Interference on projector when PC plugged into same ring final

[cornburn]

Hi all

A few installs we have done lately are picking up a bit of interference on the projector image. It only shows up when a computer is plugged into a nearby socket on the same ring that the projector is spurred from. If you run it off a laptop on battery it's fine, then displays horizontal lines as soon as you plug the charger in. The proximity of all the data stuff is definitely ok, all leads have been swapped over to double check. The PC and projector are obviously on the same phase.

Does anyone have any ideas? Swapping the computer to different sockets does change the level of interference, some better and some worse. We have tried several different computers and power supplies and all do the same thing so have ruled out everything apart from some interference getting through the spur.

I'm stuck :S I think the VGA cable for the projector signal has ground pins to sync the signal, these must go to earth through the laptop or PC so could it be something on the earth in the circuit? There are many parallel paths on the install, it's mostly in metal conduit fixed to a steel ceiling frame which is supported by the roof girders.

 

[Notsosmart]

Computers are notorious for introducing noise into the mains especially when a switch mode power supply is fitted (which is usually the norm)

An extract from another website: "It ( the power supply) switches at a much higher frequency (tens to hundreds of kHz) than that of the AC line (mains). This means that the transformer that it feeds can be much smaller than one connected directly to the line/mains. Switching creates a rectangular waveform that typically goes to the primary of the transformer.
Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), and a ripple voltage at the switching frequency and the harmonic frequencies thereof."

So the solution is maybe to fit a filter socket where you are plugging the projector into.

 

[cornburn]

Thanks, that's really helpful. Strange that out of about 150 installs I've only seen this twice. Some projectors must be worse affected than others. I didn't realise you could buy a filter socket? That could be the answer as it could save hours of troubleshooting and messing about.

I forgot to mention that it does this even when the PC or laptop is turned off at the wall (but still plugged in obviously) So surely the transformer hasn't got anything to leak then? I need to visit next week to sort it, any ideas of what to try? Would fitting a double pole socket to plug the PC into help at all? It seems as if it's an earthing problem whatever but I know earth continuity is good.