# Transformer question.

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#### allegro

##### New Member
It is said that in ideal transformer, Pin = Vp*Ip = Pout = Vs*Is.
On the other hand, P = V*I*PF, where PF = cos(φ).
In ideal inductor, there's no series ohmic resistor connected to each coil, therefore φ = ½Π, and therefore P = V*I*cos(½Π) = 0.

How do you settle it?

#### Hayato

##### Member
Well, you have the answer already. The real power through an ideal inductor is 0.

#### allegro

##### New Member
Thanks.
So the mentioned power is not active power but apparent power?

One other thing please, I read that center tapped transformer has a poor utilization of the transformer windings, why is that?
It uses the whole secondary winding to form a full wave rectifier.

#### Hayato

##### Member
Nope.
The mentioned power is active, not apparent.

Active power is the power that "generates work", like heat:
P = V.I.cos(phi) [W]

Reactive power is the power that doesn't "generate work":
P = V.I.sin(phi) [VAr]

Apparent power is the apparent power:
P = V.I [VA]

What do you mean by "poor utilization of the transformer windings"?
The thing is when you use a centertap winding, you parallel them, to get the desired voltage at X amps, or use them in series to get the doubled voltage at X/2 amps. But I do not see any poor utilization there.

#### grim

##### New Member
Nope.

What do you mean by "poor utilization of the transformer windings"?
The thing is when you use a centertap winding, you parallel them, to get the desired voltage at X amps, or use them in series to get the doubled voltage at X/2 amps. But I do not see any poor utilization there.
you can't parallel up a centre tap winding

as for the centre tap question, it depends how you are doing it. if the two end of the winding are driving a full wave bridge rectifier, and the centre tap is just for a 0V reference, then yes you use all the winding. If however you just stick one diode on each end of the winding and the return path is via the centre tap then you are only using half the winding at any one time. makes the transformer bigger, but does reduce the volt drop in the rectifier circuit. its also useful when you are running at very high current and have a lot of diodes in parallel, as you don't need as many, and they can be quite expensive

#### Hayato

##### Member
Yes I misplaced the term.

But it is some sort of parallelism.
You have 2 common nodes:
The center tap and the rectfied node.

#### grim

##### New Member
yes, but you only use each half of the winding for half the cycle, so they are not sharing the load, as they would if running in parallel

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