Hardware Reference
In-Depth Information
High-voltage Side
Low-voltage Side
Circuit
Latch
magnetic
coil
Circuit
Circuit
Figure 10-4.
A high-level overview of a mechanical relay
In this very high-level diagram you can see that if we applied a current flow to the circuit attached to the magnetic
coil the latch would move to the left to complete the circuit on the left-hand side. When the current is removed, the
latch moves back to the right and the circuit is broken.
The other type of relay you may come across will be a solid-state relay. Solid-state relays use MOSFETs inside
to achieve the same outcome as a mechanical relay. They have the benefit of being quicker and less prone to failure.
Unfortunately they also generate more heat and when they do fail they have a habit of failing and short-circuiting.
You are not likely to come across a solid-state relay in a mains AC switching circuit unless one of the selling points
is low noise. After all, they do cost more than their mechanical counterparts. Now that you know the most common
ways to switch the AC mains, what method will this chapter use?
The AC Mains Remote Control
All through this chapter I have said you won't need to touch the AC mains and even after all this talk of how to
switch the AC mains you won't be getting anywhere near it. You're going to be using a remote-controlled AC mains
switch (I'll show you which of the above techniques it uses in a little bit). Make sure it has a separate remote control;
I don't want you busting open the main unit that connects to the mains socket. Let me warn you about AC mains
again.
Don't ever work on live AC mains circuits.
Don't even think about opening an AC mains device unless you
know what you're doing. Remember that capacitors may hold lethal voltage for upward of 30 minutes after you
have unplugged the device. You can see in Figure
10-5
the device I have picked up. This device operates over a radio
frequency, which I feel is better than an infrared device.
