# Current of a transformer

1. Jul 18, 2015

### Guidestone

Ok guys so I have to purchase two transformers with certain voltage value and current. I think I know what voltage in a transformer means when it comes to look for one, it's only the input and the output voltage, please correct me if I'm wrong. But I can't understand the current value, is it used to specify the maximum current the coils can hold or something? Is it some kind of saturation? Thanks!

2. Jul 18, 2015

### Jony130

They give you the maximum current you can draw from secondary winding for resistive load.

3. Jul 18, 2015

### Merlin3189

I think the current rating is based on resistive losses in the windings creating heat. So if the current were varying (not just AC, but varying amplitude AC) it would be the maximum RMS value of the current over a few seconds and longer.
So if a transformer were rated at 4A continuous, ideally you could run it at 3A with peaks of 5A, provided the peaks were no more than about 43% of the time, then the heat generated would be the same. But as the temperature would be fluctuating, you would need to derate it a little, because you want to keep the peak temperature below a maximum value, not the average temperature below the maximum. This is a more complex calculation, which I have not done, so I can't give you the exact duty cycle

4. Jul 18, 2015

### meBigGuy

When you use phrases like "maximum current the coils can hold or something" I can only thing you have never read a basic electronics text, much less spent time trying to understand how a transformer works.

The maximum current rating is related to magnetic saturation of the core and heating from various losses. And. please do not ask what those losses are without trying to research it yourself.

5. Jul 19, 2015

### Guidestone

You're right I'll do some research on it.

6. Jul 20, 2015

### meBigGuy

7. Jul 25, 2015

### Guidestone

8. Jul 25, 2015

### meBigGuy

Hey, we all need to learn. I got schooled for saying " The maximum current rating is related to magnetic saturation of the core" (turns out it isn't) in this very thread. Feel free to ask questions when you get stumped.