# Choke to be wound calculation assistance request

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#### Joshua_G

I need a choke for tube preamp PSU. A common mode choke, 2 x 5H, 100mA.
There is local workshop which supplies transformers, custom made (E-I Core). They will supply any transformer I may need. Alas, the owner has no clue how to calculate a choke. He'd supple my one, should I mention the core size and number of turns.

Searching the internet for calculating a choke brought up nil practical results.

Hi tauro,
Thank you.

#### simon7000

My calculations based on the E-I cores I have used would be 2400 turns of 30 gauge wire for each winding. The amount of space required for that should determine the core size.

As this is a bifilar choke the DC current will have much less effect.

#### VaNarn

Paid Member
Hi Joshua,there is plenty of design information concerning the design of common mode chokes to be found on the web for switch mode power supplies,which I am sure you are aware of.Pertinent to your query is that they are based on a toroid style magnetic core for efficient common mode signal cancellation.This cancelling effect may not be as efficient on an EI core using say two windings side by side on the tongue section of the core stack.Actual practical winding formulae can be had from sources such as ''Radiotron Designers' Handbook"

#### Joshua_G

My calculations based on the E-I cores I have used would be 2400 turns of 30 gauge wire for each winding. The amount of space required for that should determine the core size.

As this is a bifilar choke the DC current will have much less effect.

Hi Simon,
Thank you very much.
1. How did you come at 2,200 turns? Do you have any formula for the relation between inductance in Henry per DC current in Ampere?
2. How did you come at 30 gauge? Do you have any formula for the relation between DC current in Ampere per wire gauge?
3. What do you mean by 'bifilar choke'? (The choke I have in mind will have two coils, placed side by side, on the same core).

#### AndrewT

re post6,
I have a few Babani titles.
Now I have an e doc that looks as good as the others.
Thanks.

#### Joshua_G

Hi Joshua,there is plenty of design information concerning the design of common mode chokes to be found on the web for switch mode power supplies,which I am sure you are aware of.Pertinent to your query is that they are based on a toroid style magnetic core for efficient common mode signal cancellation.This cancelling effect may not be as efficient on an EI core using say two windings side by side on the tongue section of the core stack.Actual practical winding formulae can be had from sources such as ''Radiotron Designers' Handbook"

Hi VaNarm,
Thank you very much.
As I wrote above, I searched the internet, found few websites with information on toroid chokes - none gave me practical results as for the instructions to give the choke winder.
Also, I have the Radiotron Designers' Handbook, it also doesn't have direct answer to my questions.

#### Joshua_G

Here's a text book from the 1960's / 1970's:
Coil Design & Construction Manual : B.B. Babani : Free Download & Streaming : Internet Archive
see chapter 5 Low Frequency Chokes

Hi,
Thank you very much.
That book probably answers my questions.
I looked at the relevant section, it contains a table of some chokes, with some inductance values per some current values with appropriate number of turns for each one, plus dividing ratios for either inductance or current divided by 10. I'll have to read it more carefully later on, to see if can translate that table to a choke of my specific values of inductance and current.

#### Joshua_G

re post6,
I have a few Babani titles.
Now I have an e doc that looks as good as the others.
Thanks.

Hi Andrew,
Thank you.
I don't get if what you wrote may have assistance to my query.

#### simon7000

L is proportional to N squared. So you pick a core that you think is large enough and measure the inductance for 100 turns.

In the mode you plan to use the choke one coil should cancel the DC effects of the other.

Wire gauge is just a table look up.

I used to calculate the actual properties as a student exercise. However most cores don't give you that information anymore. They just give you the simplified method.

#### Joshua_G

L is proportional to N squared. So you pick a core that you think is large enough and measure the inductance for 100 turns.

Hi Simon,
Thank you.
From the data I have so far, in a choke used for DC, like in a PSU filter, L is dependent also on the DC current.
Is that correct?
If yes, how does the current enters the L calculation?
Also, how do I measure the inductance while a certain amount of DC current is flowing through the choke?

Wire gauge is just a table look up.

Thanks, I know. My question was how do I know what gauge suits what current.

#### simon7000

If there is DC then there are two options, one is to let the inductance "swing" or shift according to the current. The other is to introduce an air gap in the core to increase the level of magnetic flux required to saturate the core.

With two coils on the choke there should be no net current into the core, so there should be no shift and an air gap would not be required.

As you don't have the magnetic data, you have to use rules of thumb or simplified methods.

The easiest way is to wind a test piece and measure it with the circuit you intend to use.

2400 turns should get you close as mentioned. Do you need an exact value or a minimum?

#### Joshua_G

Hi Simon,
Thank you.
Actually I need the maximum number of turns that will get me close (+/- about 10%) to 5H.

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