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Mark
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 Posted: Mon Dec 10, 2007 8:06 am Post subject: is there an equation for primary transformer current when se |
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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 |
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Eric Sears
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| Posted: Sat Dec 15, 2007 3:50 am Post subject: Re: is there an equation for primary transformer current whe |
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On Mon, 10 Dec 2007 02:06:54 GMT, Mark <---@--.com> wrote:
[quote]
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
[/quote]
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 |
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