will flow from negative to positive only. With alternating current,
the flow of the current will change direction according to a frequency. You can see an example of this in Figure
10-1
.
The voltage will go from +240 to -240 V.
+240v AC
Time
-240v AC
Figure 10-1.
An example of an AC voltage graph
This frequency is measured in hertz (Hz for short). An AC usually runs at a frequency of 50 or 60 Hz. For example,
a 50-Hz 240-V circuit will have its direction of current flow changed 50 times a second. There are a few very good
reasons why AC was eventually selected as the method of distribution across the world.
•
First, converting AC power to different voltage levels is very easy and requires no complex
circuits, and a simple transformer can take care of this. On the other hand, converting DC
power to different voltage levels is not an easy task. In recent years this has become easier but
at the time when AC and DC were fighting it out for dominance it was a very hard task that
required large machinery. In your home or at your place of work you will have many different
devices that use a wide range of voltage. Under the original DC power network, you would
have needed a separate feed for each different voltage device. As you can see, that would not
be very convenient or a good use of resources.
•
AC also has another big advantage. The fact that AC could easily have its voltage converted
up or down made it ideal for a large distribution range. The distribution network could run at
a very high voltage with less current, then be easily stepped down to your household voltage.
The advantage of using less current is that the potential voltage drop over longer runs of cable
is smaller. You can then use smaller wire, thus giving you less resistance, unlike DC, which
needs thicker wires for a higher current flow, resulting in more lost energy.
Your mains voltage will be somewhere from 100 to 240 V at 50 or 60 Hz. This won't matter for this project, so don't
worry that I will be using 240-V devices. With that in mind, all high voltage should be treated with a lot of respect,
whether it's AC or DC. Each has the potential to kill you and most certainly will kill your Raspberry Pi. Now that you
have some background on AC, it's time to take a look at the devices that will do the hard work for us.
Isolation Devices
First up, and my personal favorite, is full opto-isolation (devices that use this approach are also referred to as
opto-couplers).
