Re: RMS and Average Current

On 3 Jan 2006 19:21:03 -0600, The Phantom <phantom@xxxxxxx> wrote:

>On Tue, 03 Jan 2006 08:19:13 -0800, John Larkin
><jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
>>On 3 Jan 2006 04:37:16 -0800, Winfield Hill
>><Winfield_member@xxxxxxxxxxx> wrote:
>>
>>>Fred Bartoli wrote...
>>>> John Larkin wrote...
>>>>> Don Lancaster wrote:
>>>>>
>>>>>> Not even wrong.
>>>>>>
>>>>>> An average responding meter will ALWAYS lie like a rug.
>>>>>
>>>>> You mean, it won't indicate average? OK, what *does* an
>>>>> average-indicating ammeter indicate?
>>>>
>>>> It indicates average ...on the average.
>>>
>>> :-) I think Don Lancaster meant, average indication isn't
>>> very useful, in that it's rarely what you really want, so
>>> relying on its value is likely to give you the wrong answer.
>>> For example, an average-indicating meter may be calibrated
>>> to show rms, but of course doesn't. But we all knew that.
>>
>>One might also note that it's rare to have a ammeter that actually
>>indicates RMS current on a DC range, unless you have an ancient Weston
>>electrodynamic thing with a mirrored scale and brass lugs. As far as
>>DC current measurements, average is usually more meaningful.
>
> The old hot-wire ammeters indicated RMS of AC+DC. Some of the thermocouple
>type RF ammeters do it too. I always buy RF ammeters that I find at hamfests
>just because they can do this. The construction of the RF ammeters sometimes
>adds a small fraction of the applied current drop in series with the
>thermocouple output, and these can't accurately measure RMS AC+DC. They will
>often read backwards with applied DC of one polarity, and way too high with the
>other polarity. The ones I covet don't have this problem, and will read
>correctly with DC or AC (up to 65 MHz typically). I use them to measure
>currents in switching supplies. The modern DVMs are often fooled by all the RF
>flying around in a switcher, but the thermocouple RF ammeters aren't.
>
> Another type of meter that will correctly read RMS of AC+DC is the moving iron
>meter. These are often in the form of a panel meter, intended to be connected
>to the secondary of a current transformer, and typically are 5 amps full scale,
>even though the printed scale may be different. Bring a magnet near one of
>these and get a reading, so keep magnets away. They can be had surplus quite
>often for a good price. They aren't good for high frequency AC as are the RF
>ammeters, but for line frequency work, they are quite handy.
>
>>
>>John


I have a box full of flea-market thermal converters, which sort of
look like acorn tubes with flying leads. Inside is a heater and a
thermocouple. They were, maybe still are, used as ac-dc transfer
standards.

Fluke and somebody else still make thermal-transfer based bench DVMs,
very accurate and very expensive.

John

.

Relevant Pages

  • Re: RMS and Average Current
    ... >>> John Larkin wrote... ... >indicates RMS current on a DC range, unless you have an ancient Weston ... The old hot-wire ammeters indicated RMS of AC+DC. ... Another type of meter that will correctly read RMS of AC+DC is the moving iron ...
    (sci.electronics.design)
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