More on V- across multiple PS

[Dez]

I thought I was all over this and seen something that made me question the age old question of connecting V- together.

Previously I have always connected the V- when power injecting at the pixel strings when using multiple power supplies. We all know there is no question here.
I just rewired my new controller box which has 3 power supplies in it. Is it normal practise to run a seperate wire connecting all the V- across all 3 power supplies in our controller boxes as well?

Thanks.

 

[i13]

There is no perfect answer to this.

Connecting the negatives together at the power supplies and cutting them like the positives within the pixel strings: Cutting the negatives within the pixel strings avoids negative loops when you have multiple pixel strings running from one power supply to another. Negative loops can (but are very unlikely to) cause flickering. You may well have the negatives connected by design if you have them directly connected to the same pixel controller. The downside to this option is that cutting the negatives within the pixel strings means that the negative reference for the data is not as stable. More voltage drop might occur on one side of the cut than the other so flickering is possible.

Connecting the negatives together through the pixel strings and not directly at the power supply outputs: Having a continuous negative reference along each pixel string results in a stable reference for the data which helps avoid flicker. The downside of this option is that you'll get negative loops if you have multiple strings of pixels running from one power supply to another.

In reality, both options are likely to work. If you have flicker and you can't figure out why after exhausting all other possibilities, try changing from one option to the other.

I see no need to connect the negatives in both places (through the pixel strings AND at the power supplies) at the same time. This will create even more negative loops and there's nothing to gain compared to connecting them through the pixel strings only.

 

[TerryK]

I thought I was all over this and seen something that made me question the age old question of connecting V- together.

Previously I have always connected the V- when power injecting at the pixel strings when using multiple power supplies. We all know there is no question here.
I just rewired my new controller box which has 3 power supplies in it. Is it normal practise to run a seperate wire connecting all the V- across all 3 power supplies in our controller boxes as well?

Thanks.

For your situation as I understand it, I would say yes. With 3 supplies and controller in the enclosure, the enclosure is the focal point of the display so one would I think want the V-/GND to be common (no voltage variances between V-s [V minuses]) and solid (reliable) in operation. But, as i13 indicated, trying to define a rule of thumb or policy with so many possibilities of variations in everyone's display is difficult if next to impossible.

As to your situation, I would only recommend not connecting the V- on a supply if the supply was dedicated to a separate string or strings and use a device to handle any possible voltage level shifts such as a receiver or another controller.
For what it's worth, I do not break the V- at string power injection points. I want the Data Line signal to be as robust as possible and that means keeping the V- wire contiguous. It also means possible loop problems in the V- but I can engineer around any. I also do not break the V+ but that's another debate.

 

[Dez]

Thanks all.

 

[pixelated]

I am not yet totally clear about power injection using multiple power supplies. I have checked the 5VDC power supply I have and the V- terminals are not connected to GND. This makes it a floating dc supply with no reference to GND. V- can be connected to GND and I was wondering if that is what is done. I do not see this on any drawings I have seen. One reason for not connecting V- to GND is so the power supplies can be connected in series. In my readings it appears that the 12VDC HP server power supplies have V- connected to GND. There are articles that describe how to isolate the V- on these power supplies so that two can be connected in series to make a 24VDC high output power supply. With the DC supply floating how would ground loops be created? I would think that ALL the V-s should be connected to give a common reference for the data signal. This is what I have seen in the drawings I have come across so far. Perhaps it is induced noise on the data line that causes problems. As the data line is not shielded, care needs to be taken in its routing. The data line should not be run along side any ac cables such as those going to remote power supplies. I am sure many configurations will work and problems will not be experienced. Any clarification would be appreciated. Thank you.

 

[TerryK]

Based on how you are discussing GND I assume you are referring to the Mains ground or Chassis Ground. Few if any DC supply manufacturers connect any DC output terminals to Chassis Ground. I seem to recall encountering a few where the manufacturer 'strapped' the V- to GND leaving it to the user to remove the strap if desired.

For RGB pixels, the connections are labeled DC+, DI or DO, and for some odd reason GND rather than DC-, DC Com, or Com. I am not sure how many individuals tie V- to the supply Chassis GND. I do not. As you indicated, the V-s of multiple supplies are almost always tied together. There is a some debate on the best place to do that, out in the strings, the supplies, or both.

 

[Dez]

So from what I’m reading, I guess I will continue to keep the V- connected in the string of pixels but I will not double up by connecting them at the actual power supply

 

[pixelated]

Thank you for the clarification regarding V- and GND. I now understand how they are referenced. I appreciate the help.