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From: john larkin <jlArbor.com>
Newsgroups: sci.electronics.design
Subject: Re: Wideband ammeter
Date: Sun, 23 Mar 2025 10:05:31 -0700
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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.