Hi all,
I'm in the final stages of a circuit design, and thought it best to get some advice before going any further.

On this design there are two devices that need to be about 1 meter apart that hard wire together via a cable. Basically a power supply and main controller. The power supply board also has a couple of relays triggered by mosfet and a current transformer to measure the amperage of the main device the relays control. The main device it's controlling can draw up to 12 Amps AC on 120VAC line voltage.

The main controller has a microprocessor outputting to a character lcd and inputting from a membrane keypad. As well, I have some analog devices for the current measurement function on the controller pcb as well.

I'm not sending any data on the cable such as I2C, so what I've proposed that would physically work best to easily connect the two is a 28 AWG 6 conductor ribbon cable. Two conductors feed 3.3 volts to the controller board which draws about 130 mA, 3 on/off control conductors also carry low voltage DC, and one line with a very low voltage 60htz sine wave coming from the current transformer to be measured on the controller board.

Does anyone see any pitfalls such as RFI radiation, interference pickup or any other problems using simple unshielded untwisted flat ribbon cable in this application?

 

A sketch speaks a thousand words,
please

 

okay see here

 

I'd make the current transformer a diff pair. What is its output current? Industrial ones are 1 Amp, and that would be a lot to have flowing in a ribbon cable. Even a few hundred milliamps is going to put noise on the earth.

 

Have you looked for ribbon cable connectors? There are not a lot of 6 conductor versions. 10 is a much more common sight.

That being said, you should take advantage of the extra conductors on a 10 conductor cable.

28 AWG just seems a little small for my preference, even at only 130 mA. If it were me, I'd probably double up on those conductors.

Ian0 has a very good suggestion. For any device, the conductor carrying current out should be paired with the conductor carrying current in. In the case of your current transformer, it will reduce external interference. Use an extra conductor for that as well.

Lastly, also in an effort to reduce possible interference on your control lines, use the remaining conductor for a signal return for your DC control lines.

 

Thank you Lo_volt and Ian0.

Your ideas and suggestions are extremely helpful!