Question about transformers

[Sbampato12]

Hi,

I have a question in my mind, is there any improvement if I choose to use a toroidal transformer in a power supply, or not?
Without change anything except the transformer.

Thanks!

[Rigby]

Toroidal transformers are more efficient and emit far less stray magnetic fields.  The toroidal core basically contains nearly all of the magnetic field, whereas the E I core is pretty lossy.

[T3sl4co1l]

Worth noting, toroidial transformers are more easily saturated, which means much stronger turn-on current surges.  A somewhat larger fuse, or slow-blow type, or NTC, may prove useful to handle or mitigate this.

They're also apparently much better at wideband performance, so you can use a 240VAC 60Hz transformer down to 80VAC 20Hz, and at least as much voltage beyond 20kHz, often to 100kHz.  Which is handy for audio transformer applications, particularly tube amps.

Tim

[Sbampato12]

So the pro are: efficiency, less magnetic fields out the tranformer, and less weight and volume. But on other hand there are the 'problems' that T3sl4co1l.

I've never heard about the problems with turn-on current surges.

Thanks by the help!

[SeanB]

Use a large variac ( modified toroid) and you will see this often. In most cases a NTC thermistor will solve it along with a slow blow fuse.

[Kremmen]

Regarding NTCs just be aware that they run rightelously hot so avoid placing them near say electrolytic caps or other temp sensitive components.
Another downside is that rapid reconnects will not be surge protected. I.e. if you disconnect the device and reconnect it before the NTC has cooled down, its resistance will still be low and you may blow the fuse anyway.

[calexanian]

Also generally toroids weigh less per watt. One thing of note however for younger players, the isolation of toroids is typically not as good for breakdown as a split bobbin transformer. If your design requires that it can become a consideration, but in general use it is not an issue. Just insure you are using the rubber isolation pads supplied with the toroid. Also do not over tighten them. I have shorted windings by getting on them with a big wrench and over tightening the mounting bolt. As far as leakage goes. generally toroids leak less, but due to their lower reactance and lower overall inductance they can be electrically lossy. Generally it is not an issue, but can be in some critical situations. As said before just to saturate the core takes quite a bit depending on when you flip that switch on the AC input waveform and can really cause a big surge current. I have a 30 amp 240 volt variac that is a craps shoot every time you power it up that is does not blow something and thats running at 120! Variacs are just a toroid where one surface has had the enamel removed and a contact wipes across. I find myself using toroids more and more as the price drops on them. In the lower wattages (Sub 100VA) traditional EI core transformers are lower in cost and have better availability. Above 100VA   toroids are becoming more common and affordable. The weight savings alone account for a lot of it.

[wiss]

1500 VA tors can be a bit tricky... will blow 10 A fuse on turn on...

[johansen]

The reason toroids weight less per watt is because they are able to run the flux density at 1.7T or even higher, and a transformer's power density given constant current density follows flux squared.

Furthermore the core loss is nearly insignificant for a toroid that weighs less than 100 pounds. Most of the time its not even considered for thermal limits, and the windings being fully exposed offer perhaps 3 times the surface area compared to the same volume of wire filling an EI transformer.

the air gap being less than the insulation thickness (which is often just the oxide layer) means that the turn on current spike is of course, very high.

gigawatt sized E-I transformers can take more than 2 minutes for that turn on saturation to disappear, the reason why is because they have almost no air gap either. (relative to core length.. 4 meters of steel +1 mm of air corresponds to 1 part in 4000. try that with a 10 cm long core from a small EI transformer)

[peter.mitchell]

interwinding capacitance on a toroid is absolutely shocking.

[SeanB]

True, but putting a simple copper shield as a part of manufacturing is easy, or you just have a single layer winding and use it as a shield layer.

[Sbampato12]

The project in question, is a power supply, just the 100VA mentioned.
But I think I will go with the EI core, since it will cost me a little less.

[calexanian]

100VA and below is typically where EI is more cost effective.

[Richard Head]

Toroids are also low profile which can be very useful.

Dick

[peter.mitchell]

EI is easier to mount, arguably too.

[SeanB]

How wrong can you go with a single bolt? You can even use cable ties to mount low power ones very securely.