PSU sense lines with obstructions - does it matter? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 17, 2019, 04:38:56 am
content: I am thinking of making a outlet-bar where my power supplies goes to, so if I need power, I can connect leads to the banana/screw terminals on the outlet-bar without having to fiddle with cables from each power supply and all the way to my project.Now the question: how much will it matter if I also let the S+ and S- go to seperate sense terminal in the outlet-bar and then plug a banana plug in them, when I need the precision of the sense lines?The sense cables has a build in 5½ DMM to measure the compensation.Would that be reliable, do the plugs change much in what the sense cables detect?

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 18, 2019, 11:02:07 am
content: Here is a drawing:-)

author: Paul Rose
author_rank: Regular Contributor
timestamp: May 18, 2019, 11:54:32 am
content: If you are still running sense wires all the way to the load from the bar, then the extra connector in the sense path is no trouble.The sense current is tiny, and the voltage drop on the sense line is (sense current)*(sense line resistance). Because of the tiny sense current, a little bit of sense line resistance doesn't matter. This is why sense lines don't have to use heavy wire, even for high current supplies.If you connect the sense to the power at the outlet bar, then you have to worry about voltage drop in the power line between the bar and your load.

author: tggzzz
author_rank: Super Contributor
timestamp: May 18, 2019, 11:57:50 am
content: You should consider failure modes.What happens if a sense wire is disconnected or has intermittent contact?

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 18, 2019, 01:37:44 pm
content: I was thinking about, running twisted shielded sense wires from the PSU and direct to two posts that can take spade and banana plugs.From that post, do I want to plug in a wire and direct to the place I want to sense on.So the sense line will go from PSU to post, from post to sense spot.I'll try to get as good a post so that the connection is tight and solid.On the image have I drawen the post from the side, with a banana plug.

author: mikerj
author_rank: Super Contributor
timestamp: May 18, 2019, 02:01:45 pm
content: Quote from: tggzzz on May 18, 2019, 11:57:50 amA half decent supply will have the senses internally connected to the outputs via resistors to prevent unexpected maximum output voltage issues.

author: MarkF
author_rank: Super Contributor
timestamp: May 18, 2019, 02:47:03 pm
content: Quote from: FriedMule on May 18, 2019, 01:37:44 pmI like these banana jacks. Nothing to come loose.

author: bsdphk
author_rank: Regular Contributor
timestamp: May 18, 2019, 03:58:22 pm
content: If you do that, you want to have a toggle-switch next to the plugs which can connect the sense lines to the output terminals, otherwise the dangling sense lines will pick up noise if/when you only us the output terminals.

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 18, 2019, 05:55:05 pm
content: Quote from: bsdphk on May 18, 2019, 03:58:22 pmOkay diden't know that, the PSU have a way to switch the sense lines on and off, do I still have to use the external switch?MarkF: Thanks for the advice!

author: bsdphk
author_rank: Regular Contributor
timestamp: May 18, 2019, 09:16:49 pm
content: You want to use the sense lines all the time in a setup like this, so that the PSU compensates for the wire loss to your connecting-board.The point about the switch is that you can then choose to sense at the connectors, or wire them all the way down to your experiment.But do read your PSU's manual carefully, there are a usually footnotes about remote sense lines and stability.

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 19, 2019, 02:07:00 am
content: Quote from: bsdphk on May 18, 2019, 09:16:49 pmYes that makes sense, my thought was that for usage where the precision did not matter +/-1V, it was not necessary, but I can understand that this is wrong?What if I make two jumpers from sense to the power? You know like often seen from ground to earth on PSU's.

author: Rerouter
author_rank: Super Contributor
timestamp: May 19, 2019, 02:17:33 am
content: If your PSU does not have any kind of built in protection against it, fit some medium size resistors, e.g. 220 ohm in parrellel to the main terminals, this way if a lead lifts it just defaults back to not compensating for the wire drop, when the wires are there, the resistance of the sense leads is so low compared to the resistor that it measures close enough to not matter.

author: bsdphk
author_rank: Regular Contributor
timestamp: May 19, 2019, 06:54:39 am
content: The reason you want to use sense wires all the time is that it keeps the voltage (more) constant under changing loads.Using resistors instead of a switch may or may not work, not all PSU's are stable under all conditions.But as I said: read the manual carefully, there are usually instructions/footnotes about remote sense, because it is not quite as trivial as people think.For instance you should _always_ use a balanced wire pair for RS, either twisted (rj45 is fine) or twinax.Also check the manual for recommended capacitance at the remote output terminals.