Hardware Reference
In-Depth Information
Opto-Isolation
What exactly is opto-isolation? Opto-isolation stands for optical isolation, where the isolation part of this refers to
each side of the circuit, with one side being high voltage and the other side being low voltage. An opto-isolator works
by separating each side of the circuits by an optical connection. Take a look at Figure
10-2
to see what the inside of an
opto-isolator would like.
Photosenor
LED Transmitter
Low-voltage
side
High-voltage
side
High voltage
Low voltage
Figure 10-2.
A high-level design of an opto-isolator
As you can see, there is not much to an opto-isolator. The most common opto-isolator you will encounter will use
an infrared LED and a photosensor. You are most likely to have walked though many infrared photosensors as they are
often found on doorway entry systems.
In your common opto-isolator, when a current is passed through the low-voltage side the LED will emit an infrared
light. This light is detected by the photosensor on the high-voltage side. When the photosensor detects this light, it will
break or connect the high-voltage circuit depending on its current state. With your common infrared opto-isolators
there is a small delay of up to a few nanoseconds. That may sound very small; indeed, for your needs to switch a mains
device it's more than fine. It's not so fine if you wanted to use an opto-isolator to transmit sound though. The best thing
about opto-isolators is that there is no way at all the high-voltage side can get onto the low-voltage side. An opto-isolator
is a very safe way to isolate your projects and is not all that expensive.
MOSFET
Another common way to isolate the mains voltage is via a power MOSFET. MOSFETs are simply transistors used to
amplify or switch electronic signals. Power MOSFETs are a little different than normal MOSFETs: the gates inside
a power MOSFET are created vertically rather than horizontally. This allows the power MOSFET to sustain a large
blocking voltage, say, for example, a 240-V AC mains source. You will often encounter power MOSFETs in audio
amplifiers that are attached to massive heatsinks, because they tend to put out a large amount of heat. The reason you
find them in audio equipment is they have a very fast switching time, unlike an opto-isolator. Let's take a quick look
at Figure
10-3
to see how a MOSFET works at a high level.
