Re: Resistance of ammeter caused voltage drop

From: John Fields (jfields_at_austininstruments.com)
Date: 03/16/05 

Date: Wed, 16 Mar 2005 06:40:37 -0600

On Wed, 16 Mar 2005 03:37:29 -0800, "Larry Brasfield"
<donotspam_larry_brasfield@hotmail.com> wrote:

>"John Fields" <jfields@austininstruments.com> wrote in message
> news:jetf31l4prjgl1g4gif72o7mvj3511rko1@4ax.com...
>> On Tue, 15 Mar 2005 19:42:43 -0800, "Larry Brasfield"
>> <donotspam_larry_brasfield@hotmail.com> wrote:
>>
>>>"Fred Bloggs" <nospam@nospam.com> wrote in message
>>> news:423796E7.3090905@nospam.com...
>>
>>>> Or buy/modify a p.s. with external sense compensation,
>>>
>>>Spending money was an obvious option which
>>>I mentioned in several of its many forms.
>> ---
>> However, you never mentioned external sense compensation (using a
>> Kelvin connection at the load to supply feedback to the supply in
>> order to compensate for lead resistance) and had you known such a
>> thing existed you would surely have mentioned it as an "obvious
>> option".
>
>There are two silly assumptions you've made. As I have
>stated elsewhere, I thought the OP would like a solution
>utilizing what he mentioned he had on hand. I threw out
>a few spending options without pretending to exhaust
>them, merely to let him know he was not stuck with
>just what he had. So I was not inclined to spend much
>time trying to come up with a list that none of the smart
>alecs around here would be able to "improve" upon.
>So, assuming that my non-mention reflects ignorance
>of remote sense power supplies is fatuous. 

---
In light of the fact that his power supply might have been capable of
remote sensing and in view of your statement: "I thought the OP would
like a solution utilizing what he mentioned he had on hand." It seems
to me that your "familiarity" with remote sensing supplies would, at
the very least, brought forth the question of his power supply having
that capability and, if it did, a suggestion to use that capability.
---
>The other
>laughable assumption is that it should be an obvious
>option to anybody who knew of such supplies.  
---
Key phrase here is "knew of".  If the OP wasn't aware that his supply
had that capability, or how to use it, then that option would hardly
have been obvious.  You, though, knowing that such an option might
have been available didn't bring it up either.
---
>Why should the OP go spend that kind of money when he
>can simply use a shunt or measure the resistance of his
>cable and use that and a voltmeter to measure current?
---
He probably shouldn't, but that's not what you advised him to do, you
sent him on some wild goose chase to measure this and that and
calculate this and that, when all you had to tell him was to monitor
the voltage at the input of the radio and crank the supply up to make
that voltage go to 7.5V when he was transmitting, LOL!
You also wrote:
"You could also put a lower shunt resistor across
your ammeter and calibrate the combination."
which is one of the stupidest things I've ever read.  Do you know why
or would you like me to explain it to you while typing through fits of
laughter?
---
>> Now that the cat's out of the bag, though, I suspect you'll soon
>> become the expert you'd like us to believe you already were.
>>
>> Here, I'll save you a little time on Google:
>>
>> On power supplies supplied with external sense compensation there are
>> two terminals, one usually marked "+ sense" or something like that,
>> and the other one marked "- sense" or something like that.  In use, a
>> wire is connected from the "+ sense" terminal to the + input of the
>> load at the same point the supply lead is connected to the load, and
>> the "- sense" terminal is connected from the "- sense" terminal to the
>> - input of the load at the same point the supply lead is connected to
>> the load.  That way, voltage variations _at the load_ are sensed and
>> fed back to the supply where the supply voltage is automatically
>> adjusted upward to compensate for the voltage dropped across the
>> supply leads.  If sense compensation isn't needed, the sense terminals
>> are shorted to their respective supply outputs at the supply, and the
>> supply regulates the voltage at that point.
>
>Thanks, so much John.
>> You're welcome.
>
>I note that your little description omits mention of the
>1k or so resistors that normally obviate the need for
>those jumpers when remote sensing is not used.
---
??? Funny... all of the stuff I've got around here _requires_
strapping the sense terminals to the output terminals if remote
sensing isn't used, but hey, that's only HP.
---
>Not knowing how old you are, I may be actually wrong
>about this, but there is a good chance that the incident
>I relate below happened before you had any inkling of
>what a circuit is or what 'electronics' means.
>
>Before my job as an engineering tech which preceeded
>my career as an electronics design engineer, I held a
>job as a factory test tech.  One day, in return for a
>similar level of prank, I connected an RC network
>between the so far unused sense terminals and the
>output terminals on the power supply that my "pal"
>would be using after lunch to continue troubleshooting
>some equipment.  (These machines were battery
>operated but run off of a DC supply during most test
>and troubleshooting.)  After enjoying the spectacle of
>him trying to figure out what was going on as his bench
>supply was oscillating at a low level, I went and told a
>few other techs so they could come and "help" (see).
---
So you were into chicanery back then as well?  
Old habits die hard, I see.
---
>As for the novelty of 4 wire connections for dealing
>with cable and connection drops, you could pull up
>one of my patents detailing a system that relies on that
>very concept in order to operate effectively.  (I do
>not expect any such effort from you, attached as you
>are to the notion of my ignorance.  If not for that, it
>ought to suffice to plug your "soon become" spew.)
---
Hey, sure, I'll look it up. I always enjoy a good laugh.
What's the Patent Number?
-- 
John Fields

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