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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Instead scopes
Date: Tue, 3 Sep 2024 15:11:40 +1000
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On 3/09/2024 1:31 am, john larkin wrote:
> On Mon, 2 Sep 2024 17:13:59 +1000, Bill Sloman <bill.sloman@ieee.org> wrote:
>> On 2/09/2024 6:32 am, john larkin wrote:
>>> On Mon, 2 Sep 2024 02:20:42 +1000, Bill Sloman <bill.sloman@ieee.org> wrote:
>>>> On 2/09/2024 12:09 am, john larkin wrote:
>>>>> On Sun, 1 Sep 2024 17:45:46 +1000, Bill Sloman <bill.sloman@ieee.org> wrote:
>>>>>> On 30/08/2024 2:21 am, Jan Panteltje wrote:
>>>>>>> On a sunny day (Fri, 30 Aug 2024 00:43:39 +1000) it happened Bill Sloman
>>>>>>> <bill.sloman@ieee.org> wrote in <vaq1f2$jdj$1@dont-email.me>:

<snip>

>>>> And you don't simulate them either.
>>>
>>> Only when they matter.
>>>
>>>  Simulation is - in part - about
>>>> letting the math throw up unexpected effects that appear when you hook
>>>> up a bunch of components. Knowing when it matter relies on the
>>>> simulation inside your head.
>>>>
>>>>> I designed this surface-mount inductor for my Pockels Cell driver,
>>>>> after several tries using commercial parts. They all smoked.
>>>>
>>>> So you didn't read the data sheets carefully enough.
>>>
>>> Sure I did. They should have worked, based on the data sheets.
>>
>> Based on your understanding of the data sheet, which was obviously
>> inadequate. It's revealing that you don't post links to the data sheets
>> or specify the number that you relied on when you assumed that they
>> ought to have worked.
>>
>>> It's not a part
>>>> that would usually be described as "surface mount". If you'd scraped the
>>>> enamel off the bottom of the coil and soldered each turn down onto an
>>>> isolated copper pad on the board, it probably would qualify as surface
>>>> mount, and would have had better thermal contact with the board.
>>>
>>> I did that on the ends. I think the gap-pad works better thermally
>>> than soldering every turn to the board.
>>
>> Solder is metal, and has a higher conductivity than your gap-pad
>> material. You can over-fill the joint, which would help.
> 
> It wouldn't help much to conduct heat into a PCB pad. FR4 is a
> terrible heat conductor.

That's where your gap-ad dumps the heat. It may be a terrible conductor 
(if a lot better than air) but it was clearly good enough for you.

>>> Have you ever used a surface-mount coil that soldered every turn to
>>> the board? Got a link?
>>
>> No. It's merely an obvious possibility.
> 
> Ha.

So not obvious to you.

>>>> You might have had to make it as a sintered metal 3-D printed structure to
>>>> get this to work - the wound coil looks a bit irregular.
>>>
>>> Losses would be crazy.
>>
>> What makes you think that? The fact that the part is sintered doesn't
>> mean that you won't get close to solid metal electrical conductivity.
> 
> Skin depth is about 30 microns here, and we need a smooth, homogenous,
> annealed surface. Ask a chemist.

Ask a guy with a pulsed laser, who can melt the top 30 microns of the 
part for enough milliseconds for it flow into a smooth surface. I've 
always been puzzled why people haven't resumed silver plating RF parts 
when you can now fuse the surface electro-plated crystals into a 
continuous film of silver

>>>> Lost wax casting could have worked too.
>>
>> And even you must concede that that wouldn't have been lossy.
> 
> Impractical, 

Why?

and cast copper is probably a worse conductor than annealed.

So anneal it.

>>>>> It's wound on a specially marked Sharpie pen that we have carefully
>>>>> reserved.
>>>>
>>>> That defines it diameter. Measuring that with a vernier caliper would
>>>> give you a number you could document.
>>>>
>>>>> https://www.highlandtechnology.com/Product/T850
>>>>
>>>>> The grey gap-pad gives it some extra cooling. The board has lots of
>>>>> thermal vias down to the water-cooled baseplate.
>>>>
>>>> If you'd wound it with copper tube you could have pumped water through
>>>> the tube, or made it a heat pipe.
>>>
>>> And supply a water tank and a pump and water connectors?
>>
>> Heat pipes don't need that. A closed system doesn't need a water-tank,
>> and lots of top-end computer coolers do rely on circulating water.
> 
> My gadget is cheap and easy and works.

But looks cheap. And a hand-wound coil is never easy.

>>>> A 3-D printed structure would have offered more options.
>>>
>>> Again, massive losses.
>>
>> Imagined massive losses.
> 
> Certainly imagined. Please make a 3D fabbed inductor and measure its Q
> and report back to us.

If you'll pay for it, and the time I'd have to spend to dig out somebody 
to do it. Back when I was working I had access to people who would know 
somebody who would do it, but I moved back to Australia, and those 
people are less accessible (and some of them have died).

>>> My inductor is cheap and simple and works.
>>
>> It's hand-wound, so it looks cheaper than it is.
> 
> This Pockels Cell driver is maybe 1/20 the volume of competitors' and
> uses a few per cent of the power. The inductor is a detail.

But a crucial one.

> Most drivers dissipate
> 
> F * C * V^2
> 
> in the driver itself, but it should take zero energy to charge and
> discharge a capacitor.

Sadly. capacitors can be lossy, and the current involved has flow 
through conductors which are resistive - more so when the skin effect 
kicks in.

>>> If I get a gigantic order, I'll have a coil winding company make them
>>> and retire the Sharpie.
>>
>> Or come up with a more sensible solution?
> 
> More sensible than winding an inductor from magnet wire?

Why do you think that the Coilcraft inductors were wound with flat wire?

The skin effect means that most of your magnet wire is unused.

With flat wire, you also have more surface area to couple the wire to 
the air you are relying on to cool it.

 From a practical point of view, changing the layout to accomodate 
enough of the Coilcraft parts to dissipate the heat would give you a 
much tidier-looking product, if marginally bulkier.

If what you've got has 5% of the volume of its competition, you've got 
room to do that.

-- 
Bill Sloman, Sydney