170422-1031 EDT
From post #1 I have no idea what the question is.
1. From the title we have a question of voltage drop and something to do with USB.
2.
I have an existing installation with 18awg 2 conductor cable previously installed in wall. It was for an old low voltage control system for lighting. I need to upgrade the system without the ability to run new wires (like Cat5).
No information on load voltage or current requirement. This seems to imply --- that we have two wires already installed, and that the effort to replace them would be great. The two wires were used to control one or more lights. We don't know the complexity, but likely on-off of a single circuit. Could have been much more complex, but very unlikely.
What does upgrade the system mean? Are we dealing with a single on-off circuit? Are we dealing with a whole new system that will control many different circuits, and this remote location is expected to control any of the circuits, or still control only the one single circuit previously controlled.
Does the control device at this remote location have to be something compatible with the new central system? What functions are to be performed from this remote location?
There are lots of options for on-off control of a single circuit from this remote location with the existing wire.
3.
The wires are approximately 120-150 ft from the processor. I can use an Ipod to control the lighting (through WiFi) but i would need a constant USB source to power and charge the device. Problem is at that distance i am hitting some serious voltage drop.
The energy requirement has not been specified. I don't know the average power requirement of an Ipod, but suppose it is 1 W. At 5 V 1 W is a current of 200 mA. #18 copper wire is about 6.4 ohms per 1000 ft. The loop length here is about 300 ft or 1/3 of 6.4 ohms or about 2 ohms. Two ohms at 200 mA is 0.4 V.
I really doubt the average load would be 200 mA. Average standby power consumption is not going to be based on 200 mA. Peak charging current might be greater than 200 mA, but who cares if the average is a lot less? But suppose that 200 mA was the average requirement, then 0.4 V drop might not matter or increase the source voltage a little, or use either a switching or series pass regulator at the remote end from some voltage to 5 V to power the Ipod. Start with 24, 15, 12, or 9 V, all standard voltages at the input to the 2 wire cable. If a stepdown switching regulator is used, then its input current will be less than its load current. For a series pass the input current will be just slightly more than load current.
4.
Any one know of a device that can limit the voltage drop at that distance so that it will effectively keep this device powered? Or another solution with regard to a powered transformer?
My comment in (3) partly addresses this.
A simple approach is to use a GE RR relay with two DC power supplies at the controlled point; and a GE SPDT spring return to center control switch, and two diodes, at the remote point.
