Santanu:

Your use of the RS485 communications for the command and control is a good idea when the distances of 600 to 700 are involved.

Trying to distribute your regulated 5 volts over wires for 600-700 feet will lead to the problems that you have started to see. There are a number of alternatives that you should consider. Here is a list of ideas I can think of....

1) Put a power supply at each of the 10 sign locations and provide for primary to 5 volt conversion at each sign.

2) Distribute DC power down the cable but use a higher voltage like +12 volts and then add a local regulator to regulate down to a local 5 volts at each sign location. You could use one or more 7805 type low cost regulators (mounted on suitable heat sink) to do the conversion. Dividing the load of one sign across several 7805's can easily be done by powering a subset of the LEDs from each regulator.

3) As in 2) above distribute the DC power down the cable using +12V or even +24V. Then at each sign use a simple DC-DC converter to convert the DC line voltage to the local +5 volts. (one source of DC-DC converters is www.astrodyne.com).

Michael Karas