Hello!

I’ve been looking into wiring up a pressure transducer, and the data sheet says the +/- Excitation and Signal are on the same pins of the PT. Not wanting to get into too much detail about the PT, but it has me wonder more about current loops.

My question: is it possible to create a 4-20ma signal with a Wheatstone bridge or a shunt circuit. I know of a few shunt circuits to create a current loop, but how does the bridge come into play?

Thanks!

 

Hello!

I’ve been looking into wiring up a pressure transducer, and the data sheet says the +/- Excitation and Signal are on the same pins of the PT. Not wanting to get into too much detail about the PT, but it has me wonder more about current loops.

My question: is it possible to create a 4-20ma signal with a Wheatstone bridge or a shunt circuit. I know of a few shunt circuits to create a current loop, but how does the bridge come into play?

Thanks!

I think more detail is needed to answer your question. Datasheet or make/model of pressure sensor?

 

Thanks for the reply. I was trying to keep it somewhat elementary. That’s why I didn’t attach any data sheets.

The PT is made by GP:50; model 312

So, transducers do use a Wheatstone bridge circuit, correct? And generally, transducers with a 4-20ma output have a built in shunt?

 

I’ve been looking into wiring up a pressure transducer, and the data sheet says the +/- Excitation and Signal are on the same pins of the PT. Not wanting to get into too much detail about the PT, but it has me wonder more about current loops.

Many pressure sensors or pressure transducers are just simple two wire devices they are placed inline with a power supply, frequently 24 VDC and output a 4 to 20 mA current. Here is the data sheet for the model you mentioned. The model 312 is just a two wire pressure sensor having a 4 to 20 mA out proportional to the selected range. The sensor is placed inline with a DC supply of 9 to 40 VDC. As to Wheatstone Bridge, "of all the pressure sensors, Wheatstone bridge (strain based) sensors are the most common, offering solutions that meet varying accuracy, size, ruggedness, and cost constraints. Bridge sensors are used for high and low pressure applications, and can measure absolute, gauge, or differential pressure. All bridge sensors make use of a strain gauge and a diaphragm. Wheatstone Bridge is likely the most common design". Some sensors have the electronics built in and some don't. For example if you look at the model 112 in the data sheet it is clearly an unamplified bridge output of 3.0 mV per Volt of excitation at full scale. Excitation of 3.5 to 15 VDC maximum. The Model 212 outputs 0 to 5 VDC so the bridge has a built in amplifier.

You choose a transducer based on the intended application as to range, output and environmental conditions it will work in.

How To Measure Pressure with Pressure Sensors is a good read on the subject.

Ron

 

Many pressure sensors or pressure transducers are just simple two wire devices they are placed inline with a power supply, frequently 24 VDC and output a 4 to 20 mA current. Here is the data sheet for the model you mentioned. The model 312 is just a two wire pressure sensor having a 4 to 20 mA out proportional to the selected range. The sensor is placed inline with a DC supply of 9 to 40 VDC. As to Wheatstone Bridge, "of all the pressure sensors, Wheatstone bridge (strain based) sensors are the most common, offering solutions that meet varying accuracy, size, ruggedness, and cost constraints. Bridge sensors are used for high and low pressure applications, and can measure absolute, gauge, or differential pressure. All bridge sensors make use of a strain gauge and a diaphragm. Wheatstone Bridge is likely the most common design". Some sensors have the electronics built in and some don't. For example if you look at the model 112 in the data sheet it is clearly an unamplified bridge output of 3.0 mV per Volt of excitation at full scale. Excitation of 3.5 to 15 VDC maximum. The Model 212 outputs 0 to 5 VDC so the bridge has a built in amplifier.

You choose a transducer based on the intended application as to range, output and environmental conditions it will work in.

How To Measure Pressure with Pressure Sensors is a good read on the subject.

Ron

Thanks, Ron! I’ve saved that link to National Instruments site. NI has a ton of good stuff on their site. I didn’t even think to look there as I’ve used their stuff in the past.

I know I bit more about current loops now, mainly the 4-20ma. I’m actually getting interested in building a simple PT with a 4-20ma output just so I understand it that much more.

 

Probably all versions of that pressure transducer have the same wheatstone bridge inside. The 312 model with 2-wire 4-20mA probably has a front end IC like this which powers itself, the bridge, and an internal measurement circuit, all from the loop (without drawing >4mA) and adjusts the output current according to feedback from the bridge.

 

Thanks, Ron! I’ve saved that link to National Instruments site. NI has a ton of good stuff on their site. I didn’t even think to look there as I’ve used their stuff in the past.

I know I bit more about current loops now, mainly the 4-20ma. I’m actually getting interested in building a simple PT with a 4-20ma output just so I understand it that much more.

NI does have some great stuff, we used quite a bit of it, including software before I retired going on 5 years ago. Comes with a price but really good stuff. NI also has some great application engineering support when needed.

As to the 4 to 20 mA loops, while there are analog input modules designed for 4 - 20 mA I frequently would just run the loop through a precision 250 or 500 Ohm resistor, get a voltage drop of 2 to 10 or 1 to 5 volts and run it into an ADC doing the math in the software. That got me any engineering units I wanted.

Ron

 

A

I frequently would just run the loop through a precision 250 or 500 Ohm resistor, get a voltage drop of 2 to 10 or 1 to 5 volts and run it into an ADC doing the math in the software. That got me any engineering units I wanted.

Amazing how well Ohm's law works

 

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Amazing how well Ohm's law works

Yeah, it just keeps working and working. You can actually put it on your breakfast cereal. You just have to love it.

Ron

 

Probably all versions of that pressure transducer have the same wheatstone bridge inside. The 312 model with 2-wire 4-20mA probably has a front end IC like this which powers itself, the bridge, and an internal measurement circuit, all from the loop (without drawing >4mA) and adjusts the output current according to feedback from the bridge.

Your comment (without drawing >4mA) is what clarified it for me. I know I can read a datasheet and a print, and I wire up sensors to what the print looks like, but that doesn't mean someone actually understand what's going on. I was confused because how would I have an input (supply voltage) and an output (4-20mA) be on the same pins. I understand it for proximity sensor circuits, but didn't think it could work in a current loop.

 

You need to design the electronics so that it runs on less that 4mA. Then the output of that drives a current controller. Therefore the measured signal controls the current. 0% = 4mA and 100% = 20mA.
Have a look at the LM10 data sheet ....

http://www.linear.com/docs/1738