DMX voltage spike issue

[djgeorgef]

Hi all, I’m just after some clarification on an issue I’m having with some DMX rgb dimmer packs please. It’s a very basic setup: computer > DMX dongle (enttec usb pro) > 3 x rgb dimmer units (24vdc) 5m apart running some rgb floodlights. 

 

so the story goes…..all working lovely until a random rcd went somewhere else in the venue unrelated to the lighting setup, but on the same distro board (probably a toaster or something!!)! This resulted in 1 of the dimmer packs failing and also the enttec DMX dongle failing. Obviously very annoying!!

After looking into it, pin 1 on the DMX dimmer pack is also connected to the metal chassis inside the rgb units and hence earthed! So my theory is a spike on the earth found it’s way through to the ground on the DMX and blew the comms chip in unit and dongle? So my questions are:

1. Does this sound plausible? No faults found on any of the 24v circuits.

2. I’m assuming that the mains earth should be open circuit from the DMX ground on pin 1. Hence a badly designed/made rgb unit

3. I’ve also noticed the DMX cables (qtx brand) have the shields link to pin 1 in the xlr connectors. I’ve read that the shields should be separated from pin 1 also but quite contradicting depend on various posts

 

I’ve obvious got a niggly one here, sorry there’s a lot there but trying to future/idiot proof this so it doesn't happen again! I’m also looking into optical isolators also. Any advice would be greatfully received. Thank you in advance. 

Edited November 20, 2022 by djgeorgef

[Brian]

1 hour ago, djgeorgef said:

1. Does this sound plausible? No faults found on any of the 24v circuits.

2. I’m assuming that the mains earth should be open circuit from the DMX ground on pin 1. Hence a badly designed/made rgb unit

3. I’ve also noticed the DMX cables (qtx brand) have the shields link to pin 1 in the xlr connectors. I’ve read that the shields should be separated from pin 1 also but quite contradicting depend on various posts. 

1. Yes.

2. Whilst frowned upon, grounded receivers are permitted in the DMX spec.

3. The cable screen needs to be connected to pin 1. Without it there would be no connection, end to end, between pin 1s.

[sandall]

30 minutes ago, Brian said:

3. The cable screen needs to be connected to pin 1. Without it there would be no connection, end to end, between pin 1s.

Think he means connector shields, not cable screens (common problem with Chinese cables)

Edited November 20, 2022 by sandall

[djgeorgef]

1 hour ago, sandall said:

Think he means connector shields, not cable screens (common problem with Chinese cables)

Hi, thanks for the replies. So there are 2 cores and an overall braided shield. Pin 1 has one the overall braid/shield connected but The the shield is also soldered to the connector “earth” terminal in the xlr that goes to the xlr connector. 

[DrV]

The shield must be connected to pin 1 to provide a reference for the DMX (RS485) signals. It should not be connected to the connector shield as this may connect to mains earth at different points in the building. These mains earths could be at slightly different potentials in normal circumstances and very different potentials during fault conditions.

[sunray]

I certainly don't disagree with any of the advice given here but it never ceases to amaze me how frequently it contradicts the manufactures instructions and construction methods.

I reckon at least 50% of the DMX devices I've had reason to check have pin 1 grounded to the chassis and mains earth lead and I think every commercial DMX cable has shield on pin 1 and connector body.

[J Pearce]

7 minutes ago, sunray said:

every commercial DMX cable has shield on pin 1 and connector body

That's quite a claim, and certainly not something I've found on any of the cables I've had from professional cable makers.

[djgeorgef]

Thanks all for clarifying the cables so far, I’ve re-soldered the XLRs to keep pin 1 separate from the chassis. Had a quick look and am able to separate the mains earth and DMX ground quite easily in the units. Also
 

I'm paranoid it’s going to happen again, would optical isolators help to reduce this and would I need one on the input and output of each unit or is that overkill. Also do you think it’ll be enough to keep the grounds and earths separate. 

 

Apologies, just to explain a little more, this is a setup over Xmas that I’ve been volunteered for for a local non profit organisation. I’m an electrician but when it comes to DMX I’m a plug and play kind of person with a basic understanding until it goes pop, and would like a set up we can just roll out and plug in every year. 
 

thanks again, really appreciate all the help

[Jivemaster]

Are you working with one mains supply or several? If power is coming from several places than it's possible that the power ground is at a different potential on each supply and when these are tied together with a XLR lead bad things may happen to digital things (especially with a fault that tripped a breaker somewhere). 

Perhaps having a DMX isolated splitter between your PC/dongle and everything else and feeding some things from each output will help. 

Making sure that the digital ground is always separate from the power ground/CPC would be my first choice. 

[djgeorgef]

26 minutes ago, Jivemaster said:

Are you working with one mains supply or several? If power is coming from several places than it's possible that the power ground is at a different potential on each supply and when these are tied together with a XLR lead bad things may happen to digital things (especially with a fault that tripped a breaker somewhere). 

Perhaps having a DMX isolated splitter between your PC/dongle and everything else and feeding some things from each output will help. 

Making sure that the digital ground is always separate from the power ground/CPC would be my first choice. 

Thank you, I found this really useful. At least 2 of the dimmer units are on different ring mains. I like the idea of the splitter and then run a DMX cable to each unit. At least if one goes down the rest should stay active. 👍🏻

[DrV]

I believe the enttec pro dongle is already optically isolated (could have a look in mine tomorrow) but using an optically isolated splitter would certainly minimise your risks.

[djgeorgef]

6 hours ago, DrV said:

I believe the enttec pro dongle is already optically isolated (could have a look in mine tomorrow) but using an optically isolated splitter would certainly minimise your risks.

Yes it is,  apparently it sacrifices itself to save the desk/computer in the event of a voltage spike! It looks as though Enttec will be able to refurbish it which is good 

[timsabre]

If the fixture is designed with DMX/electronics ground connected to mains earth/chassis then you should not change this. Some power supply modules have floating DC outputs which can result in quite a high voltage on the power supply DC ground relative to earth. This can blow up DMX interfaces if connected with the power turned on (and give you tingly feelings if you touch the DMX). So to get round this they link psu DC ground to mains earth (sometimes via a resistor, fuse or fusible resistor). Removing this link is unwise.

[sunray]

16 hours ago, J Pearce said:

That's quite a claim, and certainly not something I've found on any of the cables I've had from professional cable makers.

I need to clarify the wording, it often reads differently the second time around.

16 hours ago, sunray said:

I've had reason to check ... I think every commercial DMX cable has shield on pin 1 and connector body.

I haven't had reason to check every DMX cable.

[sunray]

44 minutes ago, timsabre said:

If the fixture is designed with DMX/electronics ground connected to mains earth/chassis then you should not change this. Some power supply modules have floating DC outputs which can result in quite a high voltage on the power supply DC ground relative to earth. This can blow up DMX interfaces if connected with the power turned on (and give you tingly feelings if you touch the DMX). So to get round this they link psu DC ground to mains earth (sometimes via a resistor, fuse or fusible resistor). Removing this link is unwise.

I think it's fair to say most (if not all) of my own cheap LED units do have pin one earthed to mains if it has a mains earth. I find the 4 channel dimmer pack are about 50/50, all of my desks and those I have access to are floating walwarts.