Understanding voltage drop

I am supplying 3.2 VDC to a device over 26awg rj11 phone cable. The device requires only 75 mA. I can use the readily available voltage drop calculators to calculate voltage drop over various lengths of wire. If I add a single LED to the device how to I determine the current to recalculate voltage drop? If I add a current limiting resistor to limit the LED to 10 mA do I recalculate using 85 mA (75 plus 10)?

If the LED is in parallel with the existing 75mA load, the 10mA LED current adds to the 75mA. However, likely adding 10mA more will also drop the voltage at the load end of the wire, so the 75mA load current will decrease slightly, so the net current will be less than 85mA...

Just make sure that you factor in the round trip distance of the cable when doing your calculations. 20 feet of 2 conductor wire will be 40 feet of wire resistance. If there are four wires in the cable, you can always parallel them and cut your resistance in half.

Also, the little modular connectors do not handle much current. At 85mA it may not have much insertion loss, but you should keep that in mind anyway.

What has me stumped is my prototype is working when it should not. I am supplying 3.3VDC to an EM408 GPS module which lists an operating range of 3.2 - 3.6 VDC. This page suggests the module should not work below 3.0V. I am using 14' and 25' cables with an rj11 coupler for a total of 39' and the GPS module is working and the LED is lit. This calculator shows I should have 2.9V and I test at 2.8V with a load of about 85mA. Could it be the EM408 has a lower operating range than its specs suggest?

Most things (modules, ICs, Circuits etc.) will work outside of their range. However, they will not work consistently outside their parameters. If this is an amateur project then go for it, if it's going into production then stay within spec.

Mike.

I would prefer to stay within spec. I can easily put 5V on the rj11 cable instead of 3.3V then all I need is a LDO 3.3v regulator to supply the GPS module. I'll give that a try. Thanks for the help.

Be aware that the labeled current draw on most devices is the max that it needs under worst case conditions, and is often scaled up by some safety margin. And operating voltage ranges are similarly scaled.

And many operating parameters are temperature dependent. You may find that it works as is on a warm day, but not on a hot or cold day.