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x6gas

Wiring looms - best practice for data and power cables

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Hi all.

I've had some problems in the past with noise appearing on images taken with my Atik 383L+ which I think were due to interference from the power cable. I'd had all my data and power cables bound together into one loom - 3m USB cables and slightly shorter 12v power lines to the imaging camera, filter wheel, and guide cam. Things improved when I ran separate power to the camera, but I am now sorting out new cable management and want to ensure I don't waste any rare clear nights with cable problems.

I am thinking that I will run two looms:

  1. All of the data (USB) cables - imaging cam; guide cam; filter wheel control
  2. Power - mount; imaging cam; guide cam; filter wheel.

I want to ensure no interference and so no noise being introduced.

The plan is to run the two looms side by side - so they'll be in relative close proximity but not actually bound together.

That the best way or should some of the cables not be bound in to a loom?

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As an aside, I've read that the Atil 383+ is very fussy about power levels and that noise increases if the power drops. So having a separate power supply just for the camera may be helpful.

Helen

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You shouldn't get induced noise from DC power cables especially low voltage ones.

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If you make sure you use a screened power cable with the screening earth and a ferrite clamp, and the USB leads with ferrite clamp this should keep and RF from getting it to your ATIK.....I would screen and use ferrite on any USB or Power cable, there not expensive and your curing something before it is happening.....

http://www.maplin.co.uk/clip-on-ferrites-32799

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If you make sure you use a screened power cable with the screening earth and a ferrite clamp, and the USB leads with ferrite clamp this should keep and RF from getting it to your ATIK.....I would screen and use ferrite on any USB or Power cable, there not expensive and your curing something before it is happening.....

http://www.maplin.co...-ferrites-32799

Ferrite chokes on the USBs? Well that hadn't occured to me - thanks for the tip! Like you say, cheap enough. Am I right in thinking that the ferrite should be at the PC end of the cable?

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Good points above.

I would use seperate power supplies to avoid any ground loop problems.

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Ferrite chokes on the USBs? Well that hadn't occured to me - thanks for the tip! Like you say, cheap enough. Am I right in thinking that the ferrite should be at the PC end of the cable?

Stick them on the camera end although both ends won't do any harm, if you have noise(RF) getting into your CCD it could come from many sources, Mobile phone masts, or the phones them selves in fact anything that transmittes a RF signal, your USB, Power cables just acts like a ariel the Ferrites will help prevent this from happening....

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Keep the power and data cables as far apart as possible. At least 3 feet, 20 years as a an industrial process deveng, I have had this problem.

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Keep the power and data cables as far apart as possible. At least 3 feet, 20 years as a an industrial process deveng, I have had this problem.

With low voltage DC cables from a very low impedance source? Sorry but I can't see that myself. I've got a background in electronics R&D (my first profession) and I've never come across this as a problem. You're far more likely to get crosstalk between the digital signal leads than onto the power leads which is why they're screened.

That degree of separation is usual practice for heavier AC supplies in noisy environments and data/comms but not really for DC unless it's higher voltage/current stuff supplying electrically noisy systems of course which does sound like the sort of thing you'd be dealing with. ;)

Helen could well be on the right track. The single supply may not be quite beefy enough for the camera to run quietly along with all of the other accessories - but it sounds like the OP has already discovered that for himself.

The ferrite clamps are an excellent idea and will certainly do no harm - if there's interference being induced onto signal leads from external sources then this will certainly help. RFI is tricky stuff indeed. The ferrite rings will provide low impedance (at RF frequencies) damping so you'll need to place them close to the affected equipment as that's where you want the most effective damping. :)

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50 ohms isnt exactly a low impedance, but RF transformers work at this level or less. EMC testing is a bit of an eye opener. Putting ferrite rings on video cables isnt going to to do the bandwidth a lot of good.

Again. Keep power and data cables apart, end of that particular problem.

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50 ohms isnt exactly a low impedance, but RF transformers work at this level or less. EMC testing is a bit of an eye opener. Putting ferrite rings on video cables isnt going to to do the bandwidth a lot of good.

Again. Keep power and data cables apart, end of that particular problem.

Power supply and battery source impedances aren't 50 ohms - they're LOW impedances - tiny fractions of an ohm. Ferrite rings on a data lead aren't going to touch the bandwidth at all. In fact they're commonly used on both video and data leads to remove RFI.

Separate if you want but it's not going to make a massive difference. Go with the ferrite to reduce induced RFI on the screen and keep using the separate PSUs.

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http://www.cableorga...om/spiral-wrap/I like to use Spiral wrap on cables save lots of time taking them apart and you can put each cable to the right place as you go, the link is so you can see how its works.

That stuff is great for looms :) You can also get velcro cable ties - they're used for patch cable management.

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Thanks for the responses everyone.

I'm talking here about running data and power up to the mount / scope to a nearby laptop - only 3m of cable - but obviously the data and power are terminating at the same camera so it's not possible to keep the data and power three feet apart for most of the length of the run...

I have the spiral wrap so I think I'll try one for data and one for power with ferrites on all the cables (camera side for the USBs and power side for the 12v) and see how I go.

Thanks again!

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My wiring loom atm is 2 power and 2 USB, 10m in length the USB's are repeaters with there own Ferrites built in, for holding it all together i went with Self Amalgamating tape every 15" or so, very easy to trail across the patio, and when they reach the tripod legs there are are 2 Jubilee clips bottom and top , the cable look goes through 2 bits of Broom handle these are bored out and then split, placed round the loom and the S/E Tape round them either end of the wood which then gets placed in the spring clips,,,any further cables will just get some more S/E tape......

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Islander's spot on, you shouldn't have any noticeable effects with running LV DC currents along side the USB and data cables. 240V AC can be an issue over longer parallel lengths, where we typically recommend 25mm minimum separation - however this is subjective and bordering on the excessive and depends a lot on load - most manufacturers allow you to run 35m in parallel with mains circuits in dado trunking where the separation is much tighter. Mains RF interference is by no means the only source of interference, chest freezers, flourescent tubes can be far noisier.

If the concern is primarily the CCD, it might be worth putting an earthed shield around this to experiment or try isolating potential sources one at a time. DC powered equipment is generally isolated by mains transformer or running from batteries, so there should be no issues with stray current leakage between two points on earth.

If data side shielded patch leads are relatively cheap - with a shielded data cable the shielding needs to run the full length of the channel ie from device to switch. The only other thing to look for is there are quite a lot of cheap chinese import data cable, some of which are made of the dreaded CCA (copper clad aluminium) rather than pure copper - I've even seen CCA stranded cables patch cables around. The insertion losses and resistance of these take them way out of spec and give marginal overhead even on short channel lengths (15m+). If in doubt source a reputable brand/manufacturer.

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Not really an input regarding the OP's question, just a tip on a good zippered cable tidy. It stays soft even in the cold and has plenty room for all your cables. It doesn't snag either unless something needle sharp catches it. Found it on eBay.

post-16323-0-39813800-1352848811_thumb.j

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Another possibility is the mount itself, My HEQ5 used to give a tingling type feeling on my hands when I grabbed the tripod, this was down to the Switched mode laptop power supply I was using to power the mount.

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With low voltage DC cables from a very low impedance source? Sorry but I can't see that myself. I've got a background in electronics R&D (my first profession) and I've never come across this as a problem. You're far more likely to get crosstalk between the digital signal leads than onto the power leads which is why they're screened.

That degree of separation is usual practice for heavier AC supplies in noisy environments and data/comms but not really for DC unless it's higher voltage/current stuff supplying electrically noisy systems of course which does sound like the sort of thing you'd be dealing with. ;)

Helen could well be on the right track. The single supply may not be quite beefy enough for the camera to run quietly along with all of the other accessories - but it sounds like the OP has already discovered that for himself.

The ferrite clamps are an excellent idea and will certainly do no harm - if there's interference being induced onto signal leads from external sources then this will certainly help. RFI is tricky stuff indeed. The ferrite rings will provide low impedance (at RF frequencies) damping so you'll need to place them close to the affected equipment as that's where you want the most effective damping. :)

I agree, sounds far fetched. For power as a rule of thumb its about 1.5 - 2 x cable diameters of seperation.

More than likely noise between signal cables as suggested or from within the equipment itself. Possibly circulating currents along the earth if bonded/shielded at both ends.

The other post about tingling feeling from tripod, this is the cause. I get the same from my dj mixer when doing outdoor gigs, basically you are a lower impedance than the earth.

Electrical Engineer, consulting.

(but I know [removed word] all about electronics...)

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ok I'm trying to say I don't know much about electronics (or about beating a profanity filter) lol

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The concensus seems to be that close low voltage DC power cables and USB cables shouldn't be a problem, and that ferrites opposite the power source end would be a benefit. Is the advice still true if you add network cables into the mix/loom?

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Yes. You won't need ferrite on the network cables as they're twisted pair and hence inherently noise cancelling :)

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Thanks for all of the informative replies on this thread which have been very insightful.

By the sounds of it, the problem I was having before was unlikely to have been interference between the data and power cables (I use pretty good quality USB cables). Given what has been said above, I now suspect that it was probably low power at the camera - I use a 10 amp regulated supply and the major current draw on that is from the camera and a dew band (not wired into the loom but usually drawing from the same power supply) so total current is well below the rating of the supply. I reckon I simply had a duff cable or dodgy contact on the camera supply... Why they put 2.1mm barrel connectors on these things I will never know!

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