Does it make sense to twist/shield power supply cables? - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: January 05, 2016, 05:09:40 pm
content: Say you have a 6 foot (2 m) run of cables from a power supply carrying anywhere from 0.1 V to 50 V. Does it make sense to twist said cables (or shield them) to reduce (significant, e.g. >5 mV) noise pickup along the way?I know that the resistance at the source of a good power supply is very low, but these are "good length" runs of cable.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 05, 2016, 05:38:56 pm
content: Twisting also helps for radiated emissions that could get into other devices.Even a "good" power supply can be a noise source. It sounds like a benchtop adjustable power supply?I've had to wrap power supply wires through a ferrite occasionally.How much current are you expecting it to supply, and what is the nature of the load?

author: SharpEars
author_rank: Unknown
timestamp: January 05, 2016, 05:49:42 pm
content: Quote from: Tomorokoshi on January 05, 2016, 05:38:56 pmThis was meant to be a fairly general question, but the current is most likely less than 2 A (probably < 1 A).

author: Ian.M
author_rank: Super Contributor
timestamp: January 05, 2016, 06:01:01 pm
content: I've used coax for a power cable run when it was important to avoid producing a magnetic field, but in general, for moderate distances, if you minimise the loop area, the cable is not exposed to strong magnetic or RF fields and you have adequate decoupling at the load end of the cable, twisted pair's benefits for larger cable diameters are less than the inconvenience of needing a greater individual cable length to accommodate the twist and it taking up more space

author: SharpEars
author_rank: Unknown
timestamp: January 05, 2016, 06:16:54 pm
content: Whoa, look at what I found - a treasure trove, especially the figures starting on pg. 58:Noise Reduction Techniques in Electronic Systems

author: Vgkid
author_rank: Super Contributor
timestamp: January 05, 2016, 06:40:41 pm
content: Thanks for that link.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2016, 08:45:48 pm
content: Yes. Always pair currents, whether AC or DC, wherever they go. It's good practice, and it's a deeply fundamental principle: current flows in loops and voltages are differences. Control those loops.Tim

author: Jeroen3
author_rank: Super Contributor
timestamp: January 05, 2016, 08:50:46 pm
content: When I'm combining some wires for desk use other than banana plugs I put one end in the vise and the other end in the cordless drill.Twist them together, reverse a few turns. And voilà, makeshift cable! Works perfect every time.

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 06, 2016, 10:27:48 am
content: Greetings:I don't know why you would want to avoid the minor effort of twisting/shielding (except for a quick bench test), unless you were trying to shave pennies from a middling-largish production run.Unless this application has such simplistic requirements of V/t and I/t, and no chance of outside E/M getting in, what is your hesitation?If one spends a negligible time to AVOID glitches, and save perhaps HOURS of later fault-chasing, is that not reason enough?I found the writings, videos and books that Bob Pease, former lead scientist of National Semiconductor (up to somewhat after the Texas Instruments buyout; he died in a driving accident.) He was one of the Premier authors on troubleshooting, and had the attitude of "head 'em off at the pass", IOW, take precautions early and often, so as not to waste your time chasing the Gremlins (Like that soldering iron transformer) when you would rather be admiring your cleverness.My two cents, worth every penny you paid.Cheers,Dave

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 06, 2016, 10:43:35 am
content: Quote from: SharpEars on January 05, 2016, 05:09:40 pmYou have already been given some good advice. In addition, twisting the leads will reduce the inductance. The inductance can cause transients if the load is 'hot-plugged' For more details see:http://cds.linear.com/docs/en/application-note/an88f.pdfYou may need damping or clamping at the load.Regards,Jay_Diddy_B

author: sarepairman2
author_rank: Frequent Contributor
timestamp: January 09, 2016, 02:52:11 pm
content: from a philosophical standpoint, why not? it's kind of like not littering in the electromagnetic spectrum

author: calexanian
author_rank: Super Contributor
timestamp: January 09, 2016, 07:52:22 pm
content: Keep in mind you should always filter and decouple as near as possible to the load for anything that could be effected by the inductance of the power supply wires that are now effectively common mode chokes. This will only effect loads with a realtivley high current/frequency AC component to them, but I have been tricked by it before. Especially with a multi quadrant power supply!