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'[EE] Anyone have a 4-20ma loop calibrator circuit?'
2006\05\12@092742 by Roy

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part 1 659 bytes content-type:text/plain; (decoded 7bit)

Anyone have a 4-20ma loop calibrator circuit?

That is a 4-20ma source and sink.

I was thinking of using a AVR to do the control side but have not got my
head around how to source / sink 4-20 ma in circuits that have different
voltages i.e. 5v to 50v.

Is there a reference to a standard for 4-20ma testing?

_______________________________________

Feel the power of the dark side!  
Atmel AVR

Roy Hopkins
Tauranga
New Zealand
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part 2 35 bytes content-type:text/plain; charset="us-ascii"
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2006\05\12@100818 by Spehro Pefhany

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At 01:27 AM 5/13/2006 +1200, you wrote:
>Anyone have a 4-20ma loop calibrator circuit?
>
>That is a 4-20ma source and sink.
>
>I was thinking of using a AVR to do the control side but have not got my
>head around how to source / sink 4-20 ma in circuits that have different
>voltages i.e. 5v to 50v.

You make a current sink or source (your choice since it's going to look
like a two-terminal device) with sufficient compliance,
accuracy and power-handling capability for your application.

To simulate a loop-powered instrument you connect the sink/source in series
with the circuit. To source current you need an internal power supply with
sufficient voltage for all possible conditions (including the compliance of
the sink/source and the drop in your measurement circuitry) and that goes
in series with the sink/source and the load(s).

There are lots of very accurate commercial calibrators out there with lots
of features. It's cheaper to buy one than try to design one unless your
time is worth very little.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
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2006\05\12@102002 by Alan B. Pearce

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part 1 955 bytes content-type:text/plain; (decoded 7bit)

>Anyone have a 4-20ma loop calibrator circuit?
>
>That is a 4-20ma source and sink.
>
>I was thinking of using a AVR to do the control side but have not got my
>head around how to source / sink 4-20 ma in circuits that have different
>voltages i.e. 5v to 50v.
>
>Is there a reference to a standard for 4-20ma testing?

Shouldn't be too hard to do a transistor with a suitable emitter resistor
for the current desired, while the base is fed from a reference voltage. I
know guys here have used that system for putting known currents through
thermistors.

Know change the reference voltage to a suitable DAC, and you should have
something suitable. Without knowing just how accurate you want the known
current, you may be able to use a PWM to drive it, in which case you would
get away with a smalllish 16F series, maybe the top end 12F, depending on
how you talk to it to adjust the current.



part 2 35 bytes content-type:text/plain; charset="us-ascii"
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2006\05\12@134109 by Peter

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LM317L + 10 turn pot with scale

Peter

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