Transformers in a circuit, interpreting the label

[latot]

TL;DR Summary

How can we apply a real transformer to a circuit, using a transformer with a current and voltage dc output, and how interpret the data in the label

Hi hi, I was trying to use a transformer, that typical transformer that we have in house for a project, but then I notice something, well all this can be just a misunderstand from me.

Actually a transformer have a voltage and a current, at least that is what it says the labels, "18V and 3A" for example, my first question here is, the transformer works as a voltage and current power source? (both at the same time)

Actually, from the theory, the voltage is constant, because is a proportion from the number of coil loops, and how in the input there is 220V (here in my house), I suppose the output will be in that proportion, the current have a proportion but is not necessary constant, or at least I suppose, so maybe, the "3A" just says the max current we can get from the transformer.

There is other interpretation, maybe just say, "I have 18V and 3A", says to us, "I have 18V and 6Ohms", so the 3A is just the current we get when is connected to a tester, if is this case, I have the question of how to know the max current that the transformer supports.

Thx.

 

[Baluncore]

"18V and 3A"

Says it will produce 18 VAC rms. The load can draw up to 3 amps of average current from the 18 V secondary.

 

[latot]

:O Ty, so, a transformer don't come with resistance? or just too low?

 

[anorlunda]

I have the question of how to know the max current that the transformer supports.

That's usually listed separately on the nameplace. Either a max current or a max VA.

 

[Baluncore]

... a transformer don't come with resistance? or just too low?

The only important transformer resistance is the resistance of the wire in the windings.

The voltage, when unloaded, is designed to be about 5% higher than specified. As current increases, the voltage falls due to the winding resistance. At the maximum current specified, it meets the output voltage specification.
An 18V, 3A transformer will have about (18 V * 0.05 ) / 3 amp = 0.3 ohms of secondary winding resistance.

 

[DaveE]

It isn't unusual, for off-the-shelf transformers for several parameters that you may care about to not be specified. The manufacturer's application engineers may provide a more detailed specification drawing, although this seems to be going away with cheap suppliers dominating the market.

It is common for EEs to just get a sample and characterize it themselves. This is one reason it is common for big manufacturers to make their own specification drawings and order custom or semi-custom parts. They are often the same parts, but specified differently.

The short descriptions you see on the part or when you order them are really just marketing guides so you know the intended target application. As a rough guide those numbers mean that if you put a standard voltage on the primary (not what's in your house, what the manufacturer specified), and your load on the secondary draws 3Arms, the secondary voltage will be 18Vrms. This represents the maximum load current that meets their environmental/safety/ reliability conditions.

 

[latot]

Thx!, this was very very helpful, now I think I can use the transformers here, and test them when I need.

 

[DaveE]

The reason you seldom see complex data sheets for magnetics is that they are often customized for large manufacturers. They are most often specified, either by customers of the manufacturers, with what amounts to a BOM and a set of assembly instructions. i.e. Use this wire gauge, with this insulation; wind it as shown in drawings on this bobbin; assemble with this core this way, etc. Then the performance desired will result. Good drawings then have a few test requirements, but these are to screen for material/assembly errors, not to help designers in their selection.