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

I'm trying to get an idea of how 4-20ma transducers work. A simple schematic would be very helpful.

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.

Reply to
laylow
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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)

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-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

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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.

Reply to
Spehro Pefhany

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.

Reply to
laylow

quoted text -

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

Reply to
laylow

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.

Reply to
Jasen Betts

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