suggestions for signal/power cable? - Page 1
Forum section: electronics / projects

author: KTP
author_rank: Frequent Contributor
timestamp: July 04, 2012, 06:37:34 pm
content: Me again, new problem.I found my waterproof connectors. Six pin at 7.5 amps per pin, Eight pin at 6.5 amps per pin. Can take a cable up to 0.38 inch dia.I need to transmit RS485 and power over a cable through these connectors, to several daisy chained devices for a distance of up to 25 feet total. The devices can handle a voltage drop as they have the switching regulator I mentioned in another thread, although obviously I would like to minimize the drop and certainly not melt the insulation. The worst case scenario is around 14 amps at 15V, more usual would be 8 or 9 amps at 12V. I am considering using the six pin connector with two power pins, two ground pins and two pins for the 485.OMG is cable expensive. I hadn't checked in a few years...dang copper prices. What would you guys go with for a single cable solution? Obviously not great to run the RS485 right with the high power lines but maybe can get away with it. Use twisted pair for the RS485 and the other conductors for power, ground. Any ham radio wire or something that is already set up like this?

author: hlavac
author_rank: Frequent Contributor
timestamp: July 04, 2012, 07:11:20 pm
content: RS485 uses differential signaling, running it along the power should be fine with a twisted pair wire, crosstalk will cancel itself out

author: KTP
author_rank: Frequent Contributor
timestamp: July 04, 2012, 10:45:01 pm
content: Yeah, I know it is fine on twisted pair...the problem is getting twisted pair with enough pairs and large enough gauge to run my 15 amps.I am thinking outside the box now. How bad would it be to run ground return over the shield in a belden 5 pair 22 gauge cable (Belden 8305). Belden recommends 2.1amps max per 22 gauge conductor...I could probably exceed that a bit, but if I ran 12V on 6 of the conductors, ground on 2, RS485 on the last pair and then ground also on the shield, it might get me close to the 15 amps without too much drop.Otherwise I am looking at 4 pairs of 18 gauge cable...stuff is like $1500 a spool!

author: Unknown
author_rank: Unknown
timestamp: July 05, 2012, 12:52:04 am
content: Quote from: KTP on July 04, 2012, 10:45:01 pmConsider running 48V, the regulators required are trivial. 12V at 15A will always need a messload of copper. Commercial vehicles use 24V circuits for that very reason.

author: SeanB
author_rank: Super Contributor
timestamp: July 05, 2012, 05:14:06 am
content: Have you considered multicore telecoms cable? 20 pairs will handle the current, and give enough twisted pairs to do the data as well on a 600 ohm impedance cable.

author: amyk
author_rank: Super Contributor
timestamp: July 05, 2012, 06:57:48 am
content: Quote from: Uncle Vernon on July 05, 2012, 12:52:04 amIt's also why the power grid uses ultra-high voltages - lowers the current so the wire can be thinner. Use as high of a voltage as you can for the application.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: July 05, 2012, 12:13:22 pm
content: If you need enough of this cable, then you could consider a custom cable. Moq starts at 1000-3000meters, depending on supplier. And at those quantities, pricing mostly depends on the amount of copper, so not relly that much more expensive than standard cable.

author: KTP
author_rank: Frequent Contributor
timestamp: July 05, 2012, 12:50:30 pm
content: Thanks for the replies. The application is outdoors near/on water so I would like to keep the voltage low (like you find in typical low voltage garden lighting). There is also the fact that I have already designed the boards to operate off of 5V to 15V DC using the 12003 regulator and I would rather not add on another layer of voltage regulation to step down from 48V.I think since the wiring is in open air and since I will never be pulling max current for very long, I don't really have deep concerns now about melting the wiring or excessive voltage drop (plus the load is distributed, so we are not talking 12 to 15 amps over the entire length). My concerns have now moved on to what type of voltage might be induced on the twisted pair and if the differential receivers can handle it. Normally twisted pair would have both positive and negative voltages induced in the wires from adjacent power wires because of the twist, but if I run power through the other twists and the twists happen to line up in sections...sigh...this is going to be one of those times where you just have to build it and test it.