Step down Transformer question

[dr.power]

Hi,

As we know the output volatge of a transformer is said as a RMS value.
Please I want to know what about the current of it? For instance when they they say that a transformer is a 6A one what they mean really? Is it the RMS or....?

The other question is how to determine of a transformer is suitable for a load?
For instance if the load needs a voltage of say 20Vrms and a current draw of say 4.5A then how to determine a good transformer with a suitable output volatge and current for it so that the transformer happily works with such a load?

Thanks

 

[Diver300]

Transformers are usually rated with sinewave voltage supply and resistive loads. So a transformer rated at 6 A can provide up to 6 A RMS.

They are usually designed to produce the rated voltage at the full load current. A 20 V 4.5 A transformer would produce 20 V at 4.5 A, and could handle that load indefinitely. The no-load voltage will be larger.

Transformer power is often quoted in VA, so a 20 V, 4.5 A transformer would rated at 90 VA. Manufacturers usually offer a range of voltages for one size of transformer, where all the transformers of that size have the same VA rating. It is only the number and diameter of the secondary that changes across the range.

https://www.electro-tech-online.com/custompdfs/2011/09/494477.pdf is an example. All of the 100 VA range have the same dimensions. The 2x20V one with the two windings in parallel would be fine for a load of 4.5A. Unless you were buying huge quantities, a standard one that is 10% too big would be cheaper than a custom one that is just the right size.

 

[Grossel]

1
Transvormer current rating is rms value. That is what mostly generating heat in the transformer.

2
Current rating for a transformer must be higher than the highest current expected to be drawn from it. But of course it's no point go get a transformer that is way too big.
Also you must ensurte that the transformer can deliver it's current rating in long periods of time without overheat.

 

[crutschow]

The current rating must be reduced below the VA rating if the transformer output is driving a rectifier load with a capacitor filter to generate a DC output. This is because a rectifier with a capacitive load draws short pulses of high current near the peak of the sinewave when the capacitor is being charged. These current pulses have a high RMS rating which heats up the winding more than a resistive load with the the same average current would.

 

[carbonzit]

The current rating must be reduced below the VA rating if the transformer output is driving a rectifier load with a capacitor filter to generate a DC output. This is because a rectifier with a capacitive load draws short pulses of high current near the peak of the sinewave when the capacitor is being charged. These current pulses have a high RMS rating which heats up the winding more than a resistive load with the the same average current would.

Is there a standard derating factor that can be applied? or does it depend on the size of the filter capacitors, total current, other parameters?

 

[ericgibbs]

Is there a standard derating factor that can be applied? or does it depend on the size of the filter capacitors, total current, other parameters?

cz,
This pdf is helpful.