DC Electrics - Switches — Page 29, Lesson 29

I want to walk you through the world of switches in aircraft DC electrical systems. Switches are how we initiate and control aircraft circuits — along with relays, they're the fundamental way we tell the electrical system what to do. Let's start with the first type. A toggle switch is a general-purpose switch common in older aircraft. Inside it has a number of isolating contacts. It can be a two-position switch — simply on or off — or it can be a multi-position switch that's spring-biased to the centre or off position. With that type, you press and hold it to select in the desired direction, and when you let go, it springs back to centre or off. Now, switch lights have largely replaced toggle switches in modern aircraft. They combine two functions into one unit: a switch with a push action, and an indicator light for the associated function. There are two basic types. The first is momentary action — you press and hold to activate, and release to deactivate. The second is alternate action — you press and release once to activate, then press and release a second time to deactivate. The indicator in the lens confirms the selected position, or it can provide a warning that requires the switch to be selected. Next we have guarded switches. Both toggle switches and switch lights can be fitted with a guard to prevent inadvertent operation. A classic example is the generator disconnect switch or the fuel dump master switch — you have to lift the guard before you can operate it, so you don't accidentally shut down a generator or start dumping fuel. Now, microswitches are still used in modern aircraft to detect the position of a particular device — for example, whether a door is opened or closed. The name "Microswitch" describes the very small movement between the 'make and break' position — that is, between the point where the switch contacts close and where they open. Microswitches can activate indications on the flight deck, or they can control relays for a sequenced operation. However, they have been largely replaced by proximity detectors on modern aircraft. Next is the bimetallic switch, also called a thermal switch. This is a temperature-sensitive switch. It activates when a certain value of temperature is reached, and it provides an indication to the pilot or activates or deactivates a circuit. Examples include fire detection circuits, a battery overheat switch, and an oil temperature warning light. Finally, we have proximity detectors. These are electrical or electronic sensors that respond to the presence of a material. That electrical or electronic response is used to activate a switch, a relay, or a transistor. There are many types of proximity detectors, but the major types are inductive, capacitive, and magnetic. The inductive and magnetic sensors need the monitored material to be metal. The capacitive type, however, can monitor either metal or non-metal materials. Let's look at the inductive type in a bit more detail. This type of sensor has an inductance coil. Its inductance changes when a ferromagnetic material — what we call the target — is brought into close proximity with it. So as a metal object approaches the coil, the inductance changes, and that change is detected and used to trigger the switching action. This is the principle used in many undercarriage position sensors on modern aircraft, replacing the older microswitches.