[sdiy] Mains interference

[rburnett at richieburnett.co.uk]

Hi Tony,

You have my sympathy as this type of EMC problem can be difficult to 
diagnose!  But that's probably not what you wanted to hear, so here's a 
few thoughts.

Whenever a thermostat opens or you operate the mains switch on a piece 
of equipment there is the potential for arcing and this generates high 
di/dt usually followed by a burst of oscillation at RF frequencies.  It 
is this burst of RF that is very good at travelling along cables (or 
through the air) to some piece of sensitive equipment and causing a 
disturbance in its operation.

The trick to controlling EMC problems like this is to find the source of 
the high di/dt and then try to return the high-frequency current back to 
the source by the shortest path possible.  You can add to this by saying 
that the path taken should preferably not include going via what ever 
equipment is susceptible to RFI!  So, for example, a thermostat in a 
fridge should have a suitably rated capacitor connected across it right 
near the contacts to allow high-frequency energy to circulate and decay 
close to the thermostat.  Keeping the current loop small stops it 
affecting the supply to other things, and the small loop area also makes 
a terrible radiator of RF.

Another thing that is important is the impedance of the mains line at 
these radio frequencies.  Since you have the distribution transformer 
right outside the window you probably have quite a stiff low-impedance 
supply at low (mains) frequencies, but the impedance might be much 
higher at RF frequencies.  This is particularly so if you don't have 
many modern switch-mode appliances plugged in close to where you measure 
it.  (Switched-mode power supplies typically have X capacitors between 
live and neutral, and Y caps from each line to earth, and act to keep RF 
currents circulating locally rather than heading around your house on 
the mains wiring!)  This often causes problems to people with X10 
automation because it stops the RF propagating around their houses.

You might want to try feeding all your audio kit through a big mains EMI 
filter.  Not because you are trying to filter noise from the national 
grid from getting into the audio kit, and not because you are trying to 
stop hash from your kit getting out to the national grid, but just 
because it's internal capacitors will significantly lower the RF 
impedance on your new "filtered spur".  Any noise generated on the live 
or neutral conductors should now be strongly coupled to the other 
condutors by the capacitors inside the filter.  If there is no 
differential mode voltage between live and earth then your kit should 
not see as much interference.

The Y caps in the filter should also return common-mode noise that's 
present on the live-neutral pair to the earth conductor.  However, as 
other's have said it's worth checking that you have a good solid *LOCAL* 
earth connection.  The requirements for a satisfactory low-frequency 
mains earth are driven by safety, but such an earth can have quite a 
high RF impedance if it is located distant from your property.  Also 
check that all of the earth-bonding in your house is up to scratch as 
you don't want sections of metal pipe that aren't grounded or corroded 
connections.  Connecting the mains earth back to neutral at the 
distribution transformer only would be dangerous in the event of a 
lightning strike to the overhead lines.  It must be tied to actual "soil 
earth" somewhere for safety.  Amateur radio people know about how to get 
a good low-impedance RF earth so their literature may help.

Re: The soldering iron causing oscilloscope false-triggers, I see this 
all the time.  I've even seen it whilst working inside a screened room 
at an EMC test site, where the room and supply are constructed adhearing 
to best RF design practices!  Modern oscilloscopes only require a pulse 
of a few nanoseconds at most to trip the trigger so are very sensitive 
to radiated spikes from nearby equipment.  It's quite annoying but you 
might not be able to eliminate this!  If it really bugs you try 
disconnecting all of the probes, terminate the scope Y1, Y2 and trig 
inputs and see if it still happens.  Assuming the scope is housed in a 
decent quality metal enclosure you could try a large common-mode ferrite 
choke over it's mains lead as close as possible to where it enters the 
metal enclosure.

Good luck!

-Richie,



On 2014-03-27 10:34, Oakley Sound wrote:
> OK, this isn't specifically to do with Synth-DIY but it shows itself
> on my music gear more than on other things so I thought I'd ask.
> 
> What is it about certain properties that make audio gear within that
> property more susceptible to clicks and pops from other electrical
> items in the same property?
> 
> Since moving house some seven years ago I've noticed far more clicks
> and pops from fridges and central heating systems than any other house
> I've had. Now I have three socket ring mains around the house, but all
> three show the same problem. For example, in my office/workshop the
> soldering iron's thermostat will trigger the scope and produce a burst
> of signal on anything the probe is connected to. It will also produce
> an audible click on the computer's audio output. In my little music
> corner, switching a active monitor off will make the USB midi
> interface lose connection with the connected computer, cause another
> computer in another room to crash, and produce pops on the living room
> hi-fi.
> 
> Now I live in the middle of nowhere and the transformer sub-station is
> up a pole just six metres from my back garden fence. The single phase
> cables then travel overhead for about 30m to a pole in my front garden
> and then via underground cable to the house. The earth connection
> appears to be made from the neutral feed prior the 100A fuse - but
> it's difficult to tell for sure since it's all covered in tape. The
> rest of the hamlet share the same 240V electricity supply but the
> clicks and pops seem to be related to stuff in my house only.
> 
> Any pointers you can give would be very much appreciated.
> 
> Tony
> 
> www.oakleysound.com
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy