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Old 5th February 2004, 01:07 AM   #1
Spudnik is offline Spudnik  Canada
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Default the "core" of the problem

Hello! my first post here, im not really a diy'er or a audiophile, but i need power supply help. i figured this was the forum...

(actually a very good friend of mine lurks here, an audioNUT, hes scared to post, hehehe what a guy...)

my power supply was almost entirely his idea, it came after we had just finished a little course in elementary electronic devices. unfortunalely this course never taught me what i need to know, and niether can my buddy, so i pose it to you:

basically it is an inductive power supply, i shall demonstrate: (I) for inductor, (C) for cap, (r) rectifier, etc, etc.. please ignore the dashes, the absolute bar means the caps are joined. (ill attach a ewb circuit jpg when i get a shot)

_____(I)_____________( +35 vdc )
|------------------(C)
(r)-----------------|-----------( 0 vdc )
|_____(I)____(C)______( -35 vdc )


my question is: my inductors are home made. i made the cores, but the cores are solid toolsteel, not sliced laminated steel; and cylindrical ( 0.770 '' dia x 7 '' length ), as opposed to square. also my caps are 10000 uf used old P.O.S caps.

im using 22 gauge wire, solid core (tuna! ... disregard that... bad joke...), and im looking to make these inductors as large as possible (like 110, 120 mH ( 0.11, 0.12 H or similar ).

can anyone tell me whether the cores i have made will be good enough to do this? can i pump out that much Henry's or will i end up with significantly less Henry's because of inrush current limiting? i know i cant handle much current due to heat loss, but what can the effect of solid inductors (as mine are) be on the induction effect of my circuit?

what are the advantages to going with the spliced laminated core?


search for non-laminated, inductors, solid yealds no matches (all at same time-- inductors gets me many hits but no way i can filter them.)
-cheers all.
Spud
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Old 9th February 2004, 11:00 AM   #2
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Default Tool steel?

You could use powdered Iron cores from Micrometals. They got cores up to 6" diameter and all the app notes to design them. They're cheap too. Also, go to Toroid.com and click on their inductor link. I'm not sure but I think getting inductance close to 0.1 H would require loads of turns on a big core, therefore you would have to use smaller wire to get it to fit. Long skinny wires aren't good for high currents ie. 5 amps you would need to power an amp.
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Old 9th February 2004, 07:54 PM   #3
sreten is offline sreten  United Kingdom
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A pi filter is a far better idea, C parallel, L series, C parrallel.

You don't provide any details of load characteristics,
which are critical for a series inductor power supply,
to allow dimensioning of the inductor.

You also haven't searched on this subject, it seems.

sreten.
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Old 9th February 2004, 09:03 PM   #4
maylar is offline maylar  United States
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You need to know the relative permeability of the core material. Magnet iron is roughly "200". Dunno about tool steel...

Formulas can be found here:

http://sbvc.gran.tec.wv.us/4_inductance.html
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Old 12th February 2004, 04:59 AM   #5
Spudnik is offline Spudnik  Canada
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actually i dont have anything to power, this is a "first" attempt at a working supply. i would power something if/when i decide to build something i can use. i have very little knowledge of electronics at this point, so its safe to say i dont know what im doing, let alone what i want to do.

did i mention im using insulated wire? im not sure how much difference it makes but it makes a pretty big difference in the number of windings i need.

if i apply 150 turns on these babies i get:

0.11 = (ur * 12.56 *10^(-7) * 150^2 * 12.017) / (.1778)

or ur = 174.43

of course thats the relative permeability, but i still dont understand what the difference makes if i use a powdered/laminated/sliced core instead?

the permeability would stay the same.

what am i not getting?
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Old 18th February 2004, 01:06 AM   #6
Spudnik is offline Spudnik  Canada
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i know you dont like me to do this but i really need to know what to do and whats going on.

i redid the calculations. turns out the measurements were wrong. it was 5 inches long by .71 inches diameter.

5"=.127 m
.71" d -> = .0566 m square

applying 200 winds to the core... and wanting .11 h ...

by the formula in the above link, my relative permeability is 4.912

which sounds wierd to me. i dunno why. i dunno what im doing.
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Old 18th February 2004, 04:47 AM   #7
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Spudnik,

Firstly, welcome to the forum.
As for your question, I know we all like to use whatís handy, but I think that insulated wire of yours poses a serious bottleneck to conventional formulas.

Here's why: As you know, current flowing thru a conductor causes a magnetic field to form around it. In the case of a loop of wire, the field is shaped like a donut, with all of magnetic lines of force coming out of one side of the hole and wrapping around the donut and being sucked back in thru the other side. Now as we bring two coils together, the magnetic fields begin to interact: the magnetic field travelling down the outside of one donut meets the field travelling up from the hole of the other donut. These fields cancel-out, leaving a continuous, composite field around the whole structure. The formulas for how to calculate this composite field are well known, and thus as an offshoot, I suppose, the inductance of the composite coil.

So what does happen, when there is a gap? You get leakage flux I think, which should decrease the efficiency and maybe the inductance of the inductor. Because youíre also not getting as many windings per unit area, youíre also going to need a longer overall wire to get the same # of windings, especially if youíre going to layer the windings. This increases the series resistance of the inductor. This resistance is also a source of energy loss, which will be dissipated as heat.

Another characteristic of inductors is the winding capacitance. With the larger spaces between windings, this capacitance decreases, which Iím not sure is bad or good. It seems good in this case, but anyone have any solid answers?

As for the core material, powdered/laminated cores, in and of themselves, should not do anything to increase or decrease the inductance. This seems to be dependant only on the permeability of the material. What laminations do, is reduce eddy current losses by increasing the resistance of the core material to the eddy currents induced in them. This decreases the current, and thus overall, the amount of energy lost in the inductor to heat.

And as for the permeability of the core youíre using, Iím not sure what to tell you. I figure the carbon would decrease the permeability, but tool steel also seems to contain silicon, which increases the permeability. Iron, for reference has a permeability of around 200. Tried Googling for a while, got tired. You give it a go.

Now for the formulas. Err, you seem to be working backwards. Iím not sure where you got an inductance of 0.11H, but this should be your dependant variable. Seeing as you already have a core, youíd vary the number of turns to get your desired inductance. You could only calculate the permeability if you knew what the actual inductance was.

Iím not sure what you want to use your inductor for, but I donít see too much point in making your own 0.11H iron core inductor. You can buy even fairly high current ones pretty cheaply. Itís when the inductances and currents get high that the costs go up. Most people here make their own air-core inductors (>1H)because these donít saturate, and thus can handle more current. Great for those high power amps we all like to make.

As a final note, Iíd watch the power dissipation in your coil at high currents. I donít know how much current youíre planning on running thru this thing, but I figure itís going to be a lot, considering the size. Heat should be the only source of catastrophic problems in your scheme, if the inductor turns out to be too inefficient and the insulation on your wire begins to melt.

Hope this helps.
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Old 18th February 2004, 09:20 PM   #8
Spudnik is offline Spudnik  Canada
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ok then. i get ya. heat = bad.

so if i slap some giant heatsinks on the ends of the cores, would that do well to dissipate the heat? or is the heat in the wires?
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Old 21st February 2004, 12:27 AM   #9
Spudnik is offline Spudnik  Canada
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hi again, i said i would enclose an image of the circuit, so here it is:
http://spudnik.dragonsight.com/circuit.bmp

thank you for helping!
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Old 21st February 2004, 06:33 PM   #10
Harlan is offline Harlan  United States
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So far everything is really good reading here and helpful on this one. I want to interject one small detail please. Spudnik you started with "the "core" of the problem " and having worked with transformer design in the past, I think the one question you asked has not really been answered yet. The core "Especially tool steel" not being laminations is a big heating problem. The core material being solid is going to become a heater for you, Search induction heating properties, and the heat will destroy the magentic properties in the core and thus no output and a lot current draw and then you will have to capture the smoke to put back into the circuit!. The suggestions of Ferrite cores and related materials is necessary one here. The fight you have is the old eddy current problem with a solid steel core. Both Ulveco and Toriod Corp are great at assisting with new designs and you can get a toriod core for peanuts really to design with. I hope I am not too discouraging here, but I admit we had a lot of fun with the experiments using almost solid materials always having that fire extinguiser handy! Remeber also that tool steel likes to LOCK it's domains into a single direct, and then needs a lot of power to switch them around. For a core maybe you can fine an old power transformer that you can butcher for the laminations here and then wind to you hearts content using the simple turns per volt for the iron.
Good luck on this and keep us posted too.
Harlan
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