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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Wideband ammeter
Date: Mon, 24 Mar 2025 03:09:38 -0000 (UTC)
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john larkin <jlArbor.com> wrote:
> 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.
> 
I lied—it’s 50 MHz. 

Tektronix P6042 50MHz Current Probe

Cheers 

Phil Hobbs 


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