is there an equation for primary transformer current when secondary current = 0

[Mark]

Suppose I have a transformer such that the primary current is Ip when the
secondary current is Is, if I flip a switch on the secondary , the Is'
then is 0. I know that if you do this, the primary current does go
down but it doesn't go to 0 amps. In fact this is obviously so because
a transformer will remain warm even if the secondary is disconnected.

Is there a general equation that will estimate that or is it just something
you have to measure on the bench for each transformer? I'm
just wondering a bit about how much my various wallwarts waste when
the thing they go to are turned off.

thanks,
Mark

 

[Eric Sears]

Suppose I have a transformer such that the primary current is Ip when the
secondary current is Is, if I flip a switch on the secondary , the Is'
then is 0. I know that if you do this, the primary current does go
down but it doesn't go to 0 amps. In fact this is obviously so because
a transformer will remain warm even if the secondary is disconnected.

Is there a general equation that will estimate that or is it just something
you have to measure on the bench for each transformer? I'm
just wondering a bit about how much my various wallwarts waste when
the thing they go to are turned off.

thanks,
Mark

Just a couple of comments -
1. I suspect that you need to measure them. Most small transformers
seem to waste more (ie get hotter) as a percentage of their rated
power, than large ones in my opinion.

2. Not all "wall warts" are alike. Many today ( such as lightweight
cellphone chargers) are actually switched-mode supplies - not
transformers. They OUGHT to be less lossy - but feeling how warm they
get - I wouldn't bet on it!

Just my tuppence worth.

Eric Sears