Return path Protection - Page 1
Forum section: electronics / projects

author: NMP
author_rank: Contributor
timestamp: May 03, 2018, 10:56:05 am
content: Hi,I have a printer that have a external power supply.It communicates with my board.Connected the (-) of the board and the supply to be on the same level.If the (-) of that external Power supply is not connected it returns over my board and burns my dc/dc.Having a small value fuse i think will solve it..Do you have any suggestions?

author: JS
author_rank: Frequent Contributor
timestamp: May 03, 2018, 12:15:49 pm
content: Schematic could help, diode bridges are sometimes used as protection in this cases but hard to tell where to put it.Are you loosing just the (-) connection of your PS or both but the (-) is the one that brings trouble?JS

author: Rerouter
author_rank: Super Contributor
timestamp: May 03, 2018, 12:26:07 pm
content: First off I would ask is the external power supply floating, or has negative tied to ground, I suspect the second caseOk next up, is how your DC-DC converter is completing that ground loop, this will likely be the root cause of your problems, as if its poviding the printer a ground, most of the easy techniques dont work.As a heads up, most AC-DC power supplies are to some degree floating (no DC connection of circuit ground to mains ground) to prevent things like this. computer power supplies and a few others are an exception, where it is assumed it will tie those floating supplies to ground.

author: JS
author_rank: Frequent Contributor
timestamp: May 03, 2018, 03:49:16 pm
content: Schematic could help, diode bridges are sometimes used as protection in this cases but hard to tell where to put it.Are you loosing just the (-) connection of your PS or both but the (-) is the one that brings trouble?JS

author: NMP
author_rank: Contributor
timestamp: May 03, 2018, 05:27:05 pm
content: Quote from: Rerouter on May 03, 2018, 12:26:07 pmI think thats the problem.. the (-) is floating but the printer is in metal, attach to a grounded box.Quote from: JS on May 03, 2018, 03:49:16 pmWhen the broken link , X in the figure, happen i think it made the (-) in the PS connected to the ground through CPU.Want to avoid using diodes because it will lower the voltage in the serial communication

author: JS
author_rank: Frequent Contributor
timestamp: May 06, 2018, 01:43:23 pm
content: Quote from: NMP on May 03, 2018, 05:27:05 pmThey don't go in series, more like adding a small series resistor to the signal ground and the bridge or diodes clamp the ground. If there's a loop too high for the resistor the diodes do the heavy lifting protecting everything around.JS

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 06, 2018, 11:32:31 pm
content: Ideally, the serial comms should be common ground for signal currents only; an isolator can be used, or a few special-case options (such as with opamps to enforce current balance).Preferably, the power cable is used as ground reference for the serial, by being routed in the same cable or harness. Then the serial cable itself doesn't need to carry a ground. Alternately, the cable is shielded, and the shield is used to carry power ground.Unfortunately, a general-purpose fix is not really feasible here.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 07, 2018, 12:30:02 am
content: The Prusa MK3 has an isolator in the USB line to avoid that kind of problem.