Power use of transformers, dependent on load?

Nico wrote: I'm wondering if the power used by a transformer varies depending on the load put on the transformer.

I would say yes, definitely.  If you feel a wall wart that's plugged into the wall but not to a device, it gets a little warm.  If you plug in the device and turn it on, the wall wart gets quite a bit warmer depending on how much power the device is using.  If I remember my basics, a transformer is primarily a reactive load, which means it stores the energy it gets from the wall as a magnetic field and then returns it as the current alternates.  So if your device is off you're not consuming power -- it's just sloshing back and forth.

However, there are resistive losses from this sloshing, so things do get a little warm.  If you have a device plugged into the secondary, then you're using the energy instead of letting it slosh back.  The transfer of energy from primary to secondary is imperfect, so you get resistive losses that warm up your transformer.

I doubt you'd save anything with a smaller transformer.  It probably has thinner wires with higher resistive losses.

Power isn't my speciality, so I'm happy for others to correct me.

Nico wrote: I'm wondering if the power used by a transformer varies depending on the load put on the transformer.

I would say yes, definitely.  If you feel a wall wart that's plugged into the wall but not to a device, it gets a little warm.  If you plug in the device and turn it on, the wall wart gets quite a bit warmer depending on how much power the device is using.  If I remember my basics, a transformer is primarily a reactive load, which means it stores the energy it gets from the wall as a magnetic field and then returns it as the current alternates.  So if your device is off you're not consuming power -- it's just sloshing back and forth.

However, there are resistive losses from this sloshing, so things do get a little warm.  If you have a device plugged into the secondary, then you're using the energy instead of letting it slosh back.  The transfer of energy from primary to secondary is imperfect, so you get resistive losses that warm up your transformer.

I doubt you'd save anything with a smaller transformer.  It probably has thinner wires with higher resistive losses.

Power isn't my speciality, so I'm happy for others to correct me.

John is pretty close ....

A wall wart tends to have a DC to DC converter, with capacitors and maybe a regulator, so it will consume some power while there is no load.

More importantly the capacitors will age and especially the later inverter types without a transformer.

A straight 230 (or 120) to 12v transformer will have some losses, which tends to result in the core heating.

Therefore if you leave it there it will consume a very small amount of power, and the percentage of its full load is very small.

John is also right in the resistive losses, but you could measure the voltage to see how high it has climbed now that the load has dropped.

IMO I wouldn't bother replacing it until you have to.

For what its worth we tried those plug in power meters to check on the loads of various equipment we use, and the results were c..p.

They seem fine at high loads, but we found them to be out by 100% on the small loads.

We constructed a device that placed a resistance in the neutral, and used a portable scope to capture the voltage across it, while measuring the mains voltage.

This also included a means of capturing the first cycle to measure the inrush current, which on some 600w servers was 44 Amps.

Mark