Hello everybody

As I started doing a new project, a few fundamental questions popped up in my head. I dont think I have understood transformers properly. Please point out if I have understood something wrong. Based on what I learn here I will decide if I have to rewind my transformer or not. This is regarding energy wastage in the primary side of a transformer. How much should my primary winding be so that almost all of my power draw from the mains goes to power the load on the secondary.In my understanding at idle whatever the current passing thru the primary just goes to magnetize the core. Any current used to power the load is over and above this magnetizing current at idle. To keep this question simple, please ignore the resistive losses for now.

To make things clearer see this example :

I have a LOAD which varies its power consumption from 15 watts to 25 watts. Then I have a transformer (extracted from an old UPS, being put to use in this new project) which at no load connected, idles at 20 watts from the mains (primary coil). Suppose I now connect the LOAD to this tranny and it pulls 15 watts from the secondary for next 1 hour, is it correct if I assume that I am wasting 5 watts from this transformer under this scenario ? Will this setup be considered efficient when my LOAD is consuming between 20 to 25 watts and be considered lossy when it is consuming between 15 to 20 watts ?..... because the transformer idles at 20 watts is it correct to think that this tranny is designed for another LOAD whose minimum consumption was 20 watts ?

For my project should I rewind (increase primary winding (inductance)) this transformer to make it consume only 15 watts or less (at idle) since my LOAD at minimum consumes 15 watts ? It should also be able to deliver max power of 25 watts with minimal resistive losses I understand that.

As I started doing a new project, a few fundamental questions popped up in my head. I dont think I have understood transformers properly. Please point out if I have understood something wrong. Based on what I learn here I will decide if I have to rewind my transformer or not. This is regarding energy wastage in the primary side of a transformer. How much should my primary winding be so that almost all of my power draw from the mains goes to power the load on the secondary.In my understanding at idle whatever the current passing thru the primary just goes to magnetize the core. Any current used to power the load is over and above this magnetizing current at idle. To keep this question simple, please ignore the resistive losses for now.

To make things clearer see this example :

I have a LOAD which varies its power consumption from 15 watts to 25 watts. Then I have a transformer (extracted from an old UPS, being put to use in this new project) which at no load connected, idles at 20 watts from the mains (primary coil). Suppose I now connect the LOAD to this tranny and it pulls 15 watts from the secondary for next 1 hour, is it correct if I assume that I am wasting 5 watts from this transformer under this scenario ? Will this setup be considered efficient when my LOAD is consuming between 20 to 25 watts and be considered lossy when it is consuming between 15 to 20 watts ?..... because the transformer idles at 20 watts is it correct to think that this tranny is designed for another LOAD whose minimum consumption was 20 watts ?

For my project should I rewind (increase primary winding (inductance)) this transformer to make it consume only 15 watts or less (at idle) since my LOAD at minimum consumes 15 watts ? It should also be able to deliver max power of 25 watts with minimal resistive losses I understand that.

Last edited: