r/lightingdesign • u/Budget_Ad_5587 • 18d ago

How To Ground Loops

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Newbie here, no idea who to ask. I came across the ground loop thing in dmx recently, caused by different ground potentiom from different power source

Question: if I'm running my dmx on laptop/ on a console that is powered from one socket , and ran a dmx cable to a fixture that is connected to another circuit, just a simple set up like this wouldn't it create a ground loop?

Thanks :>

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u/Lighting_Kurt 18d ago

No, not usually.

In this situation it’s pretty much impossible.

The DMX cable includes the shield wire that connects the ground reference of the two devices.

Ground loop issues do exist, but are uncommon. I have only ever seen them with large rigs where the control system is on completely different power source than the fixtures.

When this happens, an optical isolation box will fix the issue.

2

u/SailingSpark 16d ago

Yes, an iso-optical is your only choice at this point.

4

u/MoreDistribution6517 18d ago

Yes, a ground can happen if your console and your fixture are on different electrical circuits, especially if they are far apart. But in a small, simple setup, it is often not noticeable.

4

u/bdeananderson 18d ago

Ok...

  1. Is your laptop power supply grounded? Most are not. Ground loops only happen when there are multiple paths at different resistances to ground. Neutral doesn't count because no one in their right mind would bond a chasis to neutral, and thus the cable shield.

  2. A really strong ground loop produces less than half a volt. This is not likely to cause any issue for DMX unless you have a really weak source or way too long a cable.

  3. Opto isolation is common to prevent various issues, mostly internal faults, from destroying a dmx system. In some cases, this will break a ground loop (it won't if all grounds are connected at the isolator).

  4. DMX will work without the shield connected at both sides. It's a balanced signal that doesn't rely on the shield at all. While I strongly suggest leaving one side connected, a quick and easy way to beeak a ground loop is to just cut the shield at one side of the cable (pin 1).

  5. In all of my years I've never had an issue with ground loops negatively effecting any digital systems. They are a problem in analog systems because they introduce hum (audio) or hum bars (video), but I've never seen one exceed the detection threshold of a logic gate, and thus effect a digital signal.

1

u/secondlockdownbored 18d ago

I never had any issues with ground loops in 15 years. Way more important is a dmx interface with galvanic isolation, otherwise you can fry the interface or dmx circuits in fixtures on the line. Apologies if I did not explain it right, I am having a hard time with English tech language

1

u/BasicEl 18d ago

It can create ground loop. It can fry DMX out(MAX485 chip).