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grounding noise

Posted: Sun Jan 21, 2018 8:09 am
by jward
I have an issue where the 7 segment displays occasionally alternate between displaying temperatures and 'no data'. It was suggested that noise is feeding back into the grounding wires. This makes sense as I think it happens when the chest freezer is running (aka glycol chiller). I am fairly certain I have everything wired correctly which leaves me the question of what to do to eliminate the noise?

Talking about grounding can be difficult I think because it means similar yet different things depending on the context. I am going to describe what I mean by "correct" based on lots of reading and conversations some of which were with an actual electrical engineer who designed circuits for living albeit some years ago. Good references do not mean I properly understood, haha.

DC ground:
In DC wiring you generally wire the negative terminals together. DC is based on voltage differences referenced from 0 volts. You might see some weirdness when you take actual measurements. If you measure between the + and - on a 5V circuit you should get close to 5v but between + and earth based ground it could be different. The circuit probably doesn't care as long as all the circuits reference the same voltage. I have 5v and 12v circuits in my BCS system. The words of wisdom were tie all the - terminals together and to earth ground. They will work out what 0v/ground means. Don't mix AC and DC.

AC ground:
In AC wiring the grounding wire is connected to earth ground at the electrical panel, but so is the neutral wire. The grounding wire should not carry any voltage unless you have a ground fault. So to this end, all the 120v positive, usually black, wires are switched through a relay. The neutral, often white, wires are tied together. The grounding, usually bare or green, wires are tied together.

If I assume the issue is that the compressor generates a lot of electrical noise. How does this work or rather how do I fix this? The freezer isn't using the grounding wire as the neutral. I assume the noise is not noise generated on the neutral and feeding back on the grounding wire from the panel. I assume it's feeding back directly on the grounding wire coming from the freezer.

It seems I need better grounding separation between the AC and DC circuits as they share the only earth ground available as supplied by the AC circuit. I have seen some noise filter circuits that use capacitors or such but were tricky in that you have to match the capacitor to eliminate the correct frequency of the noise. Maybe it was that technique only works on high frequency noise. The other choice seems I need to bring a second earth ground into the BCS just for the DC circuits. The problem with that is the earth ground available is still more or less the AC grounding wire from, at best, a different circuit.

Any ideas on eliminating grounding noise?

Re: grounding noise

Posted: Wed Jan 24, 2018 5:49 pm
by oakbarn
Search Grounding loops as well. You have to ground the AC to the DC of the BCS or your temps may fluctuate. Are your wires in a conduit? Are they a shielded twisted pair?

Re: grounding noise

Posted: Wed Jan 24, 2018 10:50 pm
by jward
On first build I did not have the AC and DC grounds together and did get to see the temp fluctuations. They have been. No wires are in conduit. The temp probes are shielded. I don't remember any other wires having any shielding.

Re: grounding noise

Posted: Wed Mar 14, 2018 4:06 am
by RobertG
I have the same problem with the 7-segment displays, they seem to be a royal pain in the ass to get to work properly. Even with everything else powered off except the BCS and the 7-segment displays, they still fail and display "no data" randomly...

I have tried about everything to get them to work, I tried twisting the wires, it did not help. I have installed ferrite cores on the wires, but this only seemed to make the problem worse.

Re: grounding noise

Posted: Sat Mar 31, 2018 6:09 am
by jward
Yes, the seven segment displays or I2C may be a bit twitchy.

Re: grounding noise

Posted: Sat Mar 31, 2018 7:51 pm
by oakbarn
Generally, I gave up on them a long time ago. They would work, then not. Not be a standard DIN size, there were difficult to add to an enclosure for me. I now use the API and Chomecast to put on a TV Screen of the Temp I want to monitor. Generally, I am only really concerned with the Mash. Being able to read it from across the room is a big help on brew days.