Polarity protection and constraints - Page 1
Forum section: electronics / beginners

author: mrzzarr
author_rank: Newbie
timestamp: April 18, 2025, 10:11:22 am
content: Hello!In a project I'm creating I have the following issue.I have two connectors let's call them J1 (RJ12 receptacle) and J2 (USB-C receptacle).We only care about power in this case.Both J1 and J2 have 5V power pins.There's one constraint:J2 should never deliver power.There are 4 scenarios that needs to be accounted for:Both ports are powered, J1 and J2 have the same polarity.Both ports are powered, J1 and J2 have the opposite polarity.Only J1 is powered J2 does not receive power.Only J2 is powered J1 receives power. (Polarity is not important.)So I wonder what components to use and how?Best Regards/ Rasmus

author: jwet
author_rank: Frequent Contributor
timestamp: April 19, 2025, 03:39:41 pm
content: I see you got no answers so I'll try to get something started. Your problem statement is a bit ambiguous.You probably think that you've explained this very clearly but it really isn't to an outsider.Are these "output" ports or input ports? If J2 is an output and should never deliver power, don't run 5v to it.At a simple level you can provide polarity protection with series diodes if you can take the drop or with FET's if you need low drop.Try clarifying your problem. A diagram or schematic is best. You can just write it on paper and snap it with your phone an attach.Good luck.

author: mrzzarr
author_rank: Newbie
timestamp: April 19, 2025, 05:59:09 pm
content: Hello!Thanks for the answer, yes it's easy to blind one self with thoughts.I attached the layout of the PCB I'm designing. (I called J312 -> J1 and USB321 -> J2 in my previous post as a simplification.)As I see it any type of receptacle is nether an in or out port in it self, that is decided with other components.I have some projects that (have not been produced yet, but are production ready) uses RJ12.To make things simple I allow for swapping the RJ12 pins 1-6 to 6-1 in a cable (this is common in model railroad hobby, which is the use case for my projects).Power pins on the RJ12 port are 1 and 6.So for the use cases:Use case 1: I connect my computer to the USB-C port and then one or more other projects to the RJ12 port, the computers USB port powers all connected projects.Use case 2: I connect my computer to the USB-C port and connect the RJ12 to projects that are already powered by another 5V PSU.In the layout there are Schottky diodes (D311 and D312) that protects the USB-C port from back feeding power, but they don't protect the project if the polarity is reversed.I hope this clarifies, other whys please ask again.Best Regards/ Rasmus

author: tooki
author_rank: Super Contributor
timestamp: April 19, 2025, 06:02:14 pm
content: Post the schematic. You cannot expect people to first reverse-engineer your PCB layout, and the schematic is what is needed to understand the circuit.

author: mrzzarr
author_rank: Newbie
timestamp: April 19, 2025, 10:21:51 pm
content: I did not expect anyone to reverse engineer the layout, I posted it just to show the connectors and be able to reference some components.With that said I do agree that I should have provided the schematics, so here they are.

author: Analog Kid
author_rank: Super Contributor
timestamp: April 19, 2025, 10:26:45 pm
content: Hint: the color scheme of your schematics is horrible.Try something more visible.(I simply negatived the colors here.)

author: mrzzarr
author_rank: Newbie
timestamp: April 20, 2025, 07:51:09 am
content: Here are the schematics in Kicad default theme.