single point grounding scheme and flexmod p3

[LaNeK779]

well, this is not so advanced, but it sure is technical for me. so, here goes: i am reading bill benner's projector wiring article over and over again (a huge thanks to bill for writing this!!)

in the part where he analyzes signal connections, he specifically mentions that signal levels should be derived in a differential way and that signal ground should not be connected to power ground. all is well so far.

so the question is, if one is using flexmod p3s where power ground and signal ground are the same pin (the middle one), how do we go about solving this? is the tl084 opamp he proposes in figure 5 the solution? or am i getting off to a totally wrong direction?

bill's article is here for ease of reference http://www.laserist.org/laserist2008...onnections.htm

sorry for the lame question...

[platinum]

In a recent projector, I just wired it as is and had no issues. I'm interested to see what others say though.

- Jonathan

[sbk]

I guess the differential connection is only important for the scanner amp (which have mostly diff inputs), if the laser driver don't have differential inputs just connect its ground to the central grounding point. So for the Flexmod only one negative wire will do the job... This will just not respect the ISP standard, but doesn't mean it will not works well.

The major thing to take care of is where you will attach this central grouding point : to the ILDA pin 25 (Gnd) coming from the laser controller, or to the projector earth point (the metallic parts of the case). You just need to choose only one and NOT both.

[norty303]

Ah, I've just realised where a bit of image jumpiness may be coming from, I have AC gnd connected to central gnd as well as the ILDA pin 25. I'll have to lift the pin 25 and see if it stops it.

[LaNeK779]

So for the Flexmod only one negative wire will do the job... This will just not respect the ISP standard, but doesn't mean it will not works well.

The major thing to take care of is where you will attach this central grouding point : to the ILDA pin 25 (Gnd) coming from the laser controller, or to the projector earth point (the metallic parts of the case). You just need to choose only one and NOT both.

thanks for your answer mister sbk! i'm planning to sit everything on TECs or use another method to electrically isolate parts, so i expect no phantom grounds. this way, i plan to connect the ac ground to the chasis/ baseplate and pin 25 to the central grounding point

[gottaluvlasers]

every single projector i build i tie ALL grounds (ILDA 25, AC grnds, commons, EVERYTHING) to one point. i have never had a single issue or problem.

just thought i'd throw that in there.

-Marc

[sbk]

I must admit that with the few tests I made, by connecting both ILDA 25 and AC Gnd to the same point into the projector, I noticed no problem at all in the drawed patterns. But it may be due to the fact that both laser projector and computer/laser interface was on the same electrical outlet.

My guess is with this way you may create a ground loop if you connect computer with its USB laser interface on one electrical node and the projector on a different one (i.e. with a long ILDA cable to the stage), and if your computer has its USB Gnd directly tied to AC Gnd.
I just measured this on two laptop, one (macbook pro) has 1 kOhm between USB and AC Gnd, the other (Acer) has 0 Ohm. So this last one seems to be more prone to ground loop... As on a FB3 for example the ILDA 25 is connected to the USB Gnd, so you will have a direct path between the AC Gnd of the computer and the AC Gnd of the laser projector, through the ILDA 25 wire.
If there's 1V difference between AC Gnd, this may generate a non-negligible current ground loop...

[kl79]

I connect both DB25 pin and ground to the same point.
BUT ! what may happen if a light near our lasers have a problem ? if an AC wire go to the ground ? I think before the electric breaker open, it can make a desaster inside our lasers.

[PhantomRage]

My experience with grounding is from a sound background.

I know ground loops can cause hell on gear. not sure if a ground loop can cause noise on the galvo drivers board and make a image distorted.

Maybe the issue with all in one ground will come into play when you find yourself on two different AC circuits in a venue. IE, projector plugged into circuit one on stage, and computer back in the sound booth to run laser show plugged into circuit two. Both being out of phase or on different grounds could cause a issue maybe? Since the two grounds would be tied in with the DB25 cable....

But I am very new to this projector thing. So this may not ever happen?

[taggalucci]

Poorly wired grounds i.e. not following the scheme described above, do cause image stability or noise problems. I've seen this.

Bill Benner also demonstrates this on his projector wiring page on the Pangolin website.