AC/DC common ground? - Page 1
Forum section: electronics / beginners

author: irmantus
author_rank: Newbie
timestamp: November 16, 2022, 04:02:19 pm
content: I'm building a laser cutter machine and stumbled upon this strange issue. All the electronics connected to the controller are acting a bit strange, so upon further investigation I found that my laser high voltage power supply (45kV) has common AC/DC ground.By design the controller is isolated DC only. But because it has to "talk" to a laser power supply it introduces AC ground and loads of electric noise to all components connected. Especially servo motors, they randomly start to move/jump. See my simple sketch on this setup (this is dumbed down version, there are more components)My questions:1. Are there any good reasons why this high voltage power supply was designed that way?2. Any easy way to overcome this issue without modifying the power supply? Or my best option is to modify the power supply to isolate AC and DC grounds.

author: Zero999
author_rank: Super Contributor
timestamp: November 16, 2022, 04:28:03 pm
content: 1. What's do you think is about the design? It's fairly standard for all power supplies to share the same 0V rail, bonded to mains ground.2. Why do yu want to isolate the AC and DC grounds? That sounds like a bad idea.

author: irmantus
author_rank: Newbie
timestamp: November 16, 2022, 05:05:41 pm
content: Quote from: Zero999 on November 16, 2022, 04:28:03 pm1. I just tested 5 different VDC power supplies that I have (touching power supplies metal frame and V- terminal with multimeter). None of them have continuity. So maybe we are talking about different things or I don't understand them fully. I'm a "new player" so don't beat me just yet2. I want to isolate/remove that noise. I know that this power supply introduces electrical noise into the controller and my best guess is that noise is coming from AC ground.

author: Zero999
author_rank: Super Contributor
timestamp: November 16, 2022, 05:34:36 pm
content: The power supplies are normally designed with the output isolated, but it is often tied to mains earth, when installed into the device. Removing the earth connection to 0V is likely to make the noise worse. If the power supply is powering another high voltage supply, disconnecting the 0V from ground will introduce the risk of the entire supply floating at 45kV, which will pose a safety hazard, as well as damaging it.

author: irmantus
author_rank: Newbie
timestamp: November 17, 2022, 04:31:42 pm
content: By the looks of it, the power supply has two PCB's, one for high voltage stuff and other for control logic, so I was thinking only isolate that board's GND. But ok let's leave it alone...How else can I filter/isolate that noise? I had probes attached to the controller (OC motor output while it was idle) when the laser power supply was physically disconnected and when I connected it, the signal(noise?) went to almost 5V. And that supply is not even switched ON, so it has to be AC ground. It gets so noisy that I can't even trigger on oscilloscope motor signal (which should be +5V rising edge).

author: Zero999
author_rank: Super Contributor
timestamp: November 18, 2022, 09:36:18 pm
content: You could try a capacitor between +24V and 0V.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 21, 2022, 09:31:23 am
content: I can see the problem. The controller ground comes via the Power HV supply ground. Ideally the HV supply and the controller would have indvidual ground connections back to a single point.Try to get all grounds running back to a single point If you have metal chassis ground that at the common point as well.Break the signal ground between the controller and the HV unit. A few turns on a torroid and a few nf bypass on the control lines may help- a signal CM filter is ideal. Power the HV unit via a local CM filter suitable for the current and switching frequency. Use heavy guage (10 AWG) fine stranded earth cable on the HV unit and wiring to mains inlet.If the lines from the controller to the HV unit are digital, consider isolation with optos or icouplers. This lets you separate the local grounds, this may be the most practical soltuion if noise problems perisit.Icouplers with internal power are really easy to apply. eg ADUM5210 /11

author: EPAIII
author_rank: Super Contributor
timestamp: November 21, 2022, 10:53:29 am
content: I also saw the ground loops which can act as antennae and pick up anything in the area.I like the idea of using a single point ground with the lines radiating out from there. Another thing you can try is heaver gauge wiring for the grounds. Just because the power is transmitted on a wire of a given gauge it does not follow that the same gauge is sufficient for the grounds. Try some wire of perhaps four or eight gauge sizes larger than the power and/or signal lines. Also you may want to try using shielded wires with the shield connected at one end only.Some times the only way to work out noise problems is to just try different things until it works.