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Switches and Switch-Like Devices Part 3
By Tony Bingelis (originally published in EAA Sport Aviation, December 1985)
In aircraft electrical systems, switching functions are sometimes provided by devices other than the rocker and toggle switches with which we are most familiar.
This broad family of switching devices would include a large variety of relays, solenoids, circuit breakers, micro or limit switches, and special purpose devices such as stall warning lift detectors and differential pressure switches. Even though this list is quite extensive, it is not all-inclusive. However, I don't think we need to delve too deeply into the workings of each type of switch as they all have similar characteristics. We can lump most solenoids and relays together in one group and ordinary switches in another group. That leaves us with a special group of switches that have many applications in high performance sportplanes like the Swearingen SX300, the Glasair, the Falco and most other retractable gear designs. These are the "micro switches" or "limit switches" as they are sometimes called.
The terms "micro switch" and "limit switch" are used interchangeably by most switch manufacturers, although one of them does call theirs "snap action switches". My own way of reasoning leads me to refer to this family of switches as "micro switches" until they are installed - then I call them "limit switches". Actually, both terms will be used indiscriminately.
As you might gather from the name, a micro switch is one that is operated through a very small movement of a tripping device. Ordinarily, the tripping device is a small plunger. Apply a light pressure on the plunger, and the resulting small movement of about 1/32" to 1/16" is all it takes to trip the internal circuit.
A good thing to know is that this small movement of the plunger is accompanied by a distinct audible "click" that identifies the instant that one internal circuit has opened and the other closed. This "click", you will find, can be very helpful in obtaining a precise initial positioning of the limit switch plunger during its installation against the component (landing gear or flaps, for example) whose movement needs to be limited.
Although a number of micro switch types are operated directly by means of a basic plunger, there are other types that utilize a lever or a levered roller superimposed over the basic plunger (see Figure 1).
Micro Switch or Limit Switch Applications
Already mentioned was the use of limit switches in landing gear and flap installations to limit the up travel and the down travel of the landing gear as well as the flaps. However, there are other useful applications for limit switches.
Some VariEze and Long-EZ builders even install a micro or limit switch controlled circuit connected to a light or horn to warn the pilot that his canopy is UNLATCHED. This is in addition to the limit switch installation for the usual nose gear warning proviso. Such a canopy warning system is a good idea and could be equally important in your own aircraft, especially if its windshield is integral with a swing-open canopy.
You may not realize it but the familiar stall warning, or lift detector mounted in the wing, is, in effect, nothing more than a micro switch that has its tripping lever operated by the flow of the slipstream across a blade-like vane. This switch has 3 micro switch-like terminals which are labeled COMMON, NORMALLY OPEN and NORMALLY CLOSED. The flow of air over the wing causes the activating lever to lift and close the electrical circuit inside the lift detector. The simple circuit is connected directly to a horn which sounds a warning inside the cockpit whenever a change in airflow over the wing signals an approach to a stall (see Figure 2).
Select a limit switch as you would am/ other type of switch. That is, pick a limit switch that has the electrical capacity to handle the loads to which it would be subjected in a particular circuit. For example, although a small landing gear motor may draw only 2 to 3 amperes while running unloaded, it could easily demand 10 amps or more during starting. Such a high "in rush" amperage should be considered and allowed for.
Examine a typical micro switch and you will find its electrical capacity printed or embossed somewhere on the exterior housing in much the following manner:
"15A 125-250VAC, 1 hp 125VAC".
Here's another example:
"5A 1/4 hp 125-250VAC".
As you may have surmised, in spite of all the numbers and letters, one switch has a 15 ampere rating and the other a 5 ampere rating.
Examine the micro switch housing further and you will find that it may have either two or three terminals. One terminal is always embossed with the term "COM." or "COMMON". The other two are identified as "N.C." (Normally Closed) or "N.O." (Normally Open). These terms refer to the switch's internal electrical circuit relationship. The functions of these terminals is also illustrated in Figure 3.
Incidentally, not all micro switches have screw type connector terminals. Some manufacturers produce micro switches with what they call "standard solder terminals". They do, however, also produce switches with spade terminals and with screw type terminals.
The solder terminals can be recognized by their narrow spade connector. A regular spade connector terminal is noticeably wider and will accept a standard push-on female shielded connector. Both types, however, can usually be successfully soldered if you prefer.
Soldered connections are fine, easy to make, and provide a lightweight installation for the most part, but . . .
Some of you may already have experienced the frustration brought on by a broken soldered connection while away from home. They are almost impossible to reconnect without resoldering them. Unfortunately, too, many of us have to house our aircraft in a hangar that has no available electricity. Unless we are lucky enough to have a battery powered soldering gun, there is no way to resolder broken connections without having to remove some equipment and take it someplace to have it repaired.
These are reasons enough for most builders to select screw type terminals, not only for micro switches but for most of their electrical equipment.
There is no standard way to install a limit switch. Sometimes you can secure it directly to some part of an adjacent structure or component, and sometimes a separate bracket may have to be installed. The means of attachment vary, but in all cases, the mounting should be unyielding. Otherwise, the limit switch plunger may not be tripped consistently at the right moment.
In my estimation, the neck mount type of limit switch is the easiest to install and to adjust accurately simply by varying the extent to which the long neck of the limit switch is allowed to protrude beyond the mounting bracket.
Other limit switch types don't have any adjustment provision after installation and an exact positioning is absolutely essential when you mount it. Of course, you could always install a shim behind the bracket to move the limit switch closer to the unit whose movement it is limiting. But what if the plunger is already too close?
Usually, a limit switch is installed on some fixed portion of the structure or mechanism adjacent to the moveable component. Let's take a landing gear as an example.
The limit switch plunger (lever, roller or whatever) must be located so that the movable component (landing gear in this case) will trip the limit switch circuit at the precise moment the landing gear is fully retracted or extended. Essentially, the switch must be wired so that the moment its mechanism is tripped, the circuit is opened or closed, depending upon the terminal selected (N.O. or N.C.). The tripping of the plunger instantly opens the electrical circuit and stops the gear motor immediately.
Can you imagine what would happen to a landing gear system mechanism and/or gear motor if the motor kept running after the landing gear had reached its fully extended or retracted position? If a little self destruction didn't take place it would only be because some nice circuit would have tripped and mercifully saved the day.
If you weren't knowledgeable enough to have installed a properly rated circuit breaker in the circuit, there is a very good possibility that the gear motor would have "burned out" in its persistent effort to move the immovable. Here is an instance where a circuit breaker does, in effect, serve as an automatic switch.
Yes, indeed, it is a very good idea to have the limit switch open the circuit to the landing gear motor at the exact moment the gear reaches its down and locked position. It is also a good idea to use the other terminal of the same limit switch to activate a green light to show that the gear is down and locked. If the light is on, the motor cannot be running because the light is evidence that the motor circuit is OPEN and that all is well. (See Figure 3)