A Simple Transformer Question

[desoky725]

I have a transformer 220 to 12 0 12 , and 150 mA as maximum output rating.
So I need to know what is the input current it will draw from the source If I need an output of just 10 mA,
and if I'm not connecting any thing to the output of the transformer (open circuit), what is the current it will draw.

 

[Pjdd]

It will draw a magnetising current even when there's no load at the secondary. The amount of current depends on the design and construction of the transformer and cannot be predicted without detailed data about several factors. The easiest way to find out in practice is to measure it.

 

[barry]

Assuming an IDEAL transformer (no losses in the core, windings, etc.) remember that power must be conserved (Vin * Iin==Vout*Iout). So, for 10ma, since your turns ratio is 220/24=9.2, your input current will be about 10/9.2=1.1mA

 

[goldsmith]

Dear Barry
HI
I hear that that formula is in ideal cases and we can not is that in actual causes . what do you thing about that? but i think that we can say thing that you said (approximately) . what is your idea about this?
Thanks
Goldsmith

 

[barry]

You have to look at a particular transformer to know what its characteristics are. If you need to know a precise answer, you should pick a transformer and breadboard your circuit. Assuming an ideal transformer, we calculated an input current of about 1 milliamp. A REAL transformer will probably be different, but not THAT different; it's not going to draw 5 amps!

Are you just trying to pick a transformer for the job, or determine what you need to drive an existing transformer? How precisely do you need to know what the input current will be?

 

[goldsmith]

Dear Barry
Again Hi
I agree with things that you said. but we can measure the inductance of primary and then I primary= Input voltage/Input Inductance . i think that this will be with precision . but thing that you said , is true ( with 20 or 30 percent tolerance).
Best Regards
Goldsmith

 

[barry]

Iprimary is NOT Input voltage/Input Inductance; it is a function of the secondary current. See post #3

Barry