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Old 7th October 2002, 02:29 PM   #1
BrianGT is offline BrianGT  United States
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Default Power supply inductors (what specs?)

I was down at Victoria Magnetics picking up a couple of transformers, and talked to the owner, John about possibly making power supply inductors. He has all of the wire from making the transformers, and said that he could make inductors if I was to give him a design. He even suggested that they make 12ga wire in a square form, and would help eliminate the air gaps in the windings. Does anyone have any clue about the best design for power supply inductors? What is a good tool to figure out how much inductance to put in a power supply? How many watts?

All that I have seen about inductors in Super BQ's soz:
http://www.geocities.com/super_bq/soz3.html

If we can come up with a design, this could be a good source for power supply inductors.

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Brian
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Old 7th October 2002, 03:09 PM   #2
eLarson is offline eLarson  United States
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The only rule of thumb I've seen for specifying the size of an inductor in a power supply came from the 1970 edition of the Radio Amateur's Handbook That formula was given to ensure that the inductance was sufficient to cause the filter to act as a proper choke-input supply. They didn't say how they derived it and I've been too lazy to figure it where it might have come from.

Nonetheless, here it is again:

L (given in henries) = (desired output voltage) / (current draw in milliamps)

So for a 32 volt ouput and a 2 amp current draw you would need an inductance of ...

32 / 2000 = 16 mH

It would have to be able to cope with a constant 2A draw. Current squared times its internal resistance can get you a first pass at the power handling needs.

If you aren't looking for a choke-input supply, then you can pick pretty much any value you like. I try to ensure that the combination of the inductance, capacitance and internal resistance of the choke (+ any actual resistors) is it at least critically damped.

Erik
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Old 7th October 2002, 04:19 PM   #3
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16 milliHenries might end up with more resistance than
you want. I find that in practice, anything at all helps,
but 2 mH is a pretty effective value.
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Old 7th October 2002, 04:24 PM   #4
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Question Inductor use?

Are you designing the inductor for use as the primary filtering component or as part of a larger filter network (i.e. PI filter)? Mr. Pass used a PI filter in Zen-V2 I believe after complaints of a high noise floor with the original Zen amplifier. He prescribed a value of 2-mH which maybe excessively high if calculating inductance based on ripple voltage after the first filter capacitor.

My only suggestions would be a value that would work in most projects that we see on the board--Zen's, Aleph, etc.--and that they have as low of a DCR as possible.

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Old 7th October 2002, 04:24 PM   #5
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Hi Brian,

You want a choke that can handle the maximum current/rail (peak) and the continuous current/rail. The wattage it will have to handle is determined by the DC resistance and the continuous current (I*I*R). For an Aleph2 you should, IMO, use at least 12 gauge windings, but the trick is to get enough inductance with as low a DC resistance as possible because its wasted watts. The voltage drop across the choke will fluctuate causing the rail voltage to fluctuate with variations in current draw.

How much is enough? I think thereís a lot of leeway in the amount of inductance need to suppress the ripple noise. I believe Mr. Pass suggests a 2.2 iron core. You can check some of these threads.

Problem with PI PSU
Aleph 5 PSU

Stick with the PI PSU. The Choke Input PSU is a whole different animal.

Thereís also a link to the Duncan PSU Designer II that can be helpful to quickly see proposed variations.

Hope this helps,
Rodd Yamas***a

PS Mr. Pass chimed in before I posted, but I had to run so I didn't go back through my post.
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Old 7th October 2002, 05:54 PM   #6
BrianGT is offline BrianGT  United States
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I am looking for making inductors for a PI filter for the Aleph 2. It would be in CLC formation. Will 2mH be enough? I am sure that by using heavy gauge wire, the resistance can be kept low. Should it be an air core inductor, or a iron core inductor? I was thinking that a pair of aircore inductors would be fine.

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Old 7th October 2002, 06:02 PM   #7
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Another question on PSU chokes, is it good, bad or indifferent to use common mode chokes, one half in each supply rail?

I'm talking about a C-L-L configuration in each rail.
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Old 7th October 2002, 11:58 PM   #8
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Brian,

An air core of about 2mH should work fine. I tried to find the thread where someone used 1mH and it work ok in lowering the ripple and besides, 2mH is what Mr. Pass suggests. Iím going to use 1mH Alpha Air Cores with a DC resistance of 0.12Ohms for my Aleph5. I may go 2mH yet, but I havenít decided if I want to spend the $140 for more Alphas. Hereís where I got the Alphas.

http://www.partsconnexion.com/catalog/inductors.html

You could roll your own. Itís pretty tedious work unless you have a machine and theyíll never be as neat as you would like them to be. I made the coils for my first x-over for my horns. They worked fine, but it was 16 gauge. 12 gauge will be a lot tougher to work with. I know youíve been DIYiní everything you can so youíll probably make your own no matter what I say, so Iíll shutup about it.

Bertie,

Take a look at the link I posted earlier in this thread. The bottom line on Common Mode Chokes is that they wonít work for PSUís.

Rodd Yamas***a
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Old 8th October 2002, 05:34 AM   #9
Variac is offline Variac  United States
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This has been discussed a lot. You could search and read for an hour!(worth doing!) Even for 2 mH you need at least 12 ga wire. 10 is better. Nelson likes the steel core Erse from Zylatron. They are appealing cause of the low resistance and can handle a lot of current.
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Old 8th October 2002, 04:47 PM   #10
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I've just realised what a numbskull question asking about common mode chokes was, sorry.

Getting back to the real world and engaging brain am I right in thinking that air core chokes are best, followed by iron then ferrite?

Also anyone know where I can get some 2mH 4 amp chokes for use in the Aleph 5 PSU in the UK?
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