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 and 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,

 

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.

Without a doubt I'm getting a mains noise, and it's by far the biggest one. I was trying to play with small single single transistor amplifiers, and while still failing i would think I had success with a nice big wave until I figured out I was just pumping mains 50hz through the circuit.

I absolutely do have hf noise as well, which makes for a scribbly looking signal particularly below 300mv pp. I think I should probably try to learn how to rig up some low-pass filters for that, but I kind of worry that some amount of this noise is just going to be persistent while the circuit is on a breadboard with so many long wires everywhere. (I am to understand breadboards have capacitance and I have no caution with the length of my jumpers which certainly leads to all kinds of oscillation on the breadboard, I am not sure how much I can expect to "filter" but since I'm going to play in khz with beginner stuff, I suppose I can imagine it should be easy to have some goto components that go with every singal feed., just thinking out loud).

 

Breadboards have capacitance, but not enough to be a huge problem at audio frequencies.

Again, the length of the conductors are less of a problem with low impedance circuits.

 

Certainly there is a bit of missing information here. And more certainly there are MANY different types of transformers, for MANY different applications. It is possible to provide adequate shielding for a transformer, BUT THAT IS NEITHER SIMPLE NOR EASY!
One other thing is that there are different application-types for transformers, and that audio transformers seldom serve well as power transformers. That fact is seldom mentioned or discussed in most textbooks and never mentioned in hobby types of publications. But certainly it is real and not obvious in the transformer's outer appearance

Putting distance between the power transformer and the rest of the circuit is the simple and effective way to avoid coupling the magnetic field from the transformer back into the rest of the circuit.

 

One precaution on shield/covering toroidal types, if the cover is attched to the centre mounting bolt, it can cause a shorted turn.
Shielding not usually required on toroidal versions.

 

No external radiation

"No" is a really strong word.

Considerably better than EI laminations, but, not "no".

 

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 do not believe that your audio problems are being caused by the magnetic field radiated from a power transformer. The only time I have ever experienced a magnetic interference problem with a power transformer was when it was assembled close to a CRT. If you are using full wave rectification in your power supply, your "50Hz" pickup in your circuits is more likely to be 100Hz from not enough smoothing.
When you use long wires in your circuits, they will pick up high frequency radiated noise from lights, motors, dimmers and a host of other appliences. Use shielded wires or twisted pairs to reduce it.

 

K.W. Is correct!! The majority of "noise" from the mains is mains frequency HUM. That is usually 50 or 60 Hz, or the harmonic, 100 or 120 Hz, if you have rull ave rectifiers working. The main transformer magnetic field will be nearby, within a few inches, unless you have really big transformers, much larger than what you can lift, and some sort of coil in the circuit, like a tape head or a guitar pickup coil.

 

Certainly there is a bit of missing information here. And more certainly there are MANY different types of transformers, for MANY different applications. It is possible to provide adequate shielding for a transformer, BUT THAT IS NEITHER SIMPLE NOR EASY!
One other thing is that there are different application-types for transformers, and that audio transformers seldom serve well as power transformers. That fact is seldom mentioned or discussed in most textbooks and never mentioned in hobby types of publications. But certainly it is real and not obvious in the transformer's outer appearance

Putting distance between the power transformer and the rest of the circuit is the simple and effective way to avoid coupling the magnetic field from the transformer back into the rest of the circuit.

I am a gutter installer, and as a result there was some scrap steel at the shop, I took it home. It's got to be like ,, idk 20g or 18g, I don't think it's 22g. I didn't measure. It's floppy at the size it is, normally it's like a sheathing steel, it's galvanized I think, but probably a degree of stainless as well.

Here's my current plan. I have a 3 prong chord which allows me to plug to powers earth ground. I will make a box a few inches cube with a lid I can remove. I will want to swap transformers in and out of it, and have spare room inside to put whole solder kit power supplies into as well. So I will just place like a 1A transformer into it, but it will be several inches larger in all 3 dimensions, and I will probably make it so I can mount the transformer if I want to, but will mostly just put it in there for a given experiment and have the wires come out a lid not fully closed, or maybe I will put a hole idk. Then I will make a lug to connect earth ground to for the plug and for the project. It is "ferrous" in that magnets stick to it, so although it's a bit stainless (I think), it still is responds to magnetism. The most important "experiment" for now is to power a radio made from a mess of wires and kit pcbs, including some radio kits, and still be able to get reception of the stations while the transformer is basically right next to it. I was able to improve reception just by waving my hand between the transformer and the project, so I am fairly confident I will have some effect by boxing up the transformer and grounding it, although I would not make absolute statments I suppose.

 

Interesting indeed!! A shielded steel enclosure. That should reduce radiated electrical signals, and it probably will effect electric/magnetic radiation as well. BUT there will be the consideraation of heat transfer. Electrical equipment does produce heat, and that will result in a temperature rise. Depending on the actual amount of power involved, that might require attention.

 

Interesting indeed!! A shielded steel enclosure. That should reduce radiated electrical signals, and it probably will effect electric/magnetic radiation as well. BUT there will be the consideraation of heat transfer. Electrical equipment does produce heat, and that will result in a temperature rise. Depending on the actual amount of power involved, that might require attention.

I see, I do know this but have not thought about it yet as it might relate to box design. I don't plan on running it terribly hot, all of the transformers I have at present are 1A or less. I feel like at this rating and if running only for several minutes at a time I shouldn't have to worry about heat, just the lid not being fully closed will probably keep it from getting over warm. But, that's a good point, that if I do then try to run a stereo on it for extended periods of time, or heaven-for-bid make a phone charger included in the package, I probably don't want a sealed box on the transformer. I wonder if port holes are enough, or if I should consider the wavelength of 5-60hz when making holes, or exactly how much heat it should produce, should I say put a 3A or larger transformer into it (which I think are quite large). Right now I'm mostly pursuing the things I can run off a battery, because it is simpler in all of this respect, but technically it's setting up to be a speaker cab and a number of heads.

 

Looking back at post #1, what DOES NOT SEEM REASONABLE is that any HUM pickup is from the transformer's magnetic field effects. What IS reasonable is that there would be either electrical fields produced by mains wiring close to some audio signal wiring that is not adequately or possibly not correctly shielded, which is common, or a bit of mains frequency current flowing thru some portion of the shielding conductors. A quick check could be to move some ferrous material thru the space between the transformer and the suspected pickup area are located. If passing a shield thru does not produce any change at all, that suggests that the magnetic field is not the problem.

 

Very often, AC mains hum comes, not from magnetic fields from the transformer itself, but from 50/60 Hz ripple in the DC power rails.

 

Very often, AC mains hum comes, not from magnetic fields from the transformer itself, but from 50/60 Hz ripple in the DC power rails.

Mister Chips it totally correct. Inadequate filtering is certainly a major source of hum in many instances. That is quite different from inadequate shielding and so the fixes are also quite different. Looking back at post #1 we are not given enough information to evaluate as to if the hum that the TS mentions is from one or the other sources.

This appears to be another instance of the TS having decided what the problem is and heading off in one direction, to the exclusion of other considerations. Suggestions have been provided and no responce has appeared.