How to design a Coil Step By Step

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I will here develop this tutorial for you learn how to use simple math to design a coil and predict its behavior under pulsed conditions.

First of all when working with a transformer core we than should try use the maximum of its ability.

the  window is the first thing to think about.. the number of turns you can get into it will depend on the diameter of wire, the required isolation and coil former dimensions..

how to simplify and always get it work?

First calculate how much turns you can do for the core... for that we are goingto divide the core window area by 2 assuming we are able to use half of it with copper and the other half with space and insulation,... than divide this number by the diameter of the wire used squared since we are dealing with area... the result is the maximum number of turns you can do with this wire you have...

the  isolation depends on the material you use but the bests are nomex... or poliimide films...

this films are required to isolate the coils layers at worst case condition...

here we find that the coil will occupy a maximum of space in the core...

the power a transformer can delivered is limited to the amount of copper you can wind on it...

the maximum voltage this transformer will be able to get out will depend on the primary turns and the power suppy voltage youhave

so for example we found that we can put 500turns plus the primary few turns in the core... how many  turns can we use?

here you must calculate according to the core crossectional area, voltage, frequency and the proprieties of the core

number of turns of the primary for a pulsing dc voltage is N = to V/Bmax/Area/freq/2

above this voltage or bellow this freq the core will saturate

so the maximum pulse on voltage for your maximized transformer is the ratio between this turns of the coil and the primary and the input voltage

during pulse off the colapse voltage will depend on the resistance across the primary (for a simpler control of it) and the current it was charged to, and consequently the secondary coil will receive this voltage in proportion to the turn relation again.. so its a multiplification effect...flyback

If the secondary has a blocking diode like a flyback the pulse on is blocked so no current flow and thereto the collapse will be maximum since the secondary coil didnt developed any contrary magnetic field that would cancel the energy accumulated into the core by the primary

the collapse voltage depends on your switching level of protection...and of course the resistance across the primary... the voltage that will apear across the primary during pulse off  it V=I*R this voltage is many times able to burn your switch...

to prevent  that the switch must be protected.. TVS and MOV techonlogy is available

to use them you must watch the amp rating and voltage rating i use them in series and parallel to increase the power proprieties

i get 1000v limit

remember you may be measuring the rms current so the peak may be more than twice your reading... watch this that can confuse you... the better way is to measure it on oscilloscope to get peak reading... if possible...

http://www.ieee.li/pdf/essay/practical_magnetic_design.pdf


(http://xtronic.org/wp-content/uploads/2010/05/copper-awg-metric-table.jpg)

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more info to come

https://www.eeh.ee.ethz.ch/uploads/tx_ethpublications/dalessandro_self-capacitance_HV_transformers.pdf

http://www.ti.com/lit/ml/slup125/slup125.pdf

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Power

To design a transformer is important to think about how much voltage drop there will be on it such that the load receive the proper power. 

how do we do that?

well it has to do with choosing the wire gauge...

Normally a power dissipation of 5% can be allowed on the secondary and 5% on the primary so geting from worst case situation is possible to determine how much resistance can be allowed on the coils and so according to window size and crossection area of the core the number of turns must be calculated first... than a estimation of the lenght of wire needed for that turns.. than you go for a table and find the wire that fits that resistance for the estimated lenght of wire required for the calculated turns... than check if the wire will actually fit the transformer window... and your done..

the thing is the voltage drops must be calculated such that the primary has the right number of turns to provide the right power.

for example we need a 9v300ma output under 30ohm load

how much will be the open circuit voltage of the transformer?

well it depends...

but if you take as an example a reversed engineering case... my transformer from the probe

it has 3 ohms on the secondary and so a current of 300ma will create a voltage drop of 0,9v under maximum  load... so the power dissipated in the secondary is 0,27w and the load 2,7w

this case 10% is allowed under load at the secondary... and probably other 10% at the primary...

this is a restricted current output transformer example... its made in this way to get such a voltage drop under worst case condition...

so we would need to design the secondary to have 9,9v at first glance to offer 9v 300ma to the 30ma load...

but there is something missing... the primary voltage drop!

we must account for the voltage drop in the primary at loaded condition to know the right number of turns that will compensate for the resistance of the wire...

is easy

you can consider basically the primary under load... assuming 3,3w at 230v it would result 14ma to get 14ma to dissipate 0,27 w is required a resistance of 1377ohms

the secondary turns than must account for the voltage drop of the primary before the value can be used.. so in our case... vdrop is 19,6v so the resulting voltage useable for induction was 230-19,6=210,4v  so

remembering the secondary must receive 9,9 v to give 9 under the rated load, you need to use 1:  210,4/9,9 turn ratio... 21,25

Now we only need to find the right core for this watts..

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tables

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wire types

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There are some different types of isolation used for transformers...

there are at least 5 things to be considered, volume, dielectric strenght, max temperature, dielectric constant, compatibility with oil, price, specific resistance... availability, time....

The one that is most important is the availability, if you have few to choose, get the higher streghnt that fits your pocket

theres

nomex

poliproplilene

mylar

paper

oil

encapsulation

The air problem is only solved by encapsulating while outgassing or with oil but making the vaccumm to take the ar out of the coils (outgassing)... those are the best way to go... oil willgive from 10kv/mmm to 40kv/mm or higher isolation depending on the oil used... kitchen canola oil proved to be very helpful in the case you cant get proper mineral or even beter silicon oil..  oil conducts heat well and help maintain the coil stability.. it also has a greater dielectric constant than air.

Encapsulating resin can give higher voltage ratings but may be poorer in heat dissipation.. but is more pratic to use... the temperature of operation may be below 180 degree..

Polyester would melt at 130 also so is little dangerous...

the best options are nomex or paper and oil or combinations of paper plastic layers sold already for rolling motors and transformers

if the wire isolation survive 180 degree the isolation between layers must do the same

there are diferences in the comom resin and the resin for encapsulating also...

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Why impregnation is needed in highvoltage?

Oil is the most used in the high voltage lines as it provides good heat dissipation and stability... it has from 15 to 70kv/mm strenght,

to Use oil is not simply to fill the transformer cavity with it...

oil must get into the coils to work because the all point is to get air out of the coil spaces...

air at high voltage ionizes and create a situation where the coils burn the insulators and than arcs

the air has a low strenght of 3kv/mm so as you reach this electric fields the air develop corona discharges around the coil destroying it

to get the oil to fill a process is made with vaccuum where the coil is dip into oil inside a vaccum chamber such that the air is forced to go out and that the oil is forced to go in when atmosferic pressure rises again...

the ideal is to apply vaccum first than dip the coil than bring positive pressure...

the same must be done when doing resine encapsulament

theres some different types of oils and resin for encapsulation

there is oils that are called parafinic that cannot be used bellow 0degress celcius

the ideal is to go for a liquid with good proprieties of isolation and low viscosity

i tried canola oil... and worked very nice... for reducing the viscosity i heated in mary bath ... during vaccum... the process last many hours (12) if you use paper as insulator between the coils because paper holds increadible amounts of air...is increadible how much air come out of it... i did a spíral generator with copper foil and kraft and coils with 30 layers using kitchen cooking paper... you may need to add enought layers of paper to get the thickness you want.. better have a micrometer on hands..

the funiest part is cutting the isolators

i found that it can be used but is better to use nomex or some other high temperature isolator... or even polyester sandwiched with paper..

oil expands so dont encapsulate it without letting a breath for pressure to not rise inside the coil and it not explode..


 oil can cause severe fire! be carefull specialy with high voltage...


there are epoxy resins specially developed for transformers where they prevent fire and conduct somewhat better the heat

i believe the lowest viscosity will be the ideal for filling...

the coils must have spaces for the air to come out and oil to get in to be able to do it faster and better. .

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http://www.daycounter.com/LabBook/Mutual-Inductance.phtml