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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Curve Tracer
Date: Thu, 6 Feb 2025 03:51:48 +1100
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On 6/02/2025 3:18 am, john larkin wrote:
> On Wed, 05 Feb 2025 15:33:11 +0000, Cursitor Doom <cd@notformail.com>
> wrote:
> 
>> On Wed, 5 Feb 2025 08:27:01 -0000 (UTC), Ian
>> <${send-direct-email-to-news1021-at-jusme-dot-com-if-you-must}@jusme.com>
>> wrote:
>>
>>> On 2025-02-04, john larkin <jl@glen--canyon.com> wrote:
>>>> On Tue, 04 Feb 2025 23:03:34 +0000, Cursitor Doom <cd@notformail.com>
>>>> wrote:
>>>>
>>>>> On Tue, 04 Feb 2025 10:19:40 -0800, john larkin <jl@glen--canyon.com>
>>>>> wrote:
>>>>>
>>>>>> On Tue, 04 Feb 2025 16:51:22 +0000, Cursitor Doom <cd@notformail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Gentlemen,
>>>>>>>
>>>>>>> I've decided a nice project would be to build a curve tracer which
>>>>>>> would (because I'm a vintage nut) use a CRT display of an
>>>>>>> oscilloscope. From time to time I have these Big Ideas and it could
>>>>>>> conceivably become the latest in a long line of my unfinished
>>>>>>> projects.
>>>>>>>
>>>>>>> Which curves would be essential to include for a decent, functional
>>>>>>> design? I know what the obvious ones are; just wondering if there are
>>>>>>> any more obscure ones which would be advantageous to plot. Finally,
>>>>>>> are there any additions one could make to a classic curve tracer's
>>>>>>> functionality which were omitted from the early designs (modulation
>>>>>>> feature at 1Mhz,100Mhz or whatever) for example?
>>>>>>>
>>>>>>> CD.
>>>>>>
>>>>>> I'd like to see pulsed avalanche behavior of mosfets, preferably to
>>>>>> destruction.
>>>>>>
>>>>>> C-V curves would be great too.
>>>>>>
>>>>>> Stuff that's not on the data sheet.
>>>>>>
>>>>>> The thing about an analog CRT scope is that it isn't quantitative and
>>>>>> forgets instantly. A digital scope would fix that. A curve tracer
>>>>>> should be able to trigger the scope and do short single-shot events.
>>>>>
>>>>> DSOs have their own disadvantages for this sort of thing.
>>>>> These people make this little thing with a CT function, but it needs a
>>>>> computer screen and is only capable of applying up to 10V! The stupid
>>>>> fucks have only used a single AAA battery for its power source. Can
>>>>> you believe that? I suspect there'll be a revised model along in due
>>>>> course which has a far better voltage range.
>>>>>
>>>>> https://www.peakelec.co.uk/acatalog/dca75-dca-pro-semiconductor-analyser.html
>>>>
>>>> Given a digital scope, most measurements can be done in a millisecond
>>>> or less, so a curve tracer can have some tiny power supplies and a few
>>>> big caps.
>>>>
>>>> People don't use bipolar transistors much any more, so one could
>>>> emphasize tracing mosfets and GaN fets and SiC parts.
>>>>
>>>> When I need to characterize a part, I hack up a setup with power
>>>> supplies and pulse generators and stuff.  We have a Tek scope with 4
>>>> isolated channels, which is handy snooping drain currents and such.
>>>>
>>>
>>> (crossing threads... )
>>>
>>> I've sometimes wondered if a gadget could be made that analyses a component
>>> and spits out a spice model for it.

Probably not.

>>> Maybe if fed with multiple samples of the same component it could average
>>> out the parameters.
>>
>> That is actually a really interesting idea if it could be implemented.

Less interesting if you realise how hard it would be to implement

>> However it goes beyond my pay grade so I'll have to defer to those
>> more knowledgeable about generating models for an answer to that.
>>
>> John, that was the other aspect I meant to say could use updating:
>> instead of having rows of switches and level controls, would it not be
>> better to let a Pi or an Arduino do all the heavy lifting. The
>> programming for such a straightforward task to automate things would
>> be pretty elementary and make the use of the device so much faster.

It wouldn't in the least elementary. If the device was clever enough to 
work out what kind of part it was testing, which is a big if, it would 
still have to work what kinds of current it would need to put through it 
(without blowing it up) and what kinds of voltages it would need to 
apply across it (again without blowing it up).

 >> Oh - one other feature: a 'subtract' function to analyze a batch of
 >> 'identical' devices to quickly select matched-pairs. Again that would
 >> require some computerization.
 >
 > Fun, but hardly anyone needs matched pairs these days, with cheap
 > opamps having microvolt offsets and nanovolt drift.

You can used matched pairs in other application beyond making low 
off-set op-amps. They are handy if you want to make a current mirror, 
and a matched pair inside an op amp isn't much use for that.

-- 
Bill Sloman, Sydney