Circling Around the Military and Commercial Market
By Lynda Nolen, Bishop & Associates Inc.

Circular connectors have been used for decades as a means of transferring signal and power outside the box. Accounting for approximately 6.7 percent of the total 2009 connector market, and ranked fourth behind PCB (printed circuit board), I/O rectangular, and application-specific connectors, circular connectors can be found in every end-use equipment sector. Circular connectors offer excellent environmental sealing at the connector interface, a reliable but easily implemented coupling system, and an extensive array of contact arrangements and contact styles.

Although the basic design and concept behind many of the more popular circular connectors has changed little over the last several decades, their use has continued to rise. What has allowed this connector family to maintain and grow in popularity, alongside more recently developed connector types, is this family’s ability to change and adapt to the ever-changing world of electronics.

Traditionally, circular connectors have been divided into mil-spec types and commercial types. With the onset of COTS (commercial-off-the shelf) requirements imposed by many OEMs, and in particular military and government purchasing entities, the line dividing these two types has all but vanished. Today, mil-spec type circular connectors are routinely used in commercial or industrial applications, and ruggedized versions of commercial circulars are used in military and other high-reliability applications.

A number of factors have led to the demise of this dividing line, all initiated primarily by technological advancements. New and improved materials for shells, insulators, contacts, and hardware, and the ability to modify existing coupling methods and contact arrangements, are some of the factors that have allowed circular connectors to expand beyond traditional applications. Now circular connectors can be used in applications that demand high current-carrying capabilities or board-mounted connectors. They offer the flexibility of mixing copper and fiber, signal, power, and coaxial contacts, can easily be filtered, can provide high-quality EMI shielding, and can be adapted to handle a multitude of connector interfaces.


Connector Composition
The composition of shells and insulators has greatly impacted the use of circular connectors, especially in harsh environment applications such as aerospace, industrial, medical, and automotive. In aerospace applications, as well as other applications where weight is crucial, shells manufactured from brass, nickel aluminum bronze, aluminum, stainless steel, or titanium have been replaced by alternative shell materials, primarily composite types.

Just as steel can be made stronger by increasing the carbon content, have greater tensile strength by adding nickel and manganese, or become more resistant to corrosion by adding chromium, the addition of fibers, primarily glass to a host matrix material such as epoxy resin, polyimide, or polydicyclopentadiene (PDCPD), can improve the properties of these materials. Composite materials are generally stronger than steel, provide excellent resistance to corrosion, and offer greater durability—yet weigh substantially less than their steel counterparts, up to 50 percent less.

In highly corrosive and harsh industrial applications, such as wastewater treatment plants, oil refineries, and on heavy-duty earth-moving equipment, the connections to valuable equipment are protected using high-impact glass-filled molded thermoplastic housings. Other improvements in circular connectors for harsh environments include the expansion of metal shell platings, beyond typical olive-drab chromate-over-cadmium or electroless nickel, to also include black zinc cobalt, black zinc nickel, and the use of anodic platings. For applications requiring less protection and more cost-conscious solutions, high-impact resistance housings made from heat-resistant, self-extinguishing thermoplastic material are available. Many of these less costly solutions offer both individual or strip-form contacts for use in high-speed crimping machines, as well as the capability to mate with higher cost metal shell connectors.

Not only have changes made in the composition of shells and housings increased the number of applications suitable for circular connectors, upgrades in insulator, gasket, and O-ring materials have also allowed the use of circular connectors to grow. The use of Viton® fluoroelastomers and APA (Advanced Polymer Architecture) Viton polymers for gaskets, O-rings, and even insulators, particularly in automotive and industrial applications, provide increased resistance to fuels, acids, lubricants, and a variety of other chemicals. Connector insulators manufactured of halogen-free materials, such as DSM Engineering Plastics’ halogen-free Stanyl® or other halogen-free neoprenes, reduce smoke density in the event of a fire and eliminate possible toxic contaminates, such as dioxins and furanes, from being expelled into the air.



In medical applications, insulators manufactured of PEEK (PolyEtherEther-Ketone) allow reusable medical connectors to withstand the extreme temperatures and harsh conditions of autoclave and gamma radiation sterilization. In medical applications where disposable connectors are utilized, medical-grade PBT (Polybutylene Terephthalate) insulators, which offer good electrical and mechanical properties, allow cheaper, disposable connectors to be mated with high-grade reusable connectors, with no degradation of signal.







Not confined to the medical industry, materials like PEEK are also used to manufacture housings for connectors destined for high-voltage applications, and in applications requiring hermetically sealed connectors, such as down-hole tools and oil and gas well completion systems.

Connector Interfaces
Because circular connectors provide excellent environmental sealing at the connector interface, and because they offer a number of coupling systems that are easy to use and familiar to most, it is no surprise that circular style shells have been chosen to replace or improve the mating of many less environmentally sealed interfaces. An excellent example of this can be found in Bulgin’s Buccaneer series. Incorporating both power and data connections, Bulgin’s Buccaneer series of IP68-rated environmentally sealed circular connectors offer USB, mini USB, FireWire, Ethernet, coax, and power connections up to 32 amps at 600 volts.

In addition to standard environmentally sealed versions that offer alternative interfaces, heavy-duty, Mil-DTL-38999 series shells are also available that can easily be fitted with standard cordsets. These greatly expand the protection of these standard cordsets, allowing them to meet IP67 requirements for dust and moisture, as well as mil-spec requirements for shock and vibration.

Coupling Mechanism
Improvements in the method of securing circular connectors when mated, often referred to as their coupling mechanism, has also had an effect on the increased popularity of the circular connector. A quick, yet secure and reliable coupling mechanism saves time and prevents damage to costly connectors and potential damage to the equipment they are connected to. Common methods for securing mated circular connectors include standard and reverse bayonet, threaded or screwed, and breech lock coupling. A coupling method that has gained in popularity over the last decade is the push/pull. Previously relegated primarily to commercial applications, improved versions of the push/pull coupling system feature options like emergency release and locking features, and have found numerous applications in the military/defense and medical markets, mostly on handheld or portable equipment. Other improvements in coupling mechanisms include self-locking mechanisms on threaded or screwed systems that prohibit the connectors from backing off in cases of extreme vibration. 

Contact Arrangements and Styles
Probably one of the most influential changes that circular connectors have undergone is their capability to integrate a mixture of contact styles and types. Today, in addition to a wide range of crimp or solder signal contacts, circular connectors can incorporate fiber optic, concentric or differential twinax, triax, standard and high-speed quadrax, high-current, high-power, screw terminals, standard and high-frequency coaxial, microwave, and thermocouple contacts. There is a broad array of printed circuit board contacts in a variety of lengths and contact diameters, as well as contacts specifically designed for flex circuitry.

Based on connector series or type, the number of contacts per connector and arrangement of these contacts is also astounding. Circular connectors are available with one to 300 contacts, in well over 500 different contact arrangements, not including polarizing options. High-density or density and one-half versions of popular mil-spec types, like Mil-DTL-38999, have also increased the number of standard insert arrangements available. Couple this with an expanded offering of straight and 90-degree factory or field-installed backshells, integrated backshells, endbells, cable clamps, adapters, and strain reliefs, as well as connector styles (cable mount, in-line, narrow or standard box/wall-mount, thru-bulkhead, and jam-nut), and the number of potential circular connector types becomes virtually impossible to count.

Similar to Newton’s third law of motion—for every action there is an equal and opposite reaction—for virtually any application where power or signal must be connected quickly and securely to the box, there is a circular connector that will fit the bill. Although previous attempts have been made to replace the reliable circular connector with other connector types, design engineers routinely return to the ever-faithful circular connector. For years, military specifications have indicated particular circular types were inactive for new designs, yet in many instances, these connectors have been used anyway, with many military specifications eventually updated. In the industrial, medical, transportation, and automotive sectors, where the integration of electronics has expanded drastically over the last decade, circular connectors have found unlimited applications. The circular connector’s ability to adapt to changing environments by incorporating improvements in every feature have allowed the use of circular connectors to grow and expand, and that will continue.

Lynda Nolen Product Specialist, Bishop & Associates Inc. Lynda Nolen has been in the interconnect industry for over 30 years. She has worked in sales, sales management, marketing, and product management for such companies as TRW Electronics Components Group, Sunbelt Components, Cinch Connectors, Arrow Electronics, PEI Genesis, and Delphi Interconnect. Nolen has extensive experience in competitive cross-referencing, drawing, web and catalog review, new product introduction programs, harness and connector assembly programs, account management, and customer service programs. Lynda received her Bachelor of Arts degree from Roger Williams University in Rhode Island in 1979, and has completed various electrical engineering courses.