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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Instead scopes
Date: Tue, 3 Sep 2024 15:20:47 +1000
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On 3/09/2024 1:45 am, john larkin wrote:
> On Mon, 2 Sep 2024 17:52:09 +1000, Bill Sloman <bill.sloman@ieee.org>
> wrote:
> 
>> On 2/09/2024 12:49 pm, john larkin wrote:
>>> On Sun, 01 Sep 2024 17:43:32 -0400, Joe Gwinn <joegwinn@comcast.net>
>>> wrote:
>>>
>>>> On Sun, 01 Sep 2024 13:17:03 -0700, john larkin
>>>> <jlarkin_highland_tech> wrote:
>>>>
>>>>> On Sun, 01 Sep 2024 15:53:46 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>> wrote:
>>>>>
>>>>>> On Sun, 1 Sep 2024 17:55:58 -0000 (UTC), Phil Hobbs
>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>>>
>>>>>>> john larkin <jlarkin_highland_tech> 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>
>>
>>>>> I'll have someone start on a SolidWorks model.
>>>>
>>>> I bet you need the standoff, so the lossy FR4 material isn't too
>>>> close.  That should be in the requirements as well.
>>
>> If the FR4 losses matter, the printed circuit board under the coil would
>> darken, which the customers wouldn't like. The Cambridge Instruments
>> 0.5nsec beam blanker did that so we swapped to a different substrate
>> that didn't discolour.
>>
>>> The turns squish down into the gap-pad gunk, which is an OK heat
>>> conductor. The PCB under the pad is a big copper pour, top and bottom,
>>> with a zillion thermal vias.  There's more gap-pad on the underside of
>>> the board to dump heat into the baseplate.
>>>
>>> At 4 MHz, skin depth is 32 microns, so most of the copper is wasted.
>>> That's why it gets so hot.
>>
>> It's a sawtooth so it has quite a lot of higher harmonic components with
>> even thinner skin depths. Baxandall's preference for sine waves has
>> incidental advantages.
>>
>> The turns are wide and flat, which reduces the effect of skin dept.
>>> I tried three of the Coilcraft 1010VS parts in series, but they
>>> smoked, probably skin+proximity effect.  Maybe parallel would have
>>> been better.
>>
>> https://www.coilcraft.com/getmedia/55a4b40a-2e02-4bf5-b0af-2ea5db75b6cf/1010vs.pdf
>>
>> There are five 1010VS parts, all rated at about 25A rms. You haven't
>> specified which one you used three of.
>>
>> That 25A rms isn't going to include any allowance for skin effect.
>>
>> They don't look as if there would be much cross-talk from one to the
>> next. Making space for more parts might have been a better approach.
>>
>> With +/-20% tolerance on inductance, putting them in parallel wouldn't
>> have been a good idea.
> 
> Don't they have the same tolerance in series?

When they are in series, the same current passes through each one.

When they are in parallel the lower inductance parts would carry up to 
40% more current and have to dissipate up to twice as much heat.

> In parallel, each would get 1/3 the current. But each would need to be
> 9x the inductance. I suspect that's a wash, something fundamental
> going on.
> 
> My coil opens itself up for a lot of air cooling, and bare copper can
> run pretty hot.

And melt the solder holding it in place?

>>>> I'd specify the coil dimensions, not the mandrel dimensions, which may
>>>> be provided as a helpful suggestion only.
>>>
>>> I could have a mandrel machined or 3D printed, to more accurately wind
>>> the inductor. The improvement would be mostly cosmetic.
>>>
>>> Inductors are a pain.
>>
>> Particularly when you don't think about what you doing.
> 
> But it works. A big laser company buys them.
> 
> Why don't you design a 1200 volt, 4 MHz pulse generator and we can
> discuss it here.

Why would I bother? I wouldn't get paid for it. More to the point, I'd 
need a lot more information than 1200V, 4 MHz and 25A peak to peak, and 
your customer wouldn't be happy if you broadcast that much detail.

-- 
Bill Sloman, Sydney