Help with Transformer Theory

[Pyskick]

I’m a beginner at electrical theory and struggling to get my head around one certain aspect of transformer theory. I’m hoping someone can explain this to me….

My understanding of transformers is that if, for example, 10 volts ac carrying 2 amps of current is applied to the primary of a 1:2 step up transformer then the output on the secondary would be 20 volts with a maximum current of 1 amp. However, and this is where I get confused, Ohms law says that the value of current depends on the resistance of the load. Hence if a load of 10 Ohms is attached to the output of the secondary then by ohms law the current provided by the secondary would be 2 amps (20 volts divided by 10 ohms).

Please can someone explain where I’m going wrong! TIA

 

[AnalogKid]

What you are missing is that the transformer core is, both in theory and in practice, an energy limiter. When the 10 V is transformed up to 20 V, the available max current is transformed down to 1 A. When calculating the current through a resistor, Ohm's Law assumes a theoretically perfect voltage source, one with a zero ohm output impedance, a perfectly stable voltage, and infinite current capability.

Under some, or even most conditions, a transformer output may appear as a constant voltage source, but it isn't. It is a constant power source. In your example, a 10 ohm resistor connected to the secondary will cause the secondary voltage to sag down to 10 V. If the transformer is oversized, and the AC voltage source is current-limited to 2 A, then that source's output voltage will sag down along a curve that is the 20 W boundary.

If the energy source for the primary is limited to 20 W, or the core can handle only 20 W before saturating, the output will be limited to 20 W.
Disclaimer: that sentence is a serious simplification of transformer mathematics. For a glimpse, see this:

Transformer - Wikipedia

en.wikipedia.org

Poynting vector - Wikipedia

en.wikipedia.org

ak

 

[bertus]

Hello,

A ideal transformer will have the same power on the output as on the input.
You can never have more power out as in.
Here is a page on transformers on AAC:

Step-up and Step-down Transformers | Transformers | Electronics Textbook

Read about Step-up and Step-down Transformers (Transformers) in our free Electronics Textbook

www.allaboutcircuits.com

Bertus

 

[Minder]

Another way of expressing the units is the Va (Kva) or power rating of the transformer.
The value applies to both the input and output and depends on a few factors such as core size and winding diameters etc.

 

[Bluejets]

............also turns ratio often used.

 

[crutschow]

Hence if a load of 10 Ohms is attached to the output of the secondary then by ohms law the current provided by the secondary would be 2 amps (20 volts divided by 10 ohms).

And the ideal transformer input current would then go up to 4A since power in equals power out (ideally).

 

[RichardGonzalez]

What you are missing is that the transformer core is, both in theory and in practice, an energy limiter. When the 10 V is transformed up to 20 V, the available max current is transformed down to 1 A. When calculating the current through a resistor, Ohm's Law assumes a theoretically perfect voltage source, one with a zero ohm output impedance, a perfectly stable voltage, and infinite current capability.

Under some, or even most conditions, a transformer output may appear as a constant voltage source, but it isn't. It is a constant power source. In your example, a 10 ohm resistor connected to the secondary will cause the secondary voltage to sag down to 10 V. If the transformer is oversized, and the AC voltage source is current-limited to 2 A, then that source's output voltage will sag down along a curve that is the 20 W boundary.

If the energy source for the primary is limited to 20 W, or the core can handle only 20 W before saturating, the output will be limited to 20 W.
Disclaimer: that sentence is a serious simplification of transformer mathematics. For a glimpse, see this:

Transformer - Wikipedia

en.wikipedia.org

Poynting vector - Wikipedia

en.wikipedia.org

ak

Thanks for those links, you made my day.