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Amp Output and DC Output

07/20/2009 8:39 AM

relation between 4-20mamp output & DC output from measuring equipment

why we are using resistance across 4-20mamp output signal in electronic measuring instruments.

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Guru
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#1

Re: relation between 4-20mamp output & DC output from measuring equipment

07/20/2009 8:45 AM

Ohms law has it covered...

V = IxR
You presumably know the value of the resistance (R) you are using, if you measure the voltage across it (V) (with a high impedance meter so that the meter draws negligable current) you can work out the current (I) flowing
Del

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#3
In reply to #1

Re: relation between 4-20mamp output & DC output from measuring equipment

07/20/2009 8:51 AM

Yep, and it's linear! One might even pick a resistor such that 4 mA => 4 volts, and 20 mA => 20 volts, but one must also be cautious not to add too much resistance to the loop, so the above is merely a suggestion. Many loops only have a compliance voltage of 24 VDC or so. Good answer, Del.

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#4
In reply to #3

Re: relation between 4-20mamp output & DC output from measuring equipment

07/20/2009 8:53 AM

Good answer, Del.

Cheers, I was up all night researching it

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#5
In reply to #4

Re: relation between 4-20mamp output & DC output from measuring equipment

07/20/2009 9:05 AM

Shucks, we were up all night watching "The Open" for the last few days. Lucas Glover, a former Clemson golfer who won the US Open, started play in the second round about 2 AM here on the east coast of the USA. It's hard to pay attention at 2 AM, so I recorded it.

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#6
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Re: relation between 4-20mamp output & DC output from measuring equipment

07/20/2009 9:10 AM

Over this side of the pond Lee Westwood won it.
Who won your version?
Del

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#7
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Re: relation between 4-20mamp output & DC output from measuring equipment

07/20/2009 9:42 AM

I shoulda been Tom Watson, but what the heck, 59 year olds aren't supposed to do such things. I know I couldn't have won the Open 2-3 years ago (or 30 years ago, for that matter ). Stewart Cink's face must have been "pasted" over Lee Westwood's face over on this side. Tell Lee I said, "Congratulations!"

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#8
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Re: relation between 4-20mamp output & DC output from measuring equipment

07/20/2009 3:31 PM

Stewart Cink's face must have been "pasted" over Lee Westwood's face over on this side.

PMSL.... excellent
Yeah come to think of it they had Lee taking the first steps onto the Moon too. Maybe I should check the sell by date on those mushrooms?
Del

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#10
In reply to #8

Re: relation between 4-20mamp output & DC output from measuring equipment

07/21/2009 8:10 AM

PMSL.... excellent

I know I may be senile (may -- my foot!), but what is PMSL?

Surely you don't believe that Lee was anywhere near the moon! It was all a hoax perpetrated by NASA, and the whole thing was filmed in a secret sound stage in Roswell, NM. If you look closely at some of the reflections in the "astronauts'" face shields, you can barely make out a director with a megaphone.

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#11
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Re: relation between 4-20mamp output & DC output from measuring equipment

07/21/2009 8:25 AM

Piss My Self Laughing...
as opposed to RAOFL roll around on floor laughing.
Sometimes it sooo funny I just fall off my chair with a <flump thud>
Del

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#12
In reply to #11

Re: relation between 4-20mamp output & DC output from measuring equipment

07/21/2009 9:20 AM

Thanks for the enlightenment! My heritage is Scots-Irish, English and Welsh, but I'm not familiar with all Brits' acronyms. As far as I recall, I never knew what PMSL was.

On the other hand, I do know what R(A)OFL is.

Have a good afternoon!

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#2

Re: relation between 4-20mamp output & DC output from measuring equipment

07/20/2009 8:48 AM

Hmmm... <Cough>

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#9

Re: Amp Output and DC Output

07/20/2009 4:13 PM

The most common use of a resistor across a 4-20 mA circuit is to turn the loop current into a voltage which may be easily measured. If the resistor is 3 ohms then the voltage will be 12 to 60 mV. If the resistor is 250 ohms then the voltage is 1 to 5 VDC. If the resistor is 500 ohms then 2 to ten volts. Those are the most common burden resistances IMO.

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