Transformer paramters questions(VA, Watt, rms)

[Lord Loh.]

Transformer Parameter

I have got a 12-0-12 transformer that is rated for 5A.

But when I rectified, filtered, and added an LM338 regulator, I get not more than 3A current at 5v.

When I filter the ripple waves, I get a higher voltage ( I have not measured this with a true RMS meter.)

I calculate the transformer to be rated 60W.

But if I get about 18V (on non true RMS meter), Should I see it as 18x3~60W

SO does this means that the current rating of the transformer is an rms value?

Is Wattage a better way to describe the transformer ?

And How is VA different from Watts?

 

[semiconductor]

Re: Transformer Parameter

I myself think that, it's not always 5A because exactly, it's the specification which is deduced from theory, but infact, there's many factors that effect it. So, it results in the fact that, you can not get 5A as indicated by manufacturer.

In addition, due to loss via heat or your rectifier can not accomodate the current of up to 5A, so you can not get a current of 5A go through your load!

It's just my opinion!

 

[jonw0224]

Transformer Parameter

Current ratings on transformers are typically RMS values. (Yours can handle 5 Amps AC current through the secondary windings). Keep in mind, that you have a center tapped transformer and 5 Amps is using the secondary in parallel.

Actually, the current is the limiting factor (because of heating in the windings). If you operate the transformer at a slightly higher voltage, you can push more watts through it. If you operate at a slightly lower voltage, you limit your watts. Despite the fact that current is the real rating, all larger power transformers are rated in VA, kVA, and MVA.

If you can REMOVE heat from a transformer you can INCREASE current through a transformer. That's why many power supplies have fans.

VA is the product of RMS Voltage times RMS current. Because of phase differences in current and voltage (power factor) VA is not necessarily Watts, but will always be greater than or equal to Watts.

 

[Lord Loh.]

Re: Transformer Parameter

Keep in mind, that you have a center tapped transformer and 5 Amps is using the secondary in parallel.

Sorry... I do not get this part...."Using the secondary in parallel".

 

[jonw0224]

Transformer Parameter

The center tap splits the secondary into two halves.

When you ground the center tap and use each half of the secondary to supply current. I call that using the secondary in parallel. In this case your transformer can supply 12V at 5 Amps (but really 2.5 Amps from each leg).

If you don't use the center tap and you ground the secondary at one end, the other end supplies 24V. I call that using the secondary in series. In this case your transformer can supply 24V at 2.5 Amps.

In short, you have a 60 VA transformer.

The terminology better applies to transformers with two secondaries. If the two secondaries are identical 12V, you can wire it to be 12V without a center tap (truely parallel), 24V without a center tap (truely series), or as a center tapped transformer like you have. Most ratings for center tapped transformer give the amp rating for a parallel configuration. I hope this clears up my terminology.

 

[eogotenks]

Re: Transformer Parameter

https://www.onlineconversion.com/forum/forum_1006664121.htm


look up here

 

[vicky]

Re: Transformer Parameter

hi!
there are many factors involved in this which are regarding to the operation and behaviour of transformer which are includes in its working for example eddy current loss, windings behaviour,magnetic field loss and othre parameters so.