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Electrical Noise

An unshielded twisted pair.

An unshielded twisted pair.

Noise is a hassle in just about everything and in electronics and electrical circuits it is no less of a concern.  However, it is something we can handle with some planning and if that fails a little knowledge will get us a long way to figure out a “work around” and living with the problem at an acceptable level.

Before I get too far into talking about noise I would like to do a project update.

Thermistor board component side.

Thermistor board component side.

I have been working on constructing the thermistor circuit. It will have six of the circuits on one board. Because I have very little experience etching PC boards I decided to build this with a little older method. I installed the components on a board with no copper foil on it and I am wiring jumpers on the back of the board.   It saved me the hassle of learning some software and then precisely drilling the holes.  I am not sure it was a good trade.

Thermistor Board Wiring Side

Thermistor Board Wiring Side

Although I did not have to worry about getting a good mask on the board, installing all those little wires is not an easy job for an old man with weak eyes. A bright light helps and only doing a few hours at a time also helps. One complete circuit is almost complete and two more are very far along. On the bright side, I am very glad I am now using a soldering station with a temperature controlled tip. I would have been a fussin’ cussin’ fool if I was still fighting the cheap simple soldering irons.

I have also completed some testing of some insulating material for the thermistor leads. That will be in a separate post.   Some work has also been done to put the Popsicle crane to bed, but there is nothing to show in pictures yet.

Now back to the subject at hand: electrical noise.  Noise is any unwanted signal, and can be broken down into two types.  First, is truly random noise.  Often this comes from thermal effects within the electronic components.  Second, is cyclic or pseudo-random signals coming from human sources.   An example of this is the problems that plagued car radios.  When the spark plugs fired it would make a crackling sound in the radio and I can remember on old CB radios hearing a whine that would change with engine speed.  This noise came from the alternator.

Noise other than the thermal noise gets into the electronics by two methods.  First there is electric field coupling or capacitance coupling.  The second method is magnetic fields or inductance coupling.    As always, lets start with the fundamentals we have learned.

Electric current flow depends upon a difference in voltage potential across a load.  If we can keep both wires feeding our signal input moving up and down at the same time with the noise, we do not have a difference in potential and the noise is “neutralized”.  This is called “common mode” because both the positive wire and negative wire are changing potential at the same time.  One answer to help prevent noise from getting into a circuit is to use twisted wires as shown in the picture.  This keeps the positive and negative wire very close to each other and helping to prevent a loop where magnetic fields can flow between the wires to induce a voltage.   It also means that both the positive and negative wire are equally capacitive coupled to any stray electric fields in the area.

The second method is to shield the wires.  A Faraday Cage, or Faraday Shield is simply enclosing something in a conductive container so any electric fields take the easy path around the device being shielded.  The same thing can be done with magnetic fields by using a material that easily passes a magnetic field such as iron or steel. Components and circuit boards are often mounted inside a metal enclosure to prevent the noise from entering there.  Wires, especially low signal level wires often are shielded to prevent the signal from entering by that path.   In automation work we normally run signal wires in shielded twisted pair cables and often these are ran in steel conduit around transformers and motors that produce strong magnetic fields.

On very long runs, it is possible that two devices that are “grounded” are not at the same potential all the time.  A very graphic example is if there is a lightening hit to a tower.  Although the tower is grounded, for a short period of time the earth near the tower is at a higher potential than other things connected to ground at different locations and current will flow in the ground wires between the two things.  It is common that slight potentials are at different ground points.  Normal practice in factory and plant wiring is to not connect both ends of the shield.  This prevents current flow along the shield and actually inducting noise into the signal wires.  The common term for this situation is a “ground loop”.

Resistance is an important consideration.  If a small random current is present it will show up as a much higher voltage if the input impedance of the load is higher.  In chemical plants where long cable runs exist, the signal runs are often 4 to 20 mA signals into 250 ohm loads.   Often signals to control motors in a factory where the cable runs are much shorter the signal is 0 to 10 V and the load is in the 10,000 ohm range.   (There are some other reasons for this, but the signal noise is a very important one.)

There are several more things to talk about here and those are directly related to using capacitors and inductors.  There is enough that I will hold off on that until probably the next post.

I am embedding a video that is a very graphic example of an electric Faraday shield in action.

As always it is my sincere hope that this has been worth your time.

Gary

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2 comments to Electrical Noise

  • Phillip Hughes

    Hi Gary’
    Firstly, thank you! This ‘sort’ of comment and explanation is an absolutely invaluable addition to help further an appreciation and better understanding of things electrical. Your conversational style, ordered and factual, is great. Please continue.
    Seconly, would you consider using and extending / alagamating the brief postings into longer and deeper ‘essays’, maybe as a downloadable PDF link for later reading?.
    Again, many thanks. Regards, Phillip Hughes

    • Gary

      Hello Phillip:
      Thank you for the very nice comment and feedback, I need that! It has taken me a while to develop a style that seems to work for me as well as for readers of this blog. I am trying to provide good information with enough details so people can really understand what is going on, without making it so boring and heavy that I kill everyone involved with details. (That is tough to do for an old nerdy guy!)

      You are right. I do need to start to work creating an e-book out of all these posts. Another thing I need to do is create a “Where do I begin” page and index for people just coming to this site. I am not sure when I will get either one of those done. My priority for right now is to complete two projects that have been on-going for too long.

      At the bottom of each individual post you will find a print – pdf button. That will create a pdf file or printout of that individual post. However, it doesn’t help with your request to have the posts combined.

      Again, thanks for the comment. Gary

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