Back to Basics - Buttons and Switches
There are many manual devices used to control an electrical system. We use the word 'Manual' to refer to how it's operated - not by another circuit, not by air, not by a computer, but directly by a human operator.
These devices are called ‘inputs’ since you put energy "IN" to the system to control it.
Sometimes, the switches may control the machine directly, like a light switch in a house.
Other times, the switches may not actually control the big heavy loads, but rather energize other electrical devices, like relays or motor starters.
This would be like a the ignition key in your car. The power for the starter doesn't actually go through the ignition switch, but the key activates a relay (solenoid) that turns on the starter.
Finally (and my favorite), they can send very low power signals to computer-controlled machines, like CNC machines or PLCs.
No matter which application we use for these control devices, they always send information or voltage "IN" to the system, and have we already saw, that makes them 'input' devices.
There is quite a variety of control devices available, for sure!
Most of them are like buttons and switches which are turned on and off when you touch them. Sometimes, there are knobs like dimmer switches, and we call these 'analog', but they aren't quite as common.
Some of these input devices always try to stay in one position. A spring will press them right back - like your ignition key in the 'start' position. As soon as you let go, it pops right back to the 'run' position. This action is called 'momentary'.
Other types will stay in one position, like a light switch. You don't need to sit there holding it, you can turn it on or off and walk away. This term is 'retentive' or 'latching'. Usually when we use the word 'switch', it also indicates this kind of retentive action. It doesn't try to return to any normal position.
Those momentary devices have two kinds of actions: Normally Open and Normally Closed.
Pushing the button allows electricity to flow. Release the button, electricity stops. It's really quite simple, but sometimes hard to identify.
Big industrial switches have removable contact blocks on the back of them. Sometimes they are color-coded, and a green block on the back means that it’s ‘Normally Open’.
When you press the button, it STOPS the flow of electricity. As soon as you release it, electricity flows again.
They are very common in emergency stop situations. In fact, Emergency Stop buttons are required by law to be Normally Closed.
A red colored block on an industrial switch would be ’Normally Closed’.
Remember, not every switch will be color-coded!
Some buttons and switches are built with light bulbs or LEDs inside.
They might light up whenever the button is pressed.
Sometimes, the light is wired separately from the button, so you can use it however you want. Maybe you need the light to turn on before the event, telling the operator when is the right time to press the button.