Strange touchpad behavior - Page 1
Forum section: electronics / projects

author: Shirakaba
author_rank: Contributor
timestamp: October 22, 2013, 12:01:02 pm
content: Hello all wise EE-people.I am using a laptop, a Sony Vaio VPCSA, which is equipped with a touchpad detected by X.org as an AlpsPS/2 ALPS DualPoint TouchPad. The machine is running Linux. When running it at work, with PSU connected to mains, the touchpad behaves very strange. The mouse pointer movement is jumpy and sometimes it seems to think that my single finger are two fingers, thus acting as the mouse wheel. When running the machine attached to the docking station, I use an external touchpad connected to USB, and every now and then it freaks out too. When the laptop is running on batteries and not connected to the wired network, there are no problems whatsoever.At home I use the very same setup, with another docking station, an external touchpad of the same brand as the one at work, PSU connected to mains and connected to a wired network, and I rarely ever experence any of the problems I have a work.It could be worth mentioning that the PSU for the laptop has connection to mains earth and the negative rail connected to the laptop seems to be mains earth referenced. The wall sockets are earthed in both locations. Just for fun, I measured the potential difference between myself as I am sitting now, and the VGA socket shield, measuring 0.9 VAC, 50 Hz.Is there anything that could be worth measuring and what could be a possible cure for this?Regards,Shirakaba

author: daveshah
author_rank: Unknown
timestamp: October 22, 2013, 12:03:28 pm
content: If you have a scope, try measuring the output noise of the PSU. Sometimes a noisy power supply can cause this.

author: Shirakaba
author_rank: Contributor
timestamp: October 22, 2013, 01:38:35 pm
content: Sounds like a good start. Too bad my scope is at home and my problems at work, but I guess it could give me some kind of idea how the PSU is behaving.

author: senso
author_rank: Frequent Contributor
timestamp: October 22, 2013, 02:33:01 pm
content: Lack of earthed sockets caused the same in my old laptop, either crappy wiring at work, or crappy PSU.

author: daveshah
author_rank: Unknown
timestamp: October 22, 2013, 05:29:17 pm
content: In general, SMPSs will have capacitors going from live/neutral and possibly earth to the output as part of the EMC filter. This may be causing any mains noise at work to pass through to the output and cause this behaviour.

author: Shirakaba
author_rank: Contributor
timestamp: October 22, 2013, 07:59:29 pm
content: It's very likely that it is crappy wiring at work and/or someone polluting the grid with noise. At work, we share the first floor of the building with a laundry room and a few other businesses, including an electronics shop. The mansion I live in is purely residental.I made a simple contraption to easily be able to plugin in various power supplies and also test the laptop with and without docking station. Also did a few measurements at home comparing the PSU that I used at work today ("Regular") with the PSU at home ("Travel"). Didn't see anything surprising really. The "regular" supply seems to be a bit noisier by design.Will make an attempt to move the test gear to work and see what shows up there.

author: Shirakaba
author_rank: Contributor
timestamp: October 24, 2013, 09:45:13 pm
content: Brought the scope to work and made a few discoveries.At one stage I got some strange pulsing patterns, which later disappeared. Could that possible be due to the large sized laundry machines running next door?I noticed that I got a lot less noise when I disconnected the network cable from the (probably cheap-ass) Netgear switch in our room. Found another path to go directly to the core switch and then most of the noise and most of my problems using the touchpad at my desk disappeared. Still trouble in the conference room though, despite being on wlan there. I would assume that the conference room is running on a different phase which is noisier. The travel PSU was less noisy compared to the regular one, so it worked fairly well.I guess it will remain a mystery what causes all the noise on the mains, but at least the situation got a little better. Thank you for your input.