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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Wideband ammeter
Date: Sun, 23 Mar 2025 18:08:21 -0000 (UTC)
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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. 

Cheers 

Phil Hobbs 
(In Austin visiting #1 daughter)

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics