By Max Peel
When earning a degree in electrical engineering in years past, students learned all about power interspersed with a few elective courses on that new fledging topic called electronics. Today`s engineers know a great deal about electronics, but power is in a lost gray area. This is not the power associated with utility or power station technology, but the power used to energize electronic subsystems or industrial equipment, particularly from 1 to 50 A.
An interesting search to conduct is to review connector catalogs and find the current rating that is listed and the supporting information validating the said rating. The search reveals the current rating but, with a few exceptions, the supporting information is nowhere to be found. For those instances where said rating is listed without the supporting data, questions should be asked as to how was the rating established. Odds are that the answer will be evasive or vague at best.
At the outset, one factor that needs to be determined is what the rating is based on.
- One contact energized in air
- One contact energized in the connector housing
- A number of contacts energized in air
- A number of contacts energized in the connector housing.
If the last is true, how many and at what location the measurement was performed at need to be determined. A second factor that needs to be determined is if the current rating that is indicated implies that all contacts in the housing can carry the current simultaneously.
Factors That Impact Current Rating
How should the current rating be established? What factors impact it? What are the basic concerns involved? The capability of a contact to carry current is a function of these variables, among others:
- Contact material
- Conductivity of the material
- Crossectional area
- Area in contact
- Contact geometry and configuration
- Connector housing/mating factors
- Normal force
The key functional element in power applications is heat generation or temperature rise that is generated as the current increases or as the same current is used in a different contact configuration. Contingent on the magnitude of heat generated, damage may occur and, possibly, a fire may be initiated. Additionally, heat generated is also exasperated by the number of contacts within a pattern carrying a given current level at the same time. Thus, temperature rise should be established and controlled to assure that the function of the connector is not constrained.
Basically, two techniques may be used to establish current ratings. The first is to establish the current that generates a given temperature rise (i.e., 20°C rise). Normally, this is accomplished by incrementally increasing the current at specified intervals until the desired rise is achieved and the current rating is established and derated accordingly.
The second technique is to generate a series of temperature rise plots at three to five current levels. This data can then be used to generate a current-carrying characteristic, which, in essence, states that the temperature rise plus the equipment operating ambient should not exceed the temperature rating of the connector. Thus, the hotter the equipment ambient, the less current a contact can carry, with the reverse being true as well.
The first technique is employed by the Underwriters Laboratory and automotive industry to control the heat generated for safety reasons and product liability. The second technique was created by the International Electrotechnical Commission (IEC) and has been adopted by the Electronic Industries Alliance (EIA). Thus, basic procedures are in place to establish the current rating of a given contact system. Whichever technique is chosen, references should be noted in the literature so that decisions can be made with the appropriate supporting information.
Thermal Runaway
This information is the essential data required to prevent damage. It is also the baseline for further analysis to determine if thermal runaway is a potential problem. Thermal runaway is a time-dependent failure mechanism. Temperature rise is a prime function of the contact interface characteristic. It generates an electrical resistance, a T-rise. If the resistance increases over time, a resultant increase in T-rise occurs. This continues until the heat generated is of such a magnitude that it creates significant damage.
Techniques are also in place to assess if thermal runaway is a potential failure mechanism. Once the current rating has been established, a current cycling test can be performed. This is an electrical stress test that can be performed for a specified period of time (usually 500 to 1,000 cycles), monitoring T-rise and millivolt drop (optional) at each cycle or at specified increments (i.e., twice a day). This determines the stability of T-rise over time. Another effective and discriminating test is vibration under load to determine if fretting will be an influencing factor.
While evaluating the contact system, other areas that may be impacted should also be evaluated, including:
- Monitoring heat within a wire bundle
- Monitoring heat on a printed circuit board (PCB) adjacent or in proximity to the current input area
- Monitoring heat on crimp joints, particularly saddle styles manufactured from strip material.
Need of Supportive Data
These are some of the technical issues that have to be resolved or that need supportive data to be made available. This is a far cry from simply indicating a current rating in its simplistic form, which leaves a lot of unanswered questions. The additional supporting information is necessary for understanding and comprehension. Yet, with a few exceptions, this data is not available.
It has become increasingly important to understand the interrelationships and interactions, as previously indicated. In many applications, signal contacts are being assigned to carry power. For example, it is not unusual to assign 10 contacts to carry 2 A (in parallel) in order to supply 20 A to a subsystem. The supportive data is rarely available that establishes performance criteria for this type of application. Most qualification specifications specify a current rating, but there is no requirement or test required to show the design capability. Even if a document indicates a requirement, in most cases, the methodology is lacking. For example, there may be no indication of thermocouple placement or of the number of contacts to be tested (i.e., single contact or multiples).
There is ample data generation to assure performance characterization and stability of signal contacts. Power, on the other hand, appears to be in dark void with little or no attempt to establish the stability of power contacts.
All is not lost, however. Currently, the EIA is attempting to expand and update its procedure for current rating. It has also been debating the establishment of a recommended test plan to determine performance criteria for power contacts. But standards tend to move slowly, and, in the meantime, a responsible approach for indicating current rating must occur. Not only should current rating be indicated, but supporting information should be provided as well. The long-term goal should be to create techniques to establish stability criteria for power contacts. Until this occurs, power will continue to languish and be a lost technology.
MAX PEEL, a Connector Specifier Advisory Board Member, is president of Contech Research Inc., 67 Mechanic St., Attleboro, MA 02703-2090; (508) 226-4800; Fax: (508) 226-6869.
