# How to chose choke in catalogue?

#### MacIntoshCZ

##### Active Member
Hi,
when i am designing inductor i am focusing on maximum voltage, on time (for magnetisation current) and peak current.
First of all i chose required inductance and peak current then i calculate ON-time t=L*I/Ub -> Ub is always 360V.
Then i chose some toroid core -> Bsat, Ae , le...
and calculate minimum turns for Bsat/2 (bsat/2 to stay in linear region) -> N= Ub*t(0,5*Bsat*Ae) ->
Then i calculate relative permeability for required inductance , i need to chose core with lower or equal permeabilty with same dimensions and Bsat...
When core with lower ue is selected i add turns...

But when i look at already fabricated chokes i can see only specs like inductance, Imax and resistance... No info about max voltage and on-time which was core designed for...

#### MacIntoshCZ

##### Active Member
I just figure it out...

#### Buk

##### Active Member
First of all i chose required inductance
How do you determine that?

#### MacIntoshCZ

##### Active Member
By datasheets calculations or myself... It depends

#### Buk

##### Active Member
Okay.

I seem to be missing something that eveyone else gets.

The formula for calculating inductance is allover the web: L=μN2A/l
Where
L = inductance in Henry (H)
μ = permeability (Wb/A.m)
N = number of turns in the coil
A = area encircled by the coil
l = length of the coil(m)

But, when choosing a core, the first thing you need to know is the "required inductance". How do I decide what inductance is required?

It's chicken and egg.

#### MacIntoshCZ

##### Active Member
L=μ*N*N*A/l
In this case - page 28

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