Power connector selection requires an understanding of their important features to optimize system performance.
The first step in selecting the right power connector system is defining its current rating. It's also important to confirm that the product has an adequate current rating for the specific application. You can also refer to current rating as current density.
Current rating refers to the maximum current, in amperes/circuit, that a terminal may carry when operating in its specified housing. You can determine the current rating by measuring the maximum current that can be carried by a terminal system without exceeding a rise in temperature of more than 30°C (86°F). It's best to test in an environment of 22°C (72°F). In addition, you should not use any secondary methods of cooling, i.e. a fan, during testing.
A housing with all circuit positions loaded with terminals determines the current rating. All terminals must be able to carry the final current rating without exceeding the 30°C rise. A greater number of circuits carrying current will generate more heat. You must derate the connector system according to the number of circuits in a housing.
When choosing the proper power connector, confirm which wire gauge and terminal material were used for testing. For example, derate the current per circuit when testing is performed with a 16 AWG wire, but the application will use a 20 AWG wire. This derating takes into account that a smaller wire diameter will have an effect on the temperature rise at a particular current level.
Base metal used for the terminal affects the current handling capacity of the connector system. For example, a power connector system may be rated for 9A, when tested with 16 AWG wire terminated to brass terminals inserted into a fully loaded two-circuit housing. The same two-circuit housing, when loaded with 16 AWG wire and phosphor bronze terminals, may only be rated for 8A. Loaded with 28 AWG wire and phosphor bronze terminals, the same two-circuit connector system may only be rated for 1A. Going back to the 16 AWG wire and brass terminals, a fully loaded 10-circuit housing may be rated for 7A instead of the two-circuit's 9A.
Circuit size is the number of circuits a connector system should have. Maximum current rating of the connector system per circuit, maximum tolerated engagement force, and physical size of the connector determines the circuit size. To meet all system requirements, some applications may use more than one connector.
Another critical factor is connector size or circuit density, which is the number of circuits a connector can offer per square inch. The design engineer can use this information to choose a connector family that offers the optimal current carrying capability versus space required to accommodate the connector's physical size.
Determine actual connector configuration by the application. Connector configurations are typically available in wire-to-wire, wire-to-board, and board-to-board. Wire-to-wire configurations may be free hanging or panel mounted.
Many applications may require you to direct power straight to a p. c. board. In these applications, a connector is soldered onto a p. c. board. This is typically known as a header and is often available in a vertical or a right angle design. Headers often come preloaded with terminals. These terminals are essentially the same as the crimp terminals — except an elongated solder tail inserted into the p. c. board replaces the crimp section for the application of solder.
The next step in choosing the proper connector system is to review wire size, which affects the connector system's current derating. A heavier gauge wire allows more current flow with less heat generated and can also increase the mechanical strength of the wire harness. Too large a wire can exert too much force on the crimps and terminals when the wire harness exits from the back of the housing. In this case, you should consider a strain relief.
For designs requiring a more flexible wire harness, you may consider a smaller wire gauge or possibly wire designed specifically for high flexing applications. Most power connectors will accommodate wire gauges from 16 AWG up to 28 AWG. The chosen terminal should be designed for the required wire gauge so you can achieve proper terminal crimp performance.
Connectors are also rated for operating voltage. Typically, power connectors are rated from 250Vrms up to 600Vrms. Voltage rating applies to ac and dc systems. You can achieve higher voltage ratings by fully isolating the contacts from one another in the connector housing. Fig. 1, on page 64, shows an isolated receptacle and header with contacts rated at 600V, 50A maximum (37A for 6 circuits).
Choosing proper terminal material and plating is also important when specifying a connector system. Typically, manufacturers offer terminals in brass or phosphor bronze base material. Brass is a cost-effective choice and offers a higher current carrying capability than a similar phosphor bronze terminal. Phosphor bronze terminals tend to be slightly higher in cost and require some current derating since the material doesn't maintain the same current capacity as brass does. However, phosphor bronze does offer a higher tensile strength rating. As a result, phosphor bronze is typically a preferred choice for applications requiring an elevated temperature (>75°C/167°F) or when you require a terminal with high cycle capabilities (100 cycles or more).
With the need for greater current carrying handling capabilities, as well as with advancements in processing of base materials, higher current terminals are available with some connector families. These materials offer the ability to carry more current per terminal contact using the same terminal design as the basic product. These high current terminals, although slightly higher in cost, can increase the current carrying ability of a connector system up to 50% — without increasing the size of the connector system.
The crimped terminal in Fig. 2, on page 66, is rated at 9A in brass, 12A in high conductivity alloy.
Connector terminal systems are available in a choice of plating options — typically tin, tin/lead or gold. Tin is less costly and generally suggested for applications carrying more than 0.5A. A Gold plating specification usually applies in applications where terminals carry signal level power (generally less than 0.5A). You can find gold plating in applications requiring high reliability, when exposing terminals to caustic conditions, and on terminals specified for high cycling.
Once you determine the terminal, investigate the method of wire termination. You can perform wire termination manually or with automated equipment. You can usually carry out manual crimping with a hand-crimp tool used according to manufacturer's instructions. Never use a pair of pliers for crimping. A hand tool does not guarantee that its user will position the terminal and/or wire properly or provide a consistent amount of force for the crimp action, so it's only recommended for a small number of crimps in a lab or early application evaluation.
You should perform wire crimping for production using automated crimping equipment available from the connector manufacturer. Although it's important to monitor setup, you'll achieve more consistent crimping. Terminals are available in reel form for automated, consistent crimping.
A free-hanging wire harness is easy to apply, since no special cutouts are required on a panel. However, free hanging makes it difficult to quickly locate a particular connector. In contrast, you can panel mount a connector, allowing easier location, access, and identification of a connector system. Panel-mounted connector systems require ears and a specified panel cutout.
As the need for current increases, some connector applications may require either large or bulky wire harnesses that can exert undue stress on the terminals. In turn, this may eventually lead to mechanical and/or electrical failures or malfunctions of the connector system. Some product families offer a strain relief to reduce the strain on the terminals and/or crimps.
A passive or positive locking latch is available depending on the application. A passive locking latch provides a slight increase in disengagement force above and beyond the disengagement force of the terminals themselves. A positive locking latch requires a user to physically activate a mechanism to disengage a connector system. A high vibration or critical application should specify a positive locking latch. The connector in Fig. 3, on page 67, has a positive locking latch.
Some applications are difficult to reach, calling for a blind mating interface (BMI) which allows engagement of the connector system without visual alignment. Blind mating connectors require specific design criteria. It's necessary to meet close adherence to the connector system specifications. In Fig. 4, the connector has blind mating interface (BMI) capability.
P.C. Board Considerations
Some connector designs require you to mount one or both halves of the connector system on a p. c. board. Here, the connector system must allow for terminals with solder tails. For additional design flexibility, the connector family should offer right angle and vertical-mounted design versions.
Where a vertical header is attached to a p. c. board, processing of the board may be critical if it's sent through a solder wash. Thus, the connector should have an option for drain holes to prevent water retention from a solder wash process in a vertical header.
When you want to attach a connector housing to a p. c. board, you must hold the housing in place before and after the solder processing. The surface mount compatible connector in Fig. 5 has metal mounting clips. Many connectors offer p. c. board pegs. These pegs allow the connector housing to snap into the p. c. board and hold the housing in alignment before solder processing. Plastic p. c. board pegs offer minimal strength to the connector/p. c. board interface after the solder process.
Power connectors are typically made of nylon or polyester. Some connectors are available in high temperature materials for surface mount assembly. Glass-filled nylon housings are also used for surface mount applications. The housing material in the connector in Fig. 6 is a high temperature plastic.
Most power connector materials, regardless of type of material, are rated as UL94 V-2 or UL94 V-0. The UL94 rating refers to the ability of the plastic to be able to self-extinguish in case of terminal overheating causing a fire to start. UL94 V-2 is the lesser rating with the UL94 V-0 indicating the housing material has less ability to sustain a fire and will thus extinguish more quickly. The UL rating is independent of the actual housing material. Many housing materials are available in UL94 V-2 and UL94 V-0.
Engagement force, another significant design decision, refers to the amount of force, in pounds per circuit, required to engage a terminal system. For example, if a 24-circuit connector must meet the application's current requirements, then the pounds per circuit terminal engagement force must be multiplied by 24 to determine the actual force required to engage all 24 circuits.
The engagement force is important in determining the operator's ease of extraction and/or damage to p. c. boards. You must address this before finalizing the design. In some connector systems, engagement force can reach 50 lb or more. Also, many connector applications are wire-to-board or board-to-board. Here one or both halves of the system are soldered onto the p. c. board. It's important to ensure that the engagement force is not so high that it damages solder joints.
Agency approvals or listings ensure connectors are tested and approved for operation per safety or performance standards. Most connectors are tested to the requirements set by UL (USA) or CSA (Canadian) agencies. If the product is intended for sale in markets outside of these governing agencies, VDE and/or TUV (European) licensing may also be required. The connector is evaluated as a connector system. Testing is completed on terminals and housings operating together.
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