Inductance 1

[Vladpl]

Hello All

I have made a choke/inductor which has 400 turns on one side and 2 x 200 turns on the other side of the divided bobbin.

My problem is that if I series the two 200 turn windings theoretically I should be getting the same inductance as the other 400 turns. But the thing is I am only getting 1.38H on the seriesed windings and the 400 turns are reading 1.4H. Can someone explain why this occurs? Thank you all in advance.

 

[itsmoked]

That isn't THAT big a difference. You can have differences with small variations in your windings. Like how tight your windings are. If there is a space between the two half windings this can cause a slightly different coupling to the core.

 

[mpparent]

That's a 1.4% difference. Seems to me that's in the margin of error and a pretty good result. Don't get too caught up in the theoretical.

Mike

 

[fsmyth]

1.4 Henries with 400T? What are you using for core?
Sounds like it's gonna saturate if you just get it
CLOSE to a source.
<als>

 

[Vladpl]

Hello All

Thank you for your replies.

I know that there isn't much difference in the inductance. But I have been asked the question why when you series the two 200 turn windings the answer is not 1.4H and is instead 1.38H. Measuring with a very accurate bridge 400 turns makes it 1.4H.

I am using EI lamminations and will not saturate.

 

[itsmoked]

Have you recounted all the windings?

 

[giuseppe]

Check the lenght of the coil wires and you'll find the answer. Two bobins should be shorter than one.

Regards.

Giuseppe

 

[rbulsara]

The world is not perfect, never will be.

The answer is they are not identical, and you are the only one looking at it and knows the details..find the difference. Would a splice make a difference? May be.

Make a few number of so called identical products and measure them all and compare the results, before you loose all of your hair scrathing your head.

 

[IRstuff]

Did you measure the individual 200T coils?

TTFN

 

[logbook]

Is the magnetic path identical on both sides of the bobbin? If not then this could contribute to the difference.

1.4/1.38=1.014, a 1.4% difference.

If there is a one less turn on the 200+200 winding this gives an inductance error of (400/399)^2= 1.005, 0.5% error. Does the winding start and end at the same side of the core? You can get non-integer turns on pot-cores and similar magnetic topologies depending on how the wires exit the core.

 

[RudyL54]

There is always that margin of error.

There is that theoretical result and the actual result. In the first place inductance varies on several factors and there is no such thing as perfect material and process.

 

[Vladpl]

Hello All

Thank you all for your inputs. I have worked out why there seems to be the phenomenon in this coil. Since I am measuring it on the bridge the only thing left is that it is due to the capacitive coupling in the windings.Basicaly it works similar as the mutual inductance but in reverse since current is required to create emf and therefore mutual inductance occurs.

Regards Vlad

 

[Warpspeed]

The answer to this is reduced mutual coupling between the two 200 turn coils.

Think about this. If you had one 200 turn coil, and another 200 turn coil that are not magnetically coupled, say they are separated by 300mm with individual ferrite cores for example. The inductance of both will be simply one added to the other.

But if the two coils are intimately coupled on the same core, in other words, act like one 400 turn coil, the inductance will be turns squared, or four times the inductance of one 200 turn coil.

In the first case you get double the inductance of each 200 turn coil.

In the second case you get four times the inductance of each 200 turn coil due to magnetic coupling.

By winding it on a divided bobbin you have effectively isolated the two individual halves by some small amount. Not all the flux lines link both coils, some will escape between the two. And this will lower the total inductance very slightly below the turns squared figure.

If both coils absolutely must be identical, wind them both on split bobbins and make them physically identical, it may need a very few extra turns to get back up to 1.4H but it will work.