AC power in a multicore cable - Page 1
Forum section: electronics / beginners

author: harps
author_rank: Regular Contributor
timestamp: November 23, 2017, 11:49:03 pm
content: hiI notice from a few designs that manufacturers do route multiple DC power lines through a multicore cable. so the transformers are external to the main project box.I wonder is it wise to route a few AC power lines in a multicore? when this power is to be used for Audio?would i need to shield each power line from one another? (use special shielded power cable braided/foil )for example.I have a few toroidals in a metal box which will send AC power to another main project box.input =230vout =24v + - E ( dual) AC50 v ( single) AC9 v (single) ACThese AC power outputs will feed the main project box which has a few regulator circuit boards and circuit boards such as, 2 x preamps, and an ADC.Thanks for help : )

author: Brumby
author_rank: Unknown
timestamp: November 24, 2017, 03:58:41 am
content: Is itwiseto send AC down a multicore? If you have audio signals, especially low level, travelling in the same multicore, I would strongly suggest the answer is no.If you really wanted to do it, I would use a separately shielded twisted pair - pretty much like a balanced line microphone cable - for each. But as these would be carrying higher currents than a mic circuit, then the electromagnetic fields are going to be significantly greater as will be the risk of crosstalk.However, the question begs....Why do you want to send AC down a multicore? Does the equipment at the other end need AC?

author: Zero999
author_rank: Super Contributor
timestamp: November 24, 2017, 09:58:12 am
content: It's a bad idea and might possibly violate certain safety standards, which is why I've never seen it done in industry.Use a separate cable for the mains AC power.

author: capt bullshot
author_rank: Super Contributor
timestamp: November 24, 2017, 10:16:11 am
content: As far as I understood, you want to use one single cable for 3 low voltage AC supplies, and nothing else inside the cable - Yes, that's not a big deal. Shouldn't matter for what the AC power is used for. Take care of proper fusing the transformer secondaries at the transformer side of the cable, so a short won't overheat or burn the cable.Depending on the rest of the circuit, shielding shouldn't be necessary. The AC wiring might pick up some common mode noise that may influence your amplifiers, depends on the grounding scheme. Anyway, DC wiring would pick up the same common mode noise in this case, so there's no gain in placing the regulators etc. at the transformer side of the cable.I guess the main purpose of the setup would be to move the transformers away from the electronics?

author: Bashstreet
author_rank: Frequent Contributor
timestamp: November 24, 2017, 10:21:36 am
content: I do not see why not...If he has audio gear that need AC he has toroidal transformer.input =230vout =24v + - E ( dual) AC50 v ( single) AC9 v (single) ACMaking the output into multi-core cable should not be a problem long as it is done following safety standards.If he would be planning to do audio signals on the rails.. then it can be a issue for audio quality and there are many further considerations.Now before you go and hack something up do some further research also some contributors here might be able to point out further safety issues that i have not thought of.Be safe find out the risks

author: dmills
author_rank: Super Contributor
timestamp: November 24, 2017, 11:33:50 am
content: Those are fairly standard sort of rails for a mixer (possibly a bit high actually, but in the region).The might do better to put the rectifiers and caps in the transformer box (Possibly as well as the regulators) and just move DC to the frame, it keeps the cap charging current pulses away from the sensitive doings and gets the regulator heat out.There is however no issue in doing what you have in mind, but I would make sure that all the transformer secondaries are referenced to a common point that is grounded, just my paranoia.I would think carefully about the order that pins make when plugging in, you probably do not want the 50V and 9V pins making before the 0V reference for example.Regards, Dan.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: November 24, 2017, 03:25:07 pm
content: There are no problems doing what you are describing, however if the cable lengths are over a meter, I would size the wire for current capabilities one size larger than you are expecting so you have no losses. I would also fuse each output in the source box, do not fuse higher than the wire current rating. I would also be sure to use twisted pair cables.

author: Brumby
author_rank: Unknown
timestamp: November 25, 2017, 07:27:24 am
content: Multicores tend to be somewhat longer than a metre.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: November 25, 2017, 09:28:17 am
content: Quote from: Brumby on November 25, 2017, 07:27:24 amI don't understand your comment. A multicore can be as long or short as you require, within reason of course. The resistance gets higher the longer that it is and the losses can become too much for current requirements. I often have to create harnesses that have very short multicores 150 mm, and some that are 10 to 15 meters long on the project I am currently working. That is what I do for a living, harness design engineering.

author: harps
author_rank: Regular Contributor
timestamp: November 25, 2017, 01:54:48 pm
content: hi thanksit is a low power preamp ADC.I thought to keep the toroidals in an external box for lower noise and as there is not much room in the main box. ( i also may experiment with an external battery box later)all sections in the main box will be partitioned with 2mm of aluminium, so the regulator section should be o.k and not induce too much noise.I have the option to have 3 separate shielded AC power lines cables/connectors (toriodals >3 cables> main box) on individual XLRs. Or one multi core power cable maybe on a multi pin DMX XLR. I just hoped to avoid any unnecessary noise.thanks

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 25, 2017, 04:41:50 pm
content: Quote from: harps on November 25, 2017, 01:54:48 pmThat explanation raises as many questions as it answers. What kind of "noise" are you trying to reduce? Acoustical buzz from the transformer itself? Low power transformers, and especially toroidal transformers should not have any acoustic hum or buzz.Magnetic noise? Are you using unshielded audio transformers (such as a microphone preamp, etc.) Toroidal transformers are noted for their minimal external magnetic field.Even the world's most respected audio test equipment where you would expect the utmost isolation from mains power noise (for example Audio Precision) uses internal mains-connected power supply circuitry. Using external transformers seems like shades of audiophoolery. It is easy enough to isolate even sensitive circuitry from internal linear (or switch-mode) power supply circuits.Furthermore, designing external mains-frequency AC power input seems incompatible with the option of using optional battery power. Perhaps you should consider how you intend to generate all the necessary voltages in battery mode before committing yourself to this scheme.I am considering a scheme where I have an internal SMPS to generate all the voltages (+5V, +/-15V, +48V) from a 12~24V DC input. That allows international users to use the DC wall-wart (or line-lump) power supply of their choice. For example re-cycling a disused laptop power brick, etc.To answer your original question, no, multiple-core AC wiring does NOT require any kind of shielding or special handling down at mains frequencies.

author: dmills
author_rank: Super Contributor
timestamp: November 25, 2017, 05:28:39 pm
content: Seconded!Unless there is a compelling reason to do the external power thing (And noise is seldom it) there is no real point.Audio Precision, Prism, R&S, it is all internal power (And usually switch-mode at that), and with decent design you cannot see it in the output.regards, Dan.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: November 25, 2017, 05:53:31 pm
content: Quote from: harps on November 25, 2017, 01:54:48 pmCable shielding is not required for 50-60 Hz. AC power lines. Twisted pair multicore is not required either, if the run is short, just hand twist your wires as pairs and add more mechanical scuff and stress protection in the form of spiral wrap or tape will do (heat shrink is nice and easy, but makes the cables very stiff). "SO" and "SOOW" style cable would do the job and is readily available and inexpensive, and already has protective sheath for scuff protection and other internal protection from mechanical stress, as well as a twist in the wire pairs. There are other cable/harness protection systems that are cheap and easy to use as well, you just have to look, the keywords here are cheap and easy. It is, however, imperative that you size your wire to carry the required current with correct safety factor and also recalculate for resistance losses for longer lengths of copper wire.

author: harps
author_rank: Regular Contributor
timestamp: November 25, 2017, 09:04:11 pm
content: Quote from: Richard Crowley on November 25, 2017, 04:41:50 pm1. i have no space in my main project box for the toroidals.2. I want to use batteries later, I do not want to carry toriodals on my back when carring 7 other pieces of gear on perhaps long walks. So why have them in the same box when i can leave them at home?3. The top of the line gear i have used such as Cadac souncraft DDA all have outboard PSU's (DC in multi-cores) so i knew DC was not an issue. I just wondered about AC.4 Yes the battery system would need to be figured out and maybe a separate multi-core input or switch for battery . I have no need for batteries now so i shall figure that out later.5. your answer to my question " no, multiple-core AC wiring does NOT require any kind of shielding or special handling down at mains frequencies. "So from what i can gather from your answer, I could go for an unshielded multicore power cable with a multi pin DMX plug and socket and not have any worries.. No additional noise can be induced into the audio system. it would be exactly the same with individual shielded power cables or an unshielded multi-core cable, the exact same S/N ratio will be achieved.regards.

author: Ian.M
author_rank: Super Contributor
timestamp: November 25, 2017, 09:44:48 pm
content: You can easily and cheaply get seven or eight wire automotive trailer lighting cable that's nominally rated for over 10A. There's no need to assemble a custom cable from individual wires.

author: dmills
author_rank: Super Contributor
timestamp: November 25, 2017, 10:20:44 pm
content: Please don't use a 5 pin XLR for this, it is an open invite to the fuckup fairy at some event.Personally for field kit I would be designing for nominal 12V input with whatever converters I needed in the box, then use 4 pin XLR or (smaller) Hirose or Lemo connectors. Apart from anything else there are industry standard pinouts for these and it opens up a whole world of 'battery belts' and the like.BTW: Have you seen the size of a power rack for an old pre SMPSU Cadac J type? There is a reason the thing is external!