Problems with mains spikes (solved)

[tomsimmons]

Certain items around the house are causing mains spikes of about 2 volts (in the 5V supply) lasting about 300 us, these seem to be really upsetting the raspberry Pi, the screen goes blank LED's flicker, but it recovers.

We see these when supplied from a mobile charged, and when supplied via a serious (professional 5A) bench power supply. We've got ferrite rings on the incoming and out going supply lines. We've tried extra smoothing caps, but it's still causing merry hell.

It's things like water pumps, soldering iron stations etc. that cause the problem.

Anyone come across this before? Any solutions?


Tom

[mahjongg]

The PI has no low impedance path to ground, (except through the HDMI cables GND wires). So when a power supply has parasitic capacities between mains and low voltage parts, very high voltage pulses will end up on the whole electric system of the PI, and through yet other parasitic capacitances between parts of the board and earth this can cause voltage differences over the board, and can transfer the disturbances through the HDMI port to the TV/Monitor if that isn't grounded either.


best solutions are providing better isolation in the PSU, and/or to short the voltage pulses directly to ground near where they enter the system.

[davidcoton]

I don't think the problem as described is due to earthing issues. If they were, the ferrite rings would help.

IMO, the likely cause of drop-out in the PSU output (you said spikes -- is the 5V going up to 7V or down to 3V?) is a drop-out of the supply. Are you in a rural area, on the end of a long cable? Has the supply been tested recently (particularly the external earth fault loop impedance, denoted by Ze, and the circuit earth fault loop impedance, denoted by Zs)? Problems there would show up when motors or large heaters start, though I would be surprised if an individual soldering iron could have any effect -- maybe ten or more switched on at once? Have you got the facilities and knowledge to monitor the mains supply safely?

Another possibility is some control gear causing RFI (radio frequency interference) to the PSU. A CE marked PSU should have some immunity, but strange things can happen. Unlikely given that you have tried different PSUs.

[aTao]

and when supplied via a serious (professional 5A) bench power supply

If your supply passes any mains spikes (short of a lightning strike) its not that serious.
Try using a 7 ohm resistor instead of the RPi connected to the supply (try both), your measuring instrument (must be a 'scope to see the pulse) and nothing else.
Then connect the 0V of the supply to the GND of the TV and try again.

[W. H. Heydt]

Other things to consider...
Is there an industrial park near by? Containing equipment with large motors that get started and stopped?
Do you have things with significant, intermittent power requirements on the same circuit like a refrigerator?

For preventative measures...
1. Surge protector (a good one).
2. Line conditioner with a fast response.
3. UPS with a fast response.

[tomsimmons]

Finally resolved this one.

There was no earth continuity through the HDMI to DVI adapter we were using. Changing to a HDMI to DVI cable and whilst the spikes are still present everything is rock solid.

Thanks for everyone's input.


Tom

[emma24xia]

Nice feedback, I have confirmed this solution worked.