# What is the difference between Transformer step up and down field line difference?

If you've got more windings on the primary (step up) then I imagine that there'll be more magnetic field lines flowing through the core, than if you had less windings on the primary (step down), to apply voltage to. Is this correct? If so, what is it that makes a step down as powerful as a step up, are the less feild lines 'stronger' or something in a step down?

Thanks very much.

Few things:
1. More winding in primary less in secondary is Step-Down transformer
2. If you apply same voltage to two different coils, A with more no. of turns and B with Less no. of turns then, the total magnetic flux created by the coils in the Core will be more by B than A. Yea, more the turns less the flux.

If you understand the Reason for no. 2 then probably you will have answer to your queries.

berkeman
Mentor

more the turns less the flux.

Why is that?

because we have E = N d(phi)/dt. If you apply equal voltage to two different coils, then the one with more turns can generate enough induced voltage to oppose the applied voltage by generating lesser amount of flux (phi).
If you pass equal currents, then ofcourse, the one with more turns will generate more flux.

Flux is directly proportional to Number of turns x Current flowing through them.

Applying same voltage to lesser number of turns will cause more current and more flux.

Few things:
1. More winding in primary less in secondary is Step-Down transformer

God, how embarassing, it was really late and I wasnt thinking.

So because the more turns you wind, the less current you get hence less flux? (nothing to do with lenz's law)

What property is it that allows for more windings on primary (step down) to create more current in the primary that is obviously beneficial, if there is less flux?

thanks very much

FOIWATER
Gold Member

Why is that?

Because the higher the turns, the more voltage is induced.

Hence less current, and it's the current which produces the flux.

So because the more turns you wind, the less current you get hence less flux? (nothing to do with lenz's law)
Ahh! not quite. Consider this.
Suppose there are two coils A and B. A has say 100 turns and is wound on high grade Iron core. Suppose B has 1000 turns but don't have any core (air core). We now apply a sinusoidal voltage of maginitude 100V to both coils.
Now for A.
E = N d(phi)/dt
100sin(wt) = 100 d(phi)/dt
solving for phi, phi = - Cos(wt). That is phi has magnitude of 1 for A.
Similarly for B
we have, phi = -0.1Cos(wt). That is phi has magnitude of 0.1 for B

So, upto now we are consistent that, the flux is less in B (more turns) than A.

Now for the current.
Current in A = Current required to setup flux of 1 in Iron
Current in B = Current required to setup flux of 0.1 in Air

Beasue magnetic permeability of Iron (u) is around 2000 times of Air, current in A will be 200times smaller than in B.

So, the lesson is, what current flows is subjective to on what you make winding. But, the flux, will be independent of on what you wind. So, Don't go for current, just remember fundamental faradays law
E = N d(phi)/dt