I received my first Raspberry Pi B+ last Monday. Everything works fine and I'm happy with it, except for an issue: often, when things are turned off or on, or plugged in or out of the mains at any place in the house, the screen goes black for about a second, before coming back. The Pi does not reset, it keeps operating normally: the power led keeps steady, USB devices don't turn off (and if there's any data transfer in progress, it is not affected), ethernet is not affected and any keyboard or mouse input takes effect during the time the screen is black.
My setup consists of having the Pi powered from a 1A power supply that came with a ZTE USB 3G/Wifi router (so, not a phone charger). Connected to the USB ports I have a keyboard, mouse and flash drive (from which Raspbian is running, but the problem persists when running other distros or RiscOS). There is no hub and ethernet is connected.
The connection to the monitor, a Dell E228WFP, is made through a HDMI to DVI adapter and then a 1.5 meters DVI-D cable. This same cable, adapter and monitor work fine with my laptop, without screen flickering issues.
Things I've done to mitigate the problem, without success:
- Searched the internet a lot and tried solutions for problems that are similar to mine, but not the same (I couldn't find anyone having the same problem);
- Set config_hdmi_boost to 4, and then to 5. At this point, I don't think increasing this value is going to be of any help (and I don't want to fry the DVI logic on the monitor with a too strong signal);
- Tried many different power supplies. Most ones I have say they give 1A, but I also borrowed a 2A original Samsung Note II charger - interestingly, with this one, the monitor blinking issues were even worse, with the screen going black randomly about every five seconds - and sometimes staying black for over two seconds. It was so bad I did "sudo halt" right after the RPi finished booting. I guess this is because this is a charger, possibly optimized to charge the Samsung phone and not a power supply for general use;
- Connected the ZTE power supply to an outlet without an extension cord or anything else connected;
- Connected the ZTE power supply through a UPS - this made the problem even worse, because blackouts would also happen whenever the UPS decided to switch supply to/from batteries.
The "blackouts" only happen when the Pi is connected to the Dell monitor. If I connect it to my TV set though HDMI (with a HDMI-HDMI cable), the image on the screen is steady. On the Dell monitor, to induce a blackout, it's as simple as turning on/off my desk lamp, turning on a fluorescent light anywhere in the house, turning on the toaster or having the dishwasher turn its pump on or off, to list some situations. If there's a problem with the electrical system, it hasn't had consequences in the last 20 years, and we've been powering all sorts of electronic devices without problems.
My theory for why it happens: whenever there's a fluctuation in the mains voltage due to things going on or off, the power supplies I have pass that fluctuation on to the Raspberry Pi (all the power supplies I have with a USB connector are of the switching type). The Pi doesn't have any problem with this fluctuation as it is certainly well within operating range, but it causes some noise on the HDMI output. My TV set has no problems with this noise and goes on happily, but my computer monitor freaks out and blacks the screen out, taking some time to recover (going to the point of saying there's no input signal, if the noise persists due to additional fluctuations on the mains voltage).
How I could fix it (but I would like other people's opinion before buying the things to attempt these steps): IMO the ideal solution would be some sort of filter/rectifier on the 5V output of my power supplies. I don't know if the "made for the Raspberry Pi" power adapters you see online would have enough quality to filter these fluctuations, and I don't feel like spending money to find out, so if someone knows, please tell. I think another solution would be to use a transformer-based power supply (I have read these give a cleaner output), unfortunately I don't have any around to test - and if I had it most likely wouldn't have a USB connector, meaning I'd need to do some trickery that could possibly cause more problems.
Does anyone have any suggestions to solve the problem? I can make a video if the situation is not clear enough...
Thanks in advance.