# Question on Transformers...

#### Marc0

Joined Nov 28, 2011
42
Hi,
I'm new to the forum (actually new to electronics)...
and I don't speak english well... so
Here's my question:
When a load is applied to the secondary of a transformer, what's the way to calculate how many amperes the primary is draining from my house electic outlet?
p.s. I know about the non-idealities of real life transformers, I just need something simple to get a rough idea.

#### crutschow

Joined Mar 14, 2008
30,128
The input current is the output current times the voltage ratio between the output voltage and the input voltage. Thus if the transformer output current is 2A @ 24VAC and the input voltage is 240VAC then the input current is 2A x 24/240 = 0.2A (assuming 100% transformer efficiency).

#### Marc0

Joined Nov 28, 2011
42

#### JMac3108

Joined Aug 16, 2010
348
Marco,

Some simple transformer formulas you should get familiar with. Hope this helps.

Vp/Vs = Np/Ns

Ip/Is = Ns/Np

Vp = primary voltage
Vs = secondary voltage
Np = primary turns
Ns = secondary turns
If you do the simple algebra to combine the two equations and eliminate the Np and Ns, you get ...

Vp/Vs = Is/Ip

Rearranging the terms gets you to ...

Vp x Ip = Vs x Is

Recognizing that Power = voltage times current we have ...

Pp = Ps

This tells us that the power in the primary equals the power in the secondary. Conservation of energy holds and life is good I went to the trouble to show this because beginners in electronics often as the question "why can't I just keep increasing the turns ratio of my transformer to get higher and higher voltage and thus unlimited power?". The equations show that when you increase (or step up) the voltage, you decrease the current capability.

#### Marc0

Joined Nov 28, 2011
42

#### studiot

Joined Nov 9, 2007
4,998
You are learning well Marc! - Keep it up When a load is applied to the secondary of a transformer, what's the way to calculate how many amperes the primary is draining from my house electic outlet?
I don't know about the regulations in your country, but in the UK every domestic and most industrial mains electric devices have to carry a 'rating plate' which clearly states

Operating Voltage
and either Current or Power
from which the third quantity may be deduced.

So use those formulae JMac has provided!