# Current loop or 4-20ma signal generator - How does it work?

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I'm trying to get an idea of how 4-20ma transducers work.  A simple

Many measurement transducers that I've seen accept a wide range of
power supply voltage yet return a controlled current signal.  They
must do more than just vary resistance or else the current provided
would vary with supply voltage.  There must be some method of
monitoring the supply voltage and returning a proportional load to the
circuit.

Does anyone have a circuit for a simple current loop signal transducer
that I can look at?  (I'm not talking about the induction coil type
that actually induces a coil through a hot wire).

Thanks.

Re: Current loop or 4-20ma signal generator - How does it work?

Most of them monitor the supply current and use negative feedback
to hold it at the the level appropriate to the property whose value
they are signalling. You can't do this with a simple circuit, though you
can do it with a single chip (such as the Analog Devices AD693)

--
Bill Sloman, Nijmegen

Re: Current loop or 4-20ma signal generator - How does it work?
On Tue, 27 Jan 2009 06:51:57 -0800 (PST), laylow

http://www.analog-innovations.com/

See SED schematics, SpehroLoopProtect2.pdf

You don't need the protection circuit with some op-amps-- Jim did this
to illustrate how to prevent latch-up with an LM324 type amplifier.

Re: Current loop or 4-20ma signal generator - How does it work?
Thanks.  I guess it's good to know that a simple circuit does not
exist so that I can stop looking.  The AD693 is very cool, too bad
they cost \$20 a pop.  The project I had in mind would have needed 16
of these.

I am going to have to model the SpehroLoop to see how it works.  It
may work well enough for my purposes if it does what I think it does.
I should be able to cut the line at Qclamp and measure 4-20ma directly
proportional to voltage at Vx.  I'm guessing that the supply voltage
doesn't really have to be 36V.  The smallest current output for the
LM324 that I see is 40ma.  Does this circuit cut that in half or will
it put out 40ma if given the voltage signal.  I guess I'll find out
soon enough.

Thanks again.

Re: Current loop or 4-20ma signal generator - How does it work?

This is cool.  I'm still playing with it to understand how it works
but it does work very nicely.

Re: Current loop or 4-20ma signal generator - How does it work?

here's the simplest one I can think of.

in   out
---[7805]--[250]-[1000]-.
gnd |             ^   |
|             |   |
------+-------------+---'

7805 is a LM7805 or similar
250 is 250 ohms resistor
1000 is 1K potentiometer.

here's a fancier one.

.............................
|                           |
---[7805]-+-[425]-+---------.         |
|           |       __|__       |
------+          _|_|     |    |    |-'C
|         |/ \      | uC |----|
|           T  3.3  |____|    |-.E
|           |          |        |
`-----------+----------+-[150]--'

uC is a microcontroller with a DC output upto 3V
the other (new) parts are a 3.3V zener (or similar)
NPN transistor and two resistors  not shown is the
sensing circuit connected to the microcontroller
and powered from the same 3.3v rail.