Hardware Reference
In-Depth Information
Gate
Source
Drain
Figure 10-3. A basic overview of a simple MOSFET
There are three main elements: a gate, drain, and source. When a current is applied across the gate, a connection
can be made from the source to the drain. When the current is removed from the gate, the circuit is broken. Unlike the
opto-isolator the gate in a MOSFET will start to conduct with a lower amount of voltage and will rise in a linear value
until the saturation point has been reached. The saturation point is when the MOSFET is in a fully open mode and
there will be a linear change in voltage at the drain side. Power MOSFETs, unlike normal MOSFETs, have much more
stable behavior at the saturation point. This won't matter to you if you are switching only a mains device but would
matter a lot if you were amplifying an audio signal. The last thing you need is unpredictable behavior at a certain
voltage. It's uncommon to find a power MOSFET in such a simple circuit to switch mains power.
Relays
Relays are quite common these days (they never used to be). In the pre-transistor-era, relays were state-of-the-art
technology along with valves. All computers during this time used relays and valves to do their calculations. This made
computers of that era very noisy, very large, and supplied with very high voltages. Nowadays I would be surprised if
you found many relays in your home computer (if you found any at all). The two main, common types of relays are
mechanical and solid-state, although there are many more types.
Mechanical relays are, as the name suggests, mechanical; they work by a magnetic force that pulls or lets go of
an armature. It's simply a magnetic field being switched on or off. This magnetic pull is what gives you that distinctive
click that you hear. Take a look at Figure 10-4 ; in this figure you can see how a basic relay would operate. In practice
there is a lot more to it.
 
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