Question about shielding termination for harness from power supply - Page 1
Forum section: electronics / projects

author: MRiddickW
author_rank: Contributor
timestamp: August 30, 2022, 12:09:28 am
content: Hi all,Can somebody tell me if one of the below shield termination strategies is better than the other, or if the difference is negligible?I have an HP/Agilent 6626A, and since the connections are all inconveniently located as screw terminals in the back, I want to run these connections to a terminal box that can sit on my bench.In the interest of over-complicating every project, I am running two shielded twisted pairs for each channel:20 AWGfor the supply,25 AWGfor the Vsense connection. I am terminating these cables to a DB25 connector; the mating connector will be installed on a metal box with binding posts for each connection including Vsense, with a DPDT toggle switch on each channel to select whether Vsense is local or remote (local in this case still being in the terminal box).The 6626A has a single grounding terminal on the back. As I see it, I have two primary strategies at my disposal for shield termination:Star at connector: Terminate the shield of one cable, say channel 1 power, to the power 6626A grounding terminal. Then in the DB25 connector, connect all the cable shields as well as the metal DB25 shell.Star at the supply: Connect the shield of every cable to the 6626A grounding terminal. Then in the DB25 connector, connect only one shield (say channel 1 power) to the metal DB25 shell.Both: Do both of the above. This seems inadvisable due to ground loops (I guess), but hey, I guess it's technically an option.Is one of these strategies better than the other, or is it negligible at 10 ft? (for a 20 ft total trip for all but channel 1 power.)Starring at the connector would certainly be the easiest since that's where all the cables are in one place anyway, but if I'm going through all this trouble anyways, I'd rather do it the best way.And like I said, I know I'mwayover-complicating this whole thing, but what else are personal projects for?Thanks!

author: inse
author_rank: Super Contributor
timestamp: August 30, 2022, 05:16:08 am
content: I am usually a fan of simplicity.Do you really need the sensing function for a lab power supply with only 50W of output power?I personally would wire the connector box directly to the screw terminals with decent cables (the same diameter you use on your bench) keeping them as short as possible.

author: MRiddickW
author_rank: Contributor
timestamp: August 30, 2022, 03:24:06 pm
content: That was actually my first thought, but with a supply as precise as that one is, it just seems a shame to not also be as accurate as possible. While 10' is not truly as short as possible, it is on the low side. Basically enough length to route down along the back edge my bench, and pull out towards me a few feet if need be (1' from supply to edge of bench, 3' down, 3' to middle of bench, 3' to front edge of bench).Also, not to fall trap to the sunk cost fallacy, but I do have the DB25 plug all soldered up at this point, so that's half the battle already done at least. The other nice thing about the plug is that I can keep the box off the bench when not in use, and just tuck the DB25 out of the way.I will say I haven't fully committed to the local/remote Vsense switch and binding posts on though. It would certainly be easier to just terminate both at the voltage source binding post and be done with it. But then I think: If I'm going this crazy, may as well go all the way.

author: donlisms
author_rank: Frequent Contributor
timestamp: August 30, 2022, 06:49:50 pm
content: The sense thing might be a decision where you do the math. What is the round-trip resistance of your wiring, the current you expect, and the voltage precision you'd like to have, given what's available? In some situations, the numbers will say "well, obviously -- yes!" Or "no!" Or "well... maybe." Maybe better than just kinda felling your way through that one. And maybe some pre-large-effort testing.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 31, 2022, 04:51:34 pm
content: Minimise current flow in the shield. Visualise it as a Farraday cage.What ever strategy achieves this is the correct one.

author: MRiddickW
author_rank: Contributor
timestamp: August 31, 2022, 05:03:57 pm
content: Quote from: donlisms on August 30, 2022, 06:49:50 pmAh, good point. Assuming 1 A, here's what it comes out to:\[\frac{10.5 \Omega}{1000 \mathrm{ft}} * 20 \mathrm{ft} * 1 \mathrm{A} = 210 \mathrm{mV}\]The 50 W outputs when in high-range mode (i.e. lowest precision and accuracy), have programming accuracy and readback accuracy of 10 mV, and average programming resolution of 3.2 mV. For these numbers, I think it's definitely worth it.The 25 W outputs in low-range mode (highest precision and accuracy) have programming accuracy of 1.5 mV, readback accuracy of 2 mV, and average programming resolution of 460 uV. Worst case for this mode come out as follows:\[\frac{10.5 \Omega}{1000 \mathrm{ft}} * 20 \mathrm{ft} * 15 \mathrm{mA} = 3.15 \mathrm{mV}\]The voltage drop in this case is much closer to the accuracy, but as long as I'm making this box, I think I still find it worth the time to bring the remote sense lines with it.

author: MRiddickW
author_rank: Contributor
timestamp: August 31, 2022, 05:11:31 pm
content: Quote from: Terry Bites on August 31, 2022, 04:51:34 pmSorry, I don't really understand what you're getting at when you say to minimize current flow in the shield. I am attempting to visualize it as a Faraday cage, I guess I just don't know enough about this subject to know which strategy achieves this.That said, if you have a recommendation of a good book/article that explains this well rather than you having to take the time to do so, I'd be more than happy with that solution.Thanks!