Circular connectors, or circular interconnects, are cylindrical, multi-pin electrical connectors containing contacts that transmit both data and power. Canon (now ITT Tech Solutions) first created circular connectors in the 1930s for military aircraft, but they can now be found in industrial applications, medical devices, and other environments where reliability is crucial, such as industrial applications and medical devices.
 
Circular connectors feature a plastic or metal shell surrounding the contacts, which are embedded in insulating material to maintain their alignment. These terminals are paired with a connector cable and makes them very resistant to environmental interference and accidental decoupling.
 
Connectors can be distinguished by their number of pins (usually three, six, or eight per terminal) and by their size, manufacturing specifications, attachment angles, and methods of connection and disconnection.
 
 A “standard” connector refers to a connector whose design conforms to either German or American military standards. American standards are MIL-Spec Connectors and are designed according to the best practices for military and aerospace applications. They are rugged, ideal for high-impact use, and are highly resistant to environmental conditions. Some MIL connectors are air and watertight thanks to an epoxy seal around the terminal. The German standard is DIN Spec Connector (Deutsches Institut fur Normung). DIN standards include high-frequency capability and hallmark features like a protective metal shell and notched, round terminations. Microminiature and nanominiature connectors featuring smaller pin and jack diameters also exist and have more narrowly spaced contacts. This construction helps save surface space across the face of the terminal, and it reduces the connector’s added weight to the component.
 
Another consideration for circular connectors components is there style of termination. The type of termination chosen determines the nature of the connections between electrical contacts in each of the connector’s components. Options vary regarding cost, ease of connection and disconnection, and resistance to tampering, wear, and environmental damage. Terminal options include:
  • Insulation displacement, where the pins in the plug pierce or push aside insulation around the corresponding jacks in the socket, making contact and forming an electrical connection.
  • Soldering: using an intervening metal with a low melting point, the metal touches both contacts to form a permanent corridor.
  • Screws or lugs: External hardware adheres to threaded (screws) or unthreaded (plugs) holes in the protective shield around each terminal to hold the connection in place.
  • Crimping: A contact barrel compresses around the conductor to complete the electrical connection.
  • Wire wrap: wires connected to the socket side of the connector attached to the other terminal. The wires wrap around the plug’s jacks, so an exposed segment touches the jack.


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Fiber optic cables get all the praise when it comes to new data transmission technology, but their efficiency greatly relies on a few small power connector components— cable connectors. The size and versatility of a fiber connector allows fiber optic cabling to transmit data across distributed avionics and eliminate the need for additional signal support equipment.
 
Fiber optic connectors hold individual fibers in place. They incorporate an associated adapter for alignment and mating of fiber ends which forms a unified connection point. Another way to think of this device is as a plug with a protruding ferrule. A ferrule protects the mating ends of a fiber from degradation, as splitting or cracking will damage the fiber’s ability to reflect light effectively.
 
While airlines formerly had to rely on an avionics bay to route data through the aircraft, the compact size of the fiber optic optical connector allows distributed avionics to put processing power closer to its point of use. Most aircraft formerly utilized copper cables, which require additional equipment for signal support, adding to overall weight. As a result, fiber optic cables offer 25% in space savings and 50% overall weight reduction. Connectors with composite shells are 40% lighter than metal connectors.
 
Along with knowing the benefits of connectors, you’ll want to know about the most common connectors that you might come across in aviation. Common 2.5 mm connectors, small form factor connectors, and multi-fiber push on connectors are the most prevalent fiber connectors used within the aviation industry. A general rule for this category of connectors is the following: common 2.5 mm connectors are the most versatile, small form factor(SFF) connectors are intended for high density purposes, and multi-fiber push ons (MPO) support high-data environments. 
 
There are three types of 2.5 mm common connectors, square connectors (SC), firewire connectors (FC), and stainless-steel connectors (ST).  As their name suggests, common 2.5 mm connectors have a 2.5 mm ferrule size and therefore can be used interchangeably, or mixed and matched with the addition of a hybrid adapter. These connectors were commonly used in the 1980’s and 1990’s, but the SC connector is still commonly seen in application with fiber optics.
 
SFF connectors have a latch design that allows for strong connections with reduced signal loss. In addition, it has a 1.25 mm ferrule giving it a compact and reliable design. The low loss connector (LC), a variety of SFF, is one of the most successful connectors in the industry.
 
To round out our connectors, the most commonly used power connector component in aviation is the MPO. With the rapid advancement of avionic technology, these connectors are necessary to provide high speed data transfer end-to-end. This connector can contain a multitude of fibers and can incorporate multimode fibers as well. MPO connectors are specified as “male” on one end and “female” on the other to ensure that all fibers are properly aligned.

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DC Power Connectors are electrical connectors used for supplying direct current (DC) power. They have standard types that aren’t interchangeable. The correct dimensions and arrangement should be chosen to prevent interconnection of incompatible sources and loads. They are used in a variety of applications, including small portable electronics and automotive accessories. There are three common types of barrel connectors: jacks, plugs, and receptacles.
 
Barrel Box connectors, or low voltage DC power connectors, have current and voltage ratings, specified by the manufacturer. The jack and the plug feature one exposed conductor and a recessed conductor. Jacks usually receive power and are mounted inside the appliance. A plug is usually located on the electrical cord and supplies power from a power supply. A receptacle is mounted on a power cord as well but receives power from a mating plug. Most engineers accept the center pin configuration as the defining factor for the gender of barrel power connectors. “Male” connectors have a center pin, and the mating receptacles are deemed “female”.
 
A common way of categorizing barrel connectors is through the diameter of the inner pin and outer sleeve. The diameter of the inner sleeve should be slightly larger than that of the mating pin. Typical mating connections are cantilevered flat springs. Jack insertion Connectors depth dimensions are often less than the plug barrel lengths for the following two reasons. First, the plug barrel may not be required to be completely enclosed by the receiving jack. Second, the depth of the chassis wall must be considered. Additional depth should be accounted for in the plug barrel length when the connectors are mated. 
 
Connector Plugs and jacks often have two conductors, one for power and one for ground. Usually, the center pin is power, and the outer sleeve is ground, but it’s possible for it to be the opposite. Some jacks have a third connector that forms a switch with the outer sleeve conductor; this is used to detect the insertion of a plug or to select between power sources.

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