OK- Another ground Question?? Sorry! :(

[allthatwhichis]

Well, I must say thanks to Marc for opening this thread. I checked my scanner ground and found it NOT connected to anything... It got pulled out at some point. I had some "noise" in my images but had no idea where it was coming from. I had it at FLEM II and have basically re-wireed the whole projector and cound not get rid of it. I think I knocked the ground wire out when I wired the DT40Pros in. The "noise" was a little more pronounced, but it was there before the ground got knocked out so I did not think to look there.

Right now pin 25 is only hooked to the "ground signal" input for my scanners. They have x and y+/x and y- and x and y ground. If I read Bill's post correctly...

Lastly, as Dave wrote, you will want to connect the ILDA DB25 pin 25 to the G -- AT THE POWER SUPPLY.

And most importantly, don't connect the ILDA DB25 pin 25 to anything else!!

I have this connected wrong. Should there be three cables going to the DC ground on the scanner power supply, coming from 1)pin 25 on the ILDA plug, 2)scanner amp power ground, 3)scanner amp signal ground (I guess two cables here, one for each axis, so 4 cables in all going to the scanner power supply ground). Am I even warm with that statement?

[gottaluvlasers]

allthat-

i hooked the 2 "G's" on the POWER SUPPLY of the galvos together. then hooked those to the GND on the galvo amp connections marked "GROUND." *NOT,* i repeat...*NOT* hooked to the lines marked "SIGNAL GROUND."

then that whole ground system is hooked to ILDA DB25 pin 25. thats ALL i have hooked to pin 25 on my projector and fired it ALL up for the first time tonight...

NOT A SINGLE PROBLEM!!!! WOOOWEEE!!! (pics coming soon!!!)

-Marc

[allthatwhichis]

`What do you have hooked to your signal ground?

[gottaluvlasers]

nada. zip.zero. zilch.

see what i mean about all this "GROUND" crap?? it gets VERY confusing and very misleading.

not to mention, NOTHING comes with any sort of a manual for wiring or anything. All the companies out there (and no offense to any single company) assume that EVERYONE hooking up a laser projector or EVERYONE building an RGB projector has an electronic engineering degree. i think thats a little unfair.

i think it is fairly safe to say, especially after becoming a member of this forum, that the laser field is becoming more and more "hobby" level. i dont think its too much to ask for a simple set of even rudimentary or basic wiring schematics for products that we purchase. ESPECIALLY this "high tech" stuff!

-Marc

[allthatwhichis]

nada. zip.zero. zilch.

[buffo]

Should there be three cables going to the DC ground on the scanner power supply, coming from 1)pin 25 on the ILDA plug, 2)scanner amp power ground, 3)scanner amp signal ground (I guess two cables here, one for each axis, so 4 cables in all going to the scanner power supply ground). Am I even warm with that statement?

No, not quite. You should have ILDA pin 25 conencted to the scanner power supply ground. You should have a wire from each power input on the scanner amps going to the power supply ground. And you should have the grounds from your laser power supplies connected there as well.

You are not supposed to have "signal ground" from the signal input on the scanner amps connected to that ground. This is what Bill Benner was talking about in the aforementioned post. It creates a ground loop (not in the classical audio-gear sense, but it's still a good term to use), and this can introduce noise into the input signal for the amps.

However, having said all that, I performed a few tests with the signal ground connected and with it disconnected, and I couldn't detect any difference in the scanned image. But then again, I only had a 9 ft ILDA cable hooked up, and everything in my projector is within a one-foot radius. Your mileage may vary, however, depending on the amount of noise in your projector, the length of your cable runs, and how robust your scanner amp power supply is.

Some of the more experienced members here replied to Bill's original post in that other thread, and they *did* experience problems whenever they had ILDA pin 25 connected to the signal ground on their scanner amps. Some of them fought the problem for years before learning the right way to hook it up. So if you're having problems, the signal ground might be a good place to start looking for the trouble.

Adam

[Pangolin]

In fact I am writing an article for the upcoming edition of The Laserist magazine, and there will be a diagram in there...

Bill

[smogthemog]

Will you be posting it on here Bill? please

This grounding business is a right mine field isn't it!

Can someone explain please what an 'ground loop' is please?
I think I understand but not 100%

[gottaluvlasers]

WOOOAAAH boy....

did YOU just open up a can of worms!! lol....lol...

[buffo]

Can someone explain please what an 'ground loop' is please? I think I understand but not 100%

Basically, it's when you have more than one path to ground, which can create circulating currents between those two paths, causing interference. It's most relevant when dealing with low voltage signals.

The classic example is a stereo system where you have a component from across the room (which is plugged into a different AC outlet) connected to the amp on the other side of the room via a long set of RCA cables. Because the shield on the RCA cable is grounded, you've just connected the ground from the outlet where the amp is powered to the ground where the other device is powered, creating a large loop antenna.

The tiny difference in potential between the two points on the ground loop induces a small circulating current in the ground wire on the RCA cable. And because line-level audio is already a *very* low voltage signal to start with, this small current gets picked up by the amp along with the rest of the signal, resulting in an audible 60 cycle hum superimposed over your music.

You can get the same effect in a projector when the projector is plugged into one outlet and the computer is plugged into another. You now have a loop (through the ILDA cable) that can induce noise in the signal going to the scanner amps. (This is one reason why ILDA ground should not be connected to the scanner amp signal input ground.)

There is another phenomenon related to ground loops that can occur inside the projector... If you connect a ground wire from the signal input ground to the scanner power supply ground, you can create a condition where one scanner's current draw can affect the signal to BOTH amps. While not technically a ground loop (more of a ground coupling), it's still an issue.

Say one of the scanners is under hard acceleration while the other is supposed to be stationary. The hard acceleration can cause a ground "lift" of a few percent on the scanner power supply. If you connect this to the signal input ground, that "lift" will be picked up by the wire connected to the signal input and coupled to BOTH scanner amp inputs, thus the stationary galvo may actually move slightly, causing distortion in what should otherwise be a straight line.

I have not personally experienced this particular problem, but several professional laserists have commented that they have encountered it many times. Clandestiny explained that he fought noise problems for years because of this exact issue. The solution is to leave the signal input ground unconnected.

For more info on ground loops, have a look at this wikipedia entry. It's not great, but it's a start...

Adam