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From: john larkin <jlArbor.com>
Newsgroups: sci.electronics.design
Subject: Re: Wideband ammeter
Date: Sun, 23 Mar 2025 13:01:20 -0700
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On Sun, 23 Mar 2025 18:08:21 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>john larkin <jlArbor.com> wrote:
>> On Sun, 23 Mar 2025 09:06:08 +0000, Cursitor Doom <cd@notformail.com>
>> wrote:
>> 
>>> On Sun, 23 Mar 2025 08:31:19 +0000, Jeff Layman <Jeff@invalid.invalid>
>>> wrote:
>>> 
>>>> On 23/03/2025 05:47, john larkin wrote:
>>>>> On Sun, 23 Mar 2025 03:59:46 -0000 (UTC), Sergey Kubushyn
>>>>> <ksi@koi8.net> wrote:
>>>>> 
>>>>>> Bill Sloman <bill.sloman@ieee.org> wrote:
>>>>>>> On 23/03/2025 12:49 pm, john larkin wrote:
>>>>>>>> 
>>>>>>>> How about thermal imaging a surface-mount resistor?
>>>>>>> 
>>>>>>> Why bother? Measuring the voltage drop across the same device is easier,
>>>>>>> and just as fast, if not faster.
>>>>>> 
>>>>>> That depends on what you actually want to measure. And "wideband" makes it
>>>>>> even more difficult.
>>>>>> 
>>>>>> Wideband current shunts made for AC-DC transfer are all of very special
>>>>>> costruction and cost arm and leg. If you want to measure the voltage drop
>>>>>> over those resistors, without making AC-DC transfer, you're up to another
>>>>>> challenge, measuring the AC voltage. Should start from the definition, what
>>>>>> IS the AC voltage? What the actual number your measurement shows means and
>>>>>> so on.
>>>>>> 
>>>>>> Look at e.g. not all that precise but much better than most LT1088 chip,
>>>>>> long obsolete. There is another one, proprietary and much better precision
>>>>>> inside e.g. Fluke 5790A Standard (which is a misnomer -- it is actually an
>>>>>> AC and DC voltmeter, 10x more precise that the venerable HP/Agilent/Keysight
>>>>>> 3458A).
>>>>>> 
>>>>>> ---
>>>>>> ******************************************************************
>>>>>> *  KSI@home    KOI8 Net  < >  The impossible we do immediately.  *
>>>>>> *  Las Vegas   NV, USA   < >  Miracles require 24-hour notice.   *
>>>>>> ******************************************************************
>>>>> 
>>>>> Many otherwise great DVMs have an AC bandwidth that doesn't even
>>>>> handle the audio range.
>>>>> 
>>>>> The resistor has great common-mode rejection too.
>>>> 
>>>> There is a skin effect in a surface-mount resistor (see 
>>>> <https://www.vishay.com/docs/60107/freqresp.pdf>). If an AC current 
>>>> flows nearer the surface of the resistor, would that make it "hotter" 
>>>> the higher the frequency? In other words, would the ammeter also be a 
>>>> frequency meter?
>>> 
>>> What's the big deal here? Just use a oscilloscope with a current
>>> probe.
>> 
>> Most such probes are AC-only, and none can clamp onto a PCB trace.
>> 
>> My current (in both senses) issue is to measure true RMS currents in a
>> high-frequency isolated power supply. The idea is to have one
>> STSPIN958 full-bridge make anti-phase 48-volt 500 KHz square waves
>> that drive some number N of DRQ127 isolation transformers and then
>> schottky bridge rectifiers, for some unknown N. Four would be nice.
>> 
>> I have an engineer, a kid right out of school, working on this. It's a
>> great educational project. He'd never heard of core satutation,
>> shoot-through current, diode reverse recovery, skin effect, The
>> Devil's Staircase, the quirks of STspice, any of that practical stuff.
>> And I'm teaching him how to Dremel and solder and shear and drill
>> holes.  This ain't bad for a first try:
>> 
>> https://www.dropbox.com/scl/fi/bje0zqt3okjbsaycp16of/STSPIN_Proto.jpg?rlkey=yzyi1hf0r05hbex8zhppxdln4&raw=1
>> 
>> No, a surface-mount bridge rectifier isn't supposed to smoke.
>> 
>> 
>> 
>
>My Tek one goes dc-20 MHz. It uses a Hall sensor and a regular current
>transformer. 

What model is that? Maybe we could put a little loop thingie on a
board and clip onto that.

>
>Cheers 
>
>Phil Hobbs 
>(In Austin visiting #1 daughter)

Check out the bats at sunset.