Part 1: Interfacing Methods For Vehicles with Incompatible Electrical Standards

North American commercial vehicles have electrical systems unlike those of Europe and the NATO military. Specifically, they operate at different voltages and use dissimilar connectors. As examples of wiring conventions, North American commercial vehicles use a 7 Pin SAE 560 system and NATO vehicles use a 12 pin system conforming to the STANAG 4007 standard. The European configuration is similar to the North American 12 V based SAE 560 system except that they are 24V based and use either a single ISO 1185 connector or one in combination an ISO 3731 connector . In addition to cross coupling vehicles from these three jurisdictions, when vehicles wired to RV standards become possible mating options to them the interfacing possibilities are further multiplied. Only electrical trailer interfaces make possible the electrical coupling of two vehicles wired with unrelated standards. To date, truck-trailer interfacing has been achieved with one of three commonly used electrical circuits: power resistor voltage dividers, centralized power switching regulators and distributed switching regulators. This three-part blog series will describe these circuit configurations, mentioning the pros and cons of each approach in the context of the electrical trailer interface application.

Electrical trailer interfaces have two functions. The first is to ensure that a signal at any given pin combination of the tractor is translated into an equivalent functional signal at the proper pins of the trailer connector. The second is to convert the voltage level of a power signal or combination of signals at the tractor output connector to a power signal of acceptable voltage at the intended pin of the trailer connector.

Power resistor voltage dividers work by dropping voltage through fixed power resistors. This method has three primary advantages relative to other electrical trailer interfaces:

It is the least expensive.
It has a simple circuit with a low component count.
If the output of one pin should fail, the others remain unaffected.
On the other hand, power resistor dividers have several disadvantages:

This approach is 50% efficient at best: For every watt of power transformed, at least one watt is dissipated as heat. As the source for the dissipated power, the tractor’s electrical system has to be able to furnish this power.
The housing to contain the power resistors has to be of large enough volume to enable internal components to remain within their operating temperature limits.
Power resistors of high dissipation capacities are also large and do not withstand shock and vibration as well as small components. Because of their high hot-spot temperatures, they are inherently unreliable.
Because the voltage drop across the resistor depends on the load current, this method has very poor regulation and can lead to sequential component failures.
Poor voltage regulation limits the number and characteristics of accessories that may be loaded on the auxiliary pin.
This method cannot be used for stepping up voltage, as in the case of a 12V towing vehicle and 24V trailer.
Part 2 of this blog series will focus on the advantages and disadvantages of another electrical trailer interface: centralized power switching regulators; then in Part 3 we’ll move on to the type used in SEC’s trailer interface models: distributed power switching regulators.

Links to Additional Information

NATO 12 pin functional description

SEC’s Electrical Conversion Products