Per:
If you find it necessary to source enough juice out your service connector that a "power" resistor is necessary then I would re-think the solution. Surely a poly-fuse or a current limited transistor shut down circuit would be way better solutions.

I am reminded of a "universal DC adapter" that was sold for use in autos off the accessory power plug (formerly known as a cigarette lighter plug). This device was specified as capable of "converting" the +12V from the car battery to 9V, 6V, 3V or 1.5V selectable through the use of a slider switch. Inside the switching circuit simply changed what power resistor was in series with the +12V!! The resistors were so under rated that they were mounted on a heat sink !! Needless to say the specified output voltages were only accurate at a single level of current draw !! Plus keep in mind that if the current draw was anything but minimal the heat sink got so hot it could actually make melt marks on the plastic interior elements in the automobile. Thank goodness that today the typical car power adapter uses a switcher type converter so that the output voltage is just what is needed and done in an efficient manner.

Per: I go along with the idea that someone else suggested to use a higher voltage out the service connector if it is available. But at the same time you need to consider how you protect the source in side your equipment. And you at least need to anticipate that that source may be given a complete short to the return for an indefinite period of time. A resistor in series with the power source would to me be acceptable if the short circuit condition resulted in just a few mA of current draw.

On the other hand consider for example that you decide to support up to 100 mA out your service connector off a 12V supply. You also decide that you could live with a connector voltage as low as 9V so that an external 7805 type regulator will have enough "head room". This means that nominally a series resistor drops 3 Volts at the 100 mA which is equivalent to a net 300 mWatts across some 30 ohms. Now if your service connector power pin experienced a direct short to the return path then the 12V is fully dropped across the 30 ohms for a short circuit current of 400 mA. Can your power supply provide power like that and still let the internal electronics run reliably? Such resistor of 30 ohms would have to be rated at 10 Watts using a typical 2::1 derating factor. Does that really make sense? Especially considering that you could select a 100 mA polyfuse that will open at somewhere over 100 mA (but surely much less than the 400 mA of the resistor solution) and have a much smaller power level impact inside your equipment.

Michael Karas