Page 1 of 2 12 LastLast
Results 1 to 10 of 17

Thread: The effect of phantom grounds on projected images

  1. #1
    Join Date
    Apr 2006
    Location
    Orlando, FL - USA
    Posts
    1,767

    Default The effect of phantom grounds on projected images

    HI everyone,

    This is something I've been talking about for a while, and even wrote an article on it, and the article has some photos, but video conveys a whole lot more, and a whole lot better than words. See the brief video I made here which shows what can happen to projected images if the connections are not made in the most optimal way:
    https://www.youtube.com/watch?v=Vm0GISQi9fE

    Bill

  2. #2
    Join Date
    Mar 2010
    Location
    Raleigh, NC
    Posts
    2,201

    Default

    Excellent video! Thanks for posting that.

  3. #3
    Join Date
    Jul 2007
    Location
    Knoxville, TN & Huntsville, AL, USA
    Posts
    3,146

    Default

    Quote Originally Posted by JohnYayas View Post
    Excellent video! Thanks for posting that.
    +1!

    ...I think I need to take another look at the wiring in my projectors...

    Thank-you Bill!
    RR

    Metrologic HeNe 3.3mw Modulated laser, 2 Radio Shack motors, and a broken mirror.
    1979.
    Sweet.....

  4. #4
    Join Date
    Feb 2007
    Location
    Fort Mill, SC USA
    Posts
    1,485

    Default

    Quote Originally Posted by Stuka View Post
    ...I think I need to take another look at the wiring in my projectors...
    Me too! +15 char

    David
    "Help, help, I'm being repressed!"

  5. #5
    Join Date
    Dec 2006
    Location
    Scotland
    Posts
    1,408

    Default

    Useful info ,will need to find a grid pattern to check
    In the beginning there was none. Then came the light - #1 UKLEM - 2007
    BUY UK LEGAL LASER POINTER :: NEW - Blue 460nm Laser Pointers

  6. #6
    Join Date
    Dec 2010
    Location
    DC/VA metro area, USA
    Posts
    554

    Default

    Excellent demonstration, very informative, thank you! When I build my projector I will definitely keep this in mind. Two questions, because I can't help being curious:

    1, Do this effect and solution manifest regardless whether the amps are running differential inputs or single-ended? Or, is there different guidance for differential vs. single-ended amp installations?

    2, Is this another one of those situations that capacitors won't fix? Clearly this is a complex system, so I'm trying to understand the circumstances of the amps... In the video, I hear that under acceleration amps "draw a lot of current," but also in the past I have heard "instantaneous currents are not that high." So, please forgive me for splitting hairs, but I am just attempting to gain a better understanding of the nuances. Obviously the best solution is to let that connection float, not to tie it to ground and then add capacitors to the system (nor would I suggest doing so!) but these are mysterious things until they are understood, so the instinct is to keep asking why things work the way they do.

    Thanks!
    Mike

  7. #7
    mixedgas's Avatar
    mixedgas is online now Creaky Old Award Winning Bastard Technologist
    Infinitus Excellentia Ion Laser Dominatus
    Join Date
    May 2007
    Location
    A lab with some dripping water on the floor.
    Posts
    9,468

    Default

    Insidious instantaneous currents can reach 6 to 9 amps or more on a modern scanner. It is more worrisome as scanner speed increases.

    This is the back EMF and forward current surges getting into the control signal ground plane on the circuit board. Good board layout practices mean much more expensive boards, so when cost cutting during design occurs, the split ground plane needed to mitigate this disappeared from clone amps.

    Say you have 0.1 ohm of resistance in your amp's ground plane and ground wire to the PSUs. At 6 amps of surge, you just lifted the board ground six tenths of a volt off ground. That is bad.

    Single ended control does not work with Bill's solution, it needs a ground reference for the signal, making it susceptible to hum. With single ended the answer is to use analog power supplies with huge surge capability and "Stiff" grounds, as well as reverse biased diodes on the supply rails to allow surges to return to the rail. Care is then taken to keep the distances from the signal source to the amplifier very short. As well as use shielded cables or cables with driven shields. "Home Run" or "Star" grounding is used with multiple ground wires to a central point. Control portions of the amp board have their own power supplies and own ground plane

    While I urge the use of external stiffening capacitors on switching power supplies, caps don't fix all of the ground return issues. Neither do reverse diodes dump all the EMF. Differntial signaling gets closer to zero error then anything short of multiple ground planes on the amp board.


    Eye Magic provides for the ground jumper on their older amps, but that does not cure all. I have Cambridge Amps for very large galvos with split ground planes, the noise content is noticeably smaller when scanning.

    Incidentally Bill's technique is somewhat different from DJs "lifting" the AC safety ground on their equipment, so please do not call differential signaling "ground lifting". Your projector should still have the safety ground on the AC line cord connected to case. "lifting" does little to solve the problem in this case, and is hazardous.

    Follow Pangolin's PDF when wiring projectors. It works.

    Steve
    Last edited by mixedgas; 10-10-2014 at 09:30.
    Qui habet Christos, habet Vitam!
    I should have rented the space under my name for advertising.
    When I still could have...

  8. #8
    Join Date
    Apr 2006
    Location
    Orlando, FL - USA
    Posts
    1,767

    Default

    Quote Originally Posted by tribble View Post
    1, Do this effect and solution manifest regardless whether the amps are running differential inputs or single-ended? Or, is there different guidance for differential vs. single-ended amp installations?
    No, you definitely need differential inputs, because that's the thing that actually solves the problem. During a period of high-current flowing through the X amplifier, *probably* the Y amplifier's "ground" will become "lifted". If you have differential inputs, that "lift" will be experienced by both the + and - input, and the common-mode-rejection capability of this input will thus, ignore this "lifting" action and not interfere with the actual images. This is the real "secret".

    HOWEVER, one thing I didn't mention and that you can verify is -- that your amp actually has good "common-mode-rejection" in the first place. When I first started working on these amps, they didn't! The test is -- feed the same signal (let's say Y+) into both the + and - inputs. You should see substantially NOTHING -- no scanning. If you see scanning, then the common-mode-rejection is crap, and you'll have to solve that problem first.

    In the case of the amps I was working on, they had 100K resistor in three (out of the four) locations of a difference amp, and 200K effective resistance in the fourth place. So I had to solve that problem. Once I got that solved, the images looked really good. Amps that started out really concerning me in terms of quality graphics, wound up delivering some of the best images I had ever seen (after all the modifications).

    Quote Originally Posted by tribble View Post
    2, Is this another one of those situations that capacitors won't fix?
    It could be that capacitors placed in some certain location would really help. But those capacitors would do largely the same thing that connecting the two amp grounds together with a huge piece of wire does. But doing it "my way" also has other benefits, and it's free. So, the way I think about it -- let's see -- huge expensive capacitors placed in some strategic location, or free. Your choice?

    Bill

  9. #9
    Join Date
    Jun 2010
    Location
    Australia
    Posts
    3,730

    Default

    Ah yes, output or supply currents in the input path... Current errors.

    Designing a projector wiring topology is no different to audio amplifiers really.
    This space for rent.

  10. #10
    Join Date
    Dec 2010
    Location
    DC/VA metro area, USA
    Posts
    554

    Default

    Steve, Bill, thanks for the detailed responses. I'm just going for, you know, more Capt. Kirk. vs. Lt. Savvik when it comes to knowing "why star (projectors) do what they do."

    Quote Originally Posted by Pangolin View Post
    Your choice?
    Bill
    Well, I stated my (obvious) choice in my original message, but one still likes to know the rationale behind the right decision so one can make the right decision in similar, but different, circumstances.
    Last edited by tribble; 10-12-2014 at 05:48. Reason: wrong smiley codes

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •