inductor sizing

[Tom Bruhns]

Does an air-core toroid make any sense? I don't think I've ever seen
one. Micrometals does have a no-op core material, u=1, made out of
phenolic or something.

John

Yes, but I believe the advantage is small enough that it seldom
outweighs the additional complexity of implementing the winding. In
an air-core solenoid coil, it's rather interesting how low the
coupling is between the first and last turns in "typical" RF coil
geometries--where the coil length is commonly equal to or greater than
its diameter.

Cheers,
Tom

 

[Harry Dellamano]

Yes, but I believe the advantage is small enough that it seldom
outweighs the additional complexity of implementing the winding. In
an air-core solenoid coil, it's rather interesting how low the
coupling is between the first and last turns in "typical" RF coil
geometries--where the coil length is commonly equal to or greater than
its diameter.

Cheers,
Tom

Hey Tom, are you going to give us a size and SRF estimation of the air core
inductor needed in the above filter. I have calculated the SRF of the
ferrite inductor to be 5.75MHz +/-20%. It would be nice to have a Rac at
200KHz number also, the ferrite should come in at Rdc< 8.0mR and Rac< 15mR.
Isat is >75A for the ferrite.
What say you,
Harry

 

[Tom Bruhns]

I totally agree with your second stage design. When I stated "60 Hz hat", I
meant the core material can be 60Hz material but the corner frequency is
still up at maybe 30KHz with low parasitic capacity to get the required
200KHz attenuation. As we both stated 60Hz THD requirements must be meat
with loop design. Wima caps are a good alternative but cost will start to
soar.
As for air cores, I do not disagree with anything you have stated above.
but size matters to further this discussion. The current levels are very
high, wiring loops areas must be keep as small as possible so they no not
act as primaries nor secondary windings. In so doing the inductor is in very
close proximity with many other windings with no room for shielding.
I would be very interested in the size of the air core inductor compared to
the gapped ferrite which is about a three inch cube. The self resonate
frequency, which I will calculate for the ferrite, would be an interesting
data point.

Regards,
Harry

Yep, I agree size does matter. I should perhaps clarify what I said
before about it being unlikely I'd actually build a supply like that
with an air core coil: if I did, it certainly would be a one-off
experiment, and not something I'd contemplate putting into
production. Since in some of my spare moments I'm playing with a
supply at about the 2kW level and about half the OP's supply voltage,
I may try pulling out my toroid inductor and putting in an air core
coil. I'm sure I'll learn a thing or two from the 'speriment. ;-)
And of course, the times I learn the most are when I discover the
error of my ways -- so when I try things that "shouldn't work" I do
tend to learn a lot.

I only have an easy way to calculate expected resonances for single-
layer solenoid coils, but for a 10uH 20 turn coil of 12AWG 1.665
inches long (about 3 mils between turns; essentially close-wound
enameled magnet wire), the predicted first parallel resonance is
39.3MHz and the first series resonance is 62.6MHz. 1.62pF effective
parallel capacitance. The closeness of the turns is probably pushing
the program beyond where it's intended to be used, so that may be in
error some.

Cheers,
Tom

 

[Tom Bruhns]

Hey Tom, are you going to give us a size and SRF estimation of the air core
inductor needed in the above filter. I have calculated the SRF of the
ferrite inductor to be 5.75MHz +/-20%. It would be nice to have a Rac at
200KHz number also, the ferrite should come in at Rdc< 8.0mR and Rac< 15mR.
Isat is >75A for the ferrite.
What say you,
Harry

Hi Harry,

I looked through the quoted material in your posting that I've
replaced with "..." above and didn't see an inductance. What
inductance did you do that estimation for? You've perhaps seen my
estimate for a particular coil which is probably not very close to
what you've estimated, so in the interest of comparing apples and, um
oranges instead of apples and hedgehogs, ...

The 10uH coil I mentioned in my other recent posting would be about 12
milliohms DCR; I expect Qu in a coil that size and at that frequency,
wound with optimal Litz wire, to be around 150. "Isat" doesn't have
much meaning for an air core coil, though losses-->heat will limit the
maximum operating current. I'm curious enough about this now that
I'll see if I can hack a coil and do a quick-and-dirty measurement
tonight--at low power, of course. I do wonder at your Rac a bit; if
it's for a 10uH inductor, it implies a Q of over 800 at 200kHz. Seems
a bit high.

Cheers,
Tom

 

[Terry Given]

Yep, I agree size does matter. I should perhaps clarify what I said
before about it being unlikely I'd actually build a supply like that
with an air core coil: if I did, it certainly would be a one-off
experiment, and not something I'd contemplate putting into
production. Since in some of my spare moments I'm playing with a
supply at about the 2kW level and about half the OP's supply voltage,
I may try pulling out my toroid inductor and putting in an air core
coil. I'm sure I'll learn a thing or two from the 'speriment. ;-)
And of course, the times I learn the most are when I discover the
error of my ways -- so when I try things that "shouldn't work" I do
tend to learn a lot.

I only have an easy way to calculate expected resonances for single-
layer solenoid coils, but for a 10uH 20 turn coil of 12AWG 1.665
inches long (about 3 mils between turns; essentially close-wound
enameled magnet wire), the predicted first parallel resonance is
39.3MHz and the first series resonance is 62.6MHz. 1.62pF effective
parallel capacitance. The closeness of the turns is probably pushing
the program beyond where it's intended to be used, so that may be in
error some.

Cheers,
Tom

Hi Tom,

how do you calculate the first series resonance? Im having a problem
with that right now...

Cheers
Terry

 

[John Fields]

As I said, what I consider to be the design process is what you seem
to define as "intentionally devious", which you have now further
morphed into "ill-gotten." This gets better and better.

---
Of course that's not what I meant, as you well know, and your
attempt to redefine "intentionally devious" into an acceptable
design process is disingenuous, at best.

But that seems to be the defense mechanism you employ in order to
try to deflect commentary which you consider to be damaging.

For example, what I was referring to wasn't the design process, it
was your response to criticism, which you generally respond to by
changing the subject or responding to the criticism flippantly in
order to try to make the criticism also seem flippant.
---

We design and build electronics. We have specs, manuals, and a price
list. If people want to buy our stuff, they can; if they don't like
the price, they can shop around for a better deal. If they want to try
a loner, to see if it works as claimed, we provide one. If anything
goes wrong, we fix it fast. If the customer isn't happy with one of
our products, we take it back and refund all their money. If they ask
a question, we tell them the truth.

The design part is a lot more "devious", but that's not the customers
concern. The customer pays for a product that performs a function.
Since we never sell designs, only products, so the customer has no
reason to see the process.

---
Then that's the difference between us.

We sell designs and supply first articles, if asked to, in order to
prove the soundness of the design.

Also, as I'm sure you've noticed, we give away a lot of stuff, pro
bono, which can be built in the real world and which will work.

And you?

You generally just play the game of the elite guru who only issues
hints.

 

[John Larkin]

---
Of course that's not what I meant, as you well know, and your
attempt to redefine "intentionally devious" into an acceptable
design process is disingenuous, at best.

But that seems to be the defense mechanism you employ in order to
try to deflect commentary which you consider to be damaging.

Damaging? How the hell could you ever damage me? Don't be absurd.

For example, what I was referring to wasn't the design process, it
was your response to criticism, which you generally respond to by
changing the subject or responding to the criticism flippantly in
order to try to make the criticism also seem flippant.

I reserve the right to make fun of anybody or anything. You can
reserve the right to be as prissy and grim as pleases you. Fair
enough?

---


---
Then that's the difference between us.

We sell designs and supply first articles, if asked to, in order to
prove the soundness of the design.

I figured out a long time ago that, if you sell engineering, you only
get paid for it once. If you design and sell products, your
manufacturing people can replicate hundreds, or in some cases
thousands, of copies, and you get paid for every one. It's like what
the military call "force multiplication."

Also, as I'm sure you've noticed, we give away a lot of stuff, pro
bono, which can be built in the real world and which will work.

And you?

You generally just play the game of the elite guru who only issues
hints.

My body will be unsuited to sheetrocking long before my mind is
unsuited to design. As far as the sheetrock thing goes, my body is
over the hill already. But my wife has decided to start her own
business, and rented an unfinished space to start up in, so we're
doing it up...

http://www.activspace.com/mission.html

This is owned by a lady from Seattle who constructs buildings with a
few hundred small offices each, and rents them out month-to-month.
They provide power, security, wi-fi, parking, and a really cool
environment. It's a bare room that you finish and decorate however you
want. It's mostly artistes and startup small biz, so I suspect there's
a lot of turnover. It would be ideal for a very small electronics
startup or something like that. This should be fun.

But as far as fallback goes, that's another advantage that selling
products has over selling design. If I quit today, my company could
keep selling what we have for 5, maybe 10 years.

John

 

[Terry Given]

Damaging? How the hell could you ever damage me? Don't be absurd.





I reserve the right to make fun of anybody or anything. You can
reserve the right to be as prissy and grim as pleases you. Fair
enough?






I figured out a long time ago that, if you sell engineering, you only
get paid for it once. If you design and sell products, your
manufacturing people can replicate hundreds, or in some cases
thousands, of copies, and you get paid for every one. It's like what
the military call "force multiplication."




My body will be unsuited to sheetrocking long before my mind is
unsuited to design. As far as the sheetrock thing goes, my body is
over the hill already. But my wife has decided to start her own
business, and rented an unfinished space to start up in, so we're
doing it up...

http://www.activspace.com/mission.html

This is owned by a lady from Seattle who constructs buildings with a
few hundred small offices each, and rents them out month-to-month.
They provide power, security, wi-fi, parking, and a really cool
environment. It's a bare room that you finish and decorate however you
want. It's mostly artistes and startup small biz, so I suspect there's
a lot of turnover. It would be ideal for a very small electronics
startup or something like that. This should be fun.

But as far as fallback goes, that's another advantage that selling
products has over selling design. If I quit today, my company could
keep selling what we have for 5, maybe 10 years.

John

would licensing designs be a good compromise?

making stuff is fun.

Cheers
Terry

 

[John Larkin]

would licensing designs be a good compromise?

Yup, as long as you will get paid for every copy, forever. That's
actually better than manufacturing copies of your IP... let somebody
else do it. We license two products, one an electrical power
metering/datalogger box, and one an eximer laser controller. Support
is minimal, and the checks keep coming in.

making stuff is fun.

Yup. The tools and toys that are available nowadays, like right out of
the Digikey catalog, are astounding.

John

 

[Terry Given]

Yup, as long as you will get paid for every copy, forever. That's
actually better than manufacturing copies of your IP... let somebody
else do it. We license two products, one an electrical power
metering/datalogger box, and one an eximer laser controller. Support
is minimal, and the checks keep coming in.

I'm having a crack at selling labour cheap + shares. we'll see how it
goes. mostly I sell labour. but I want to sell products.

Yup. The tools and toys that are available nowadays, like right out of
the Digikey catalog, are astounding.

John

I just bought a new toy - a shielded room. It was astonishingly cheap,
*but* I have to dismantle it. and then re-mantle (?) it

still, I then have the perfect excuse for buying a USB-GPIB interface
and programming my HP3585 to do EMC sweeps.

any recommendations for the USB-GPIB interface?

Cheers
Terry

 

[JosephKK]

Yes, but I believe the advantage is small enough that it seldom
outweighs the additional complexity of implementing the winding. In
an air-core solenoid coil, it's rather interesting how low the
coupling is between the first and last turns in "typical" RF coil
geometries--where the coil length is commonly equal to or greater than
its diameter.

Cheers,
Tom

Ok, i get it. Most people won't even try to think about the physics.

 

[Joel Koltner]

I just bought a new toy - a shielded room. It was astonishingly cheap, *but*
I have to dismantle it. and then re-mantle (?) it

They're cheap because it's generally impossible to get them to meet their
original specs the second time. Still, 1000x better than nothing.

We're getting around to building a new buliding (it keeps getting moved out
based on revenue ) and we figure we'll be lucky to get anyone to take it
for free, since by then it'll be the third assembly.

any recommendations for the USB-GPIB interface?

Did you see the thread on that very topic a few weeks back? This guy -->
http://prologix.googlepages.com/ seems to be a really good value if don't need
ever last bit of programming already done for you by, e.g., National
Instruments. If you're feeling a little more flush we have a few of the NI
USB adapters (something like $400/ea?) that we've never had any problems with.
We also have one NI Ethernet->GPIB converter, and unfortunately it isn't fully
compatible with all of our instruments.

---Joel

 

[Tom Bruhns]

Hi Tom,

how do you calculate the first series resonance? Im having a problem
with that right now...

Cheers
Terry

Hi Terry,

Oh, I cheat. I certainly would not want to try to think about the
physics. ;-) I use a program written by John Mezak called "AirCoil"
or something like that. But you can do as well using the calculator
on http://hamwaves.com/antennas/inductance.html.

The basic idea is to think of the coil as a helical transmission line;
if one end is open, the other shows a series resonance at odd quarter-
waves.

Cheers,
Tom

 

[Terry Given]

Hi Terry,

Oh, I cheat. I certainly would not want to try to think about the
physics. ;-) I use a program written by John Mezak called "AirCoil"
or something like that. But you can do as well using the calculator
on http://hamwaves.com/antennas/inductance.html.

The basic idea is to think of the coil as a helical transmission line;
if one end is open, the other shows a series resonance at odd quarter-
waves.

Cheers,
Tom

ah, that makes sense, and certainly matches what I am seeing.

Thanks!


Cheers
Terry

 

[Terry Given]

ah, that makes sense, and certainly matches what I am seeing.

Thanks!


Cheers
Terry

Now heres something I dont want to try & analyse - can i foil the
lambda/4 resonance by continuously varying the pitch of my helix?

Cheers
Terry

 

[John Fields]

On Mon, 10 Mar 2008 20:03:59 -0500, John Fields


Damaging? How the hell could you ever damage me? Don't be absurd.

---
I don't damage you, you damage you.

I merely point out your errors and inconsistencies and watch, with
incredulity, at the lengths you'll go to to keep from having to
admit to either.
---

I reserve the right to make fun of anybody or anything. You can
reserve the right to be as prissy and grim as pleases you. Fair
enough?

---
Nope.

You have that right, but you can't "reserve" it since everyone else
has the same right.

In the same vein, I can't reserve the right to be prissy and grim
but, of course, I have the right to be if I choose.

Interestingly, beauty is in the eye of the beholder, so if it's
_you_ that's being made fun of, you might well consider that to be
prissy and/or grim.
---

I figured out a long time ago that, if you sell engineering, you only
get paid for it once.

---
Not necessarily, since it's sometimes possible to work out a royalty
deal.
---

If you design and sell products, your
manufacturing people can replicate hundreds, or in some cases
thousands, of copies, and you get paid for every one. It's like what
the military call "force multiplication."

---
Yes, well, that's your way.

Mine is to produce a large variety of different paintings and to
sell each one instead of painting only a few and selling
reproductions.

 

[Joel Koltner]

Tom,

Do you have a copy of Randy Rhea's (of Eagleware Genesys fame) paper, "A
Multimode High-Frequency Inductor Model?" He does a pretty good job
demonstrating how most real inductors are a lot closer to a transmission line
model as you're discussing than the ECE 101 "fields and waves" model that's
often proffered.

---Joel

 

[Jim Thompson]

Tom,

Do you have a copy of Randy Rhea's (of Eagleware Genesys fame) paper, "A
Multimode High-Frequency Inductor Model?" He does a pretty good job
demonstrating how most real inductors are a lot closer to a transmission line
model as you're discussing than the ECE 101 "fields and waves" model that's
often proffered.

---Joel

Where can I get a copy of the paper? Google gives lots of referrals
to the paper, but not the paper itself.

...Jim Thompson

 

[Harry Dellamano]

Hi Harry,

I looked through the quoted material in your posting that I've
replaced with "..." above and didn't see an inductance. What
inductance did you do that estimation for? You've perhaps seen my
estimate for a particular coil which is probably not very close to
what you've estimated, so in the interest of comparing apples and, um
oranges instead of apples and hedgehogs, ...

The 10uH coil I mentioned in my other recent posting would be about 12
milliohms DCR; I expect Qu in a coil that size and at that frequency,
wound with optimal Litz wire, to be around 150. "Isat" doesn't have
much meaning for an air core coil, though losses-->heat will limit the
maximum operating current. I'm curious enough about this now that
I'll see if I can hack a coil and do a quick-and-dirty measurement
tonight--at low power, of course. I do wonder at your Rac a bit; if
it's for a 10uH inductor, it implies a Q of over 800 at 200kHz. Seems
a bit high.

Cheers,
Tom

Hi Tom, sorry for the confusion.
The ferrite inductor is 25uH in a 2.75" cube. It is designed for 35Apk,
20.0Aac and 2.0Adc in a larger core than necessary because the OP will
probably have to drop down to 100KHz in switching frequency and he needs a
lot of wiggle room at his skills level. That said, a 2.25" cube would get
the job done. My Rdc = 8.0mR is real but the Rac is a optimistic guestimate.
Though I used Litz wire, the core gaps will cause havoc with the Rac. With
the large Aac component, Rac is a dominate player and reducing it is an
ongoing task.
In winding ferrite cores with high Aac levels, we normally can not use many
layers nor fill the available window because of proximity losses, air cores
should exhibit the same or worse effects due to the flux being outside the
core. Do you have a way of measuring your Rac at 200KHz? Your #24 Litz wire
sounds too fat, maybe >#28 would serve you better. Tell us about your
dimensions and layering.

Regards,
Harry

 

[Tom Bruhns]

Now heres something I dont want to try & analyse - can i foil the
lambda/4 resonance by continuously varying the pitch of my helix?

Cheers
Terry

See http://www.coilcraft.com/bcl-s.cfm. You don't have to wind your
own. ;-)

Cheers,
Tom