Anyone good with transformers?

[Grall]

I don't remember much from my highschool physics classes, it was 25 friggin years ago now (besides, was never any damn good anyway ), and I don't know if the stuff we read covered this anyway, so here I ask, hoping someone knows:

Assume you have a transformer rated at 20W output from the secondary winding (for driving a single halogen spotlight bulb), and you then go and replace the halogen bulb with a LED spotlight rated at 4.5W instead. Is the primary winding then still drawing 20+W off the grid even with a lower draw load hooked up to it?

I assume it is, as the transformer isn't a modern switching power supply - the lighting fixture was bought in the mid-1990s when such things weren't as common as nowadays. I could measure myself, but I don't have a multimeter...

 

[Bludd]

Uh, wattage is variable. W = V * A

Assume V is constant (110V or 230V, depending on where you are) and you know W, the amperage varies when you change the bulb. The wattage rating of 20W is afaik an upper bound.

 

[Grall]

The wattage rating of 20W is afaik an upper bound.

You sure about that? Because I once had a 220-110V step-down transformer (for a minidisc player bought in Canada incidentally) that was just a lump of iron and copper, no electronics whatsoever. And it got mighty hot whenever it was plugged in, even when the player wasn't doing anything. So it had to be sitting there, burning electricity even with basically zero load on it.

 

[Rurouni]

Have you tried using the step down transformer without the player being plugged? Was it still hot? Depending on the quality of the transformer, when nothing is plugged or the device connected is on off posititon, the transformer itself should draw almost nothing. The lower quality one might draw more than the good one, but it should still be small.
Anyway, as long as the voltage rating for the halogen is the same as the LED, you should be able to plug it safely as long as it is bellow the max rated wattage of the power supply. It will definitely use less electricity, but since the power supply was designed for a higher wattage, it might not be as efficient to power a lower wattage device.

 

[London Geezer]

I'm great with Transformers