Multiple voltage supplies over long wires - check my thinking please - Page 1
Forum section: electronics / beginners

author: sokoloff
author_rank: Super Contributor
timestamp: April 15, 2017, 02:00:23 pm
content: Scenario:I am installing some landscape lighting in my yard, to be switched from a project in my house, controlled from an RPi or Arduino. The landscape lights I've selected are all LED, but some still draw almost 1A at 12V. I plan to have multiple "zones" of lighting.On the bench, I've tested lights individually and believe I have a strategy for what will work in the yard, but because I'm burying the cable, I want to make sure I'm not overlooking something before I commit to backfilling the trenches.My plan is to use 14/4 direct burial speaker cable (as a commodity product, it's relatively cheap), 3 relays initially, each driving one 12V "zone" of lighting as below.My first questions are:Assuming that zone 1 and zone 2 will be most commonly used circuits, and I can arrange zone 2 to be the highest current draw, I believe that minimizes the current on the "ground" wire.I will drive zone 1 and zone 3 from a +12V to GND supply, and zone 2 from a GND to -12V supply.(I'm assuming that when zone 1 and zone 2 are both "on" to the extent they were exactly equal current, that the current on ground wire (3) would be zero.)1. Is that right?2. If I later switch away from relays to an output that can be PWM'd, does that change anything of substance in the above line of thinking? (I don't expect to do this, but want to know.)3. Is there any productive way that I can control more zones of lighting across passive 14/4 cable?4. Should I be considering another approach here?

author: Nusa
author_rank: Super Contributor
timestamp: April 15, 2017, 02:53:44 pm
content: You're doing a lot of worrying about current, given that your combined load is maybe 4 amps. 14 awg is good for far more than that on a continuous basis. I suspect you're looking at charts that are worried about voltage drop, which for lighting can either be ignored (your lights will probably still be happy with 11 volts) or compensated for by turning up the volts at the power supply (if it has an adjustment). No need for any attempted trickery with multiple power supplies either.Whatever you do, test your whole setup above ground before you bury it, with all zones in operation. Then you can measure voltages and make any changes necessary.

author: sokoloff
author_rank: Super Contributor
timestamp: April 15, 2017, 03:08:52 pm
content: Fair comment and thank you.I should have been more clear that I am only worried about voltage drop (not the current limit or overheating concerns), as some zones will have 6 lamps on them initially and may grow to 9 or more (depending on how the landscaping plan evolves over time). Zone 2 is the highest power zone currently, with 6 10W lamps on it, and the lamps on that zone range from 10m from the supply out to 50m, so I'm worried that I can't indefinitely crank up the source voltage as I need to balance the voltage seen at lamp 1 (close to the supply) and lamp 6 (at the farthest end of the run).

author: rstofer
author_rank: Super Contributor
timestamp: April 15, 2017, 03:28:17 pm
content: 14 AWG could ordinarily be rated for 15A.The resistance of 14 AWG stranded wire is 0.5 Ohms for 200' (100' down and back). You find calculators via Google.https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-tableI would probably use 1 cable per zone just because wire is cheap compared to digging a ditch.You can do some testing at reduced voltage to see how the lights work out. In fact, you can probably do a life size example before you even start to bury wire.

author: SeanB
author_rank: Super Contributor
timestamp: April 16, 2017, 04:51:27 pm
content: If it is garden lighting use a conduit, otherwise you will be forever replacing cables that have been "gardened" with a spade, fork or whatever going through them, or just nicking them.I did a lot using just ordinary house wire, the one you pull in a conduit, as twisted pairs in conduit, as I had a few larger 250VA transformers to power them, and with this wire in conduit, and a join box ( 3 way, and filled inside with ordinary lithium grease to keep the water out) to connect the wiring to the individual lamp units. I got tired of the weekly replace mains wiring, and went low voltage so earth faults did not worry me, and 12v 50W dichroic lamps in there, later replaced with 20W ones as they lasted longer, did not run so hot and I could double the number of lamps as well, plus they did not cook the die cast housing seals as much, so they had less water in them. Must have been around 60 of them, plus a dozen 125W mercury floods as either general light or just as accent lighting, along with around a dozen PL bulkheads under plants as feature lights. I do not miss that part of that house, I think I bought a lamp supplier a good number of good quarters, just in lamps and replacement electronic ( and regular potted toroid) transformers and fixture parts, plus a lot of cable and socket outlets and plugs for those running on mains.Then changing lights in the koi pond, swimming down 2m to get them out, and changing while swimming there. Wonder if Shabby kept up with that, must look one night.