I have been assembling all kinds of little learn electronics kits. Some of them require 12vac and I have a few small transformers. But, when I do some experimenting, particularly with radios or signals, I feel that the transformer provides me with a magnetic field to think about. It can interfere with radio reception in a major way, and I can't be 100% sure it is the sole source of my noise woes, but when I make breadboard circuits with some longer wires involved (like alligator clips) I end up with a very noisy signal as soon as it is no longer measured directly from the signal generator kit.

I would like to monkey around with having a transformer in the radio builds, as some of kits are audio amps that require 12vac, but without properly building out a shielded chassis for all the components. Instead, I would like my transformer in a box, and just hook it up to one thing and another thing. Would it make sense to box in the transformer? How would I do this? Would I need to earth ground the box, or would it be enough for it to just surround the transformer? I have some thin (I think 18g) aluminum (gutter material) I would use to make the box, it would be super simple to assemble, but placebo is a powerful thing. I've shielded a guitar with aluminum tape, it worked marvelously, but that was protecting the electronics in the body from ambient noise, which is not the same thing as trying to contain a transformer. I have also built an amp kit a long time ago, where the transformers were mounted to the top of a steel chassis, and the components were wired up inside of the chassis, so that the shielding surrounded the components on 5 sides while the transformers were mounted to the furthest side from the opening. In both of these cases the shielding was grounded. In this case, I would be much easier for me to not build out an earth grounded box and get a 3 prong wire, I would prefer a box that I could then quickly chuck any variety of transformers into it and without any sort of permanent mounting or extra hookup.

Would this do anything for me? Would it be a problem for the transformer? Should I have the transformer have an open box so that the magnetic field still has a direction to stretch out, and then just try to aim it away from my project?

 

Yes, AC power transformers can radiate 50/60 Hz noise into sensitive circuits. There are many options one can choose to mitigate this problem. Let us start by looking at the external PSU solution.

The first solution is to have the PSU mounted separately from your sensitive circuit.

1. You can build the power supply as you wish in a separate housing, open or closed.
2. For small, low powered projects, you can buy an off-the-shelf AC-to-DC adapter.
3. You can buy a transformerless PSU that uses SMPS (switched mode power supply). Be aware that now you have to deal with high frequency switching noise in the DC supply.

Next, we can tackle the situation where the transformer must be mounted in the same box as the electronics.

1. Transformer location and orientation matters. In tube amplifiers, it is good practice to locate the output transformer away from the power transformer. Even the orientation of both transformers can make a difference.
2. It is also good practice to place a bulky transformer on top of the chassis and locate the electronics underneath the chassis.
3. At all times, try to locate the transformer away from electronics.
4. If you are thinking of providing some kind of metallic shielding, aluminum is a poor choice since it does not provide magnetic shielding. You want a metal that is magnetic, such as iron.
5. Transformers are built using open frame construction or with an enclosure. Select a transformer that is already enclosed. Toroidal transformers are more efficient and produce less magnetic interference.

 

when I make breadboard circuits with some longer wires involved (like alligator clips) I end up with a very noisy signal as soon as it is no longer measured directly from the signal generator kit.

As MrC noted, the magnetic field from a power transformer can generate mains 50/60Hz low frequency interference (hum) into your circuits, but not any higher frequency noise you may have.

 

When connecting AF signals, it is customary to use shielded audio cables.

 

Another thing to note is that the susceptibility of a circuit to pick up EMI is directly dependent on the impedance of the circuit. A high impedance circuit is more likely to pick up EMI. The RF antenna input of a radio receiver has high input impedance is a good place for receiving unwanted EMI. Low impedance BJT circuits are less susceptible.

 

Another precaution is to use Toroidal type mains transformers.
No external radiation,