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I'd like to ask you all experts out there regarding to inductors. How to design an inductor? Actually, I'd like to build a circuit which includes an inductor of 18uH. So, how to design this 18uH inductor? Do i have to consider about the core of the inductor, wether to use air core, toroidal core, etc?

Maybe you could provide me the formula in designing the inductor. Please help :roll:
Please be more specific.

There is a lot of difference between an 18uH inductor for use at 20MHz RF and the same value for use at 220V 50Hz 10A.

What type of circuit is it going into?

What voltage/current levels?
Well, actually, i don't really know what voltage/current level for the inductor. However, the circuit that i want to build is a boost converter which produces 30V from a 5V supply.

The circuit can be found at **broken link removed**

Thanks for your fast reply pilot

from what i see is that it is a very low current coil, since the circuit will not provide more than 30-50 mA so you can use a standard one, already buit.
Amidon Associates, among others, make/sell the cores for all sorts of inductors. I purchased or was given a handbook that they publish. It looks like designing an inductor can be a pretty involved task but "cut and try" can work too. It would not surprise me if Amidon or other core manufacturers have info to help you easily construct what you need. I have some stuff at home - email me if you don't find what you need on line.
Bogdan, how do you estimate the output current? Is it possible to modify this circuit so that it can produce large current? Say 20A?

Stevez, what do you mean by 'cut and try'? It sounds like winding the coil and then measure the inductance and then cut the coil if the measured inductance is higher than the required value, but how to measure the inductance?

:?: :?:
if you want 30V@20A, that means 600W, so if you have 100% eficency, then you need 5V@120A minimum......with routh calculation...
but if you have a 80-90% efficency then you need 5A and 135-150A...
that will envolve you to have a HUGE suply for 5V and also it will be very hard to gind components and expensive too.....
i dont think that this is possible, think of what wires you were to use.....150A!!!! you will need 7-8mm wires in leasts
getting 30V@20A from another source like the mains will be much easyer, but not easy...
what do you need it anyway for? 20A? sure you didn't put an extra '0'?
Lumpin - sorry to oversimplify too much. The behavior of an inductor depends on a number of things, among them are current, frequency, core characteristics, etc. Inductor design is a fairly well understood science so with enough effort you can design and build exactly what you need. The level of effort is just not worth it in many applications. With some rough rules of thumb you can make your own inductor by winding wire on a core - with luck you'll get close to the value you need and if not then adding or subtracting turns will help you zero in - hence my "cut and try" comment.

Toroidal inductor cores are self sheilding and are fairly easy to work with. A toroid looks like a donut. The cores are made from a variety of materials, usually powdered iron or ferrite. The selection of material is determined by the application. I think Amidon has a recommendation for your application.

The cores come in different sizes with smaller cores being used where the wire size can be small. There are tables or formulas that indicate how many turns it takes to get a particular inductance and how many turns of a given size wire will fit on the core. With a little juggling you'll find a combination that makes sense.

Wire should be magnet or similar wire that is varnished or similarly insulated. If you think the supply might be 1 amp or so then size the wire for that. You have to start somewhere. The wire has to be big enough to handle the current passing thru it. A dab of glue, RTV or similar material can be used to hold the turns in place. Spreading or compressing the turns can be of some value in changing the actual inductance. Generally the turns are spread over about 270 degrees though that is not critical.

I hope this helps.
Haha :lol: I never thought about the wire bodgan. I wish i had :lol:

Actually, someone asked me to build a circuit which produces 30V from a 7.2V supply as a permanent magnet DC motor speed booster (RC car). In his idea, when the boost button is pressed, the circuit is activated and then delivers 30V to the motor. However, the boost will be only for a short period so that the motor will not get burnt. I wonder if this idea works but i think it worth a try.

About the output current, the 20A is just my assumption. As i said, the circuit only provides a short duration of speed boost. This means, the circuit is only supposed to be activated after the vehicle gained the momentum. From what i read so far, the current is very much needed for the starting pick-up (to overcome the weight of the vehicle), and then after it reached the momentum, less current is needed. Am i right? So, i made an assumption that this circuit only delivers high voltage (30V) with lower current, which i picked approximately as 20A. Please correct me if my assumption is wrong. If wrong, please give me a rough estimation of the output current.

About the circuit, if the 74HC14 doesn't support higher than 5V as the supply voltage, i plan to use a 5V regulator and put a large heatsink on it (which i'm not sure if it works or not).

For you experts out there, any ideas are welcomed sooooo much!

wopss....seems that i posted twice....i think it's my browser's fault :oops:

sorry for the inconvenience :wink:
I'd invest a little time into understanding just how much voltage the motor can take. There is certainly a point where something will let go, even with a momentary increase in voltage. Take an older motor that you can afford to ruin and put two, then three battery packs in series. Use alligator clips and wires that will not get hot and burn you. First do no-load tests then come up with something to load the motor down a little then a lot. Perform tests for the duration of time that you might otherwise boost the voltage. Once you know how far you can go you can do a better job on your supply.

I do not know how a DC motor behaves with respect to voltage but it's got to be at least something like a resistor. If the voltage doubles so does the current.
Stevez, where can i get the tables or formulas to get that certain inductance? I've read a few books but they only give this formula for inductance:

L = (N^2 x a x UoUr) / l


N = number of turns
a = cross sectional area
UoUr = permeability of the air and core
l = length of the coil

From what i see, this formula only shows how to get certain inductance, but, what about the current? For example, the circuit needs a coil of 18uH, however, bogdan estimated the current as 30 - 50 mA. So, how to build a coil of 18uH which carries a current of 50 mA? I have heard that there was a formula to design an inductor at a certain rated current but i haven't see it. If you all know about it, please let me know.

Daviddoria, what to you mean by that? Is it crazy to run a dc motor on 20A? :) Actually, the motor that he will be using is the one which claimed to be 'no limit'. I also wonder how did they rate that kind of motor. However, i guess it will be fun to test & ruin the motor :) [/img][/code]
Defining the inductance required for a Flyback or boost regulator( which is what you want to build) requires a little bit of maths.......
Let me explain:

Based on a 78S40 Switching regulator......

Firstly you need to know a few things.
a) output voltage.......Vo
b)output current.........Io
c)Supply volts.............Vi
d)Switching frequency...Fs

If you do a search on the device for the data sheet, all the calcs are shown for you. Much easier than my rabbiting on about them.........

HTH Chip
Amidon Associates and others make/sell the toroidal cores and they publish tables that are relatively easy to interpret - so maybe a little math but not mysterious stuff. Look up Amidon Associates and sooner or later you'll find what you need - or just email/call them directly.

Info I have is in manufacturer's (Amidon) handbooks or other electronics references.
Forgot to mention that what you are proposing to do is like wanting to apply 48 volts to a 12 volt light bulb. Yes you can over-volt bulbs to get more light but at some point they change from just being brighter bulbs with shorter life to behaving more like flashbulbs. Hit the motor with enough voltage and you might not get any revolutions out of it before it goes up in smoke.
I am not sure that 50 ma is the right current but I did want to more directly answer the question - and I have a moment to do that now. You said the current was 50 ma, the desired inductance was 18 microhenries and the application suggests relatively low frequency.

30 ga wire will handle 143 ma using a standard of 700 CM/Amp. 30 ga wire is pretty small but we'll start there.

#26 powdered iron mix is good for dc to 1 mHz.

A T-50 core is 1/2 inch inside diameter and just a place to start. A T-50-26 is a T-50 core made from #26 material. The "AL" value from the table for this core is 320. You can wind as many as 81 turns of 30 ga wire in a single layer on this core.

Crank thru the formula and it tells me that 24 turns of #30 wire will yeild a little more than 18 microhenries of inductance. Since the core can handle 81 turns this solution is rational.

I've just walked you thru the simplified process of inductor design. This is by no means all encompassing but is a good method for hobbyists.

50 ma doesn't fit with the magnitude of your project so realize that once you determine what current you really need you'll have an idea of how to design the inductor.
Hmm....Actually, i don't know if the current that the inductor suppose to carry is 50mA. That was an estimated value by Bogdan. However, now i really have some ideas on how to design that inductor and thanks to you stevez :D

From what i get here,things that i should look for are the tables and formulas to get a certain inductance at a rated current and voltage for toroid based cores. Actually, that was what i'm looking for so far but i haven't get them. If you techies out there know where to get them, please inform me here or email me at

However, what about the other cores? for example air-core? What's the difference in using toroidal core and the other types of cores? Another example, what's the difference between an 18uH air-cored inductor and 18uH toroid-cored inductor at a same rated voltage and current?
An air core inductor with a value of 18 microhenries would be fairly large - maybe 2 inches in diameter and 4 inches long. It would not be self shielding so the surrouding magnetic field might impact or be impacted by other things nearby.

References that I have are ARRL Radio Amateur's Handbook and Amidon Associates handbooks. I have no idea how much of this stuff is available on-line.
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