Current and voltage sensor output

Thread Starter

alyeomans

Joined Sep 13, 2010
39
I came across a sensors which lets you calibrate the output for either 4-20mA current or 0-5 voltage output. In this sensor the outputs are mutually exclusive and I am intrigued about the circuit and was wondering if anyone had seen something like this?

I have been pondering the circuit and started by finding the below circuit from the LTC1453 datasheet.

4-20 output.png

Looking at the above example I could not see a direct way to achieve both kinds of outputs. I've put together another circuit which simplifies things a bit and just may possibly work.
different output.png
I have been considering different locations. Any ideas?
Al
 

Reloadron

Joined Jan 15, 2015
6,712
Taken from the LTC 1453 data sheet.
"The LTC® 1451/LTC1452/LTC1453 are complete single supply, rail-to-rail voltage output 12-bit digital-to-analog converters (DACs) in an SO-8 package. They include an output buffer amplifier and an easy-to-use 3-wire cascadable serial interface. The LTC1451 has an onboard reference of 2.048V and a full-scale output of 4.095V. It operates from a single 4.5V to 5.5V supply".

The output in your top example takes a Vout from the DAC and drives a simple operational operational amplifier circuit using a LT 1077 but the LT 1077 can only source 10 mA so it in turn drives the 2N3440 NPN transistor with a collector current rating up to 1.0 Amp. Now we get an output of 4 to 20 mA based on a voltage generated by our D to A converter. So what happens at Iout if I place a 250 Ohm resistor to ground? This assumes a Vloop applied of 24 VDC. What will the output be now? How about if I place a 500 Ohm resistor between Iout and ground? Now the 4 to 20 mA becomes 1 to 5 VDC out or 2 to 10 VDC out. This is why 1 to 5 VDC and 2 to 10 VDC are popular. Adding another operational amplifier I could get 0 to 5 VDC or 0 to 10 VDC. Overall it's just a matter of placing a resistor across the Iout to ground.

Ron
 

Thread Starter

alyeomans

Joined Sep 13, 2010
39
Many thanks Reloadron. Yes that makes sense however the crux of the matter is the device can be either a 4-20mA or 0-5V output without circuit change. For the 4-20mA output a resistor is used at the measurement end via a resistor of which value the sensor does not know accurately and for the voltage just reading without the resistor.

I can confirm the device supplies the power for the current loop and it is allowable join the signal negative to the sensor power negative. So does this hint to having the sensor output at R3 on my circuit?

Al
 

Reloadron

Joined Jan 15, 2015
6,712
In your lower drawing R3 is a current sense. This is really a very common and practical current source circuit using an op-amp and a transistor. R3 provides feedback to the inverting (-) input of the op-amp to maintain a constant current. This link provides a pretty good explanation of how it works. Sensors which offer the option of a voltage of current output are pretty common. Most just place a resistor across a current source which can be switched in or out. I also have some sensors laying around here which are as you mention "loop powered" which I used quite a bit for pressure and temperature sensing.

Would you be sensing anything or is this just a learning exercise?

Ron
 

Thread Starter

alyeomans

Joined Sep 13, 2010
39
...Sensors which offer the option of a voltage of current output are pretty common. Most just place a resistor across a current source which can be switched in or out. I also have some sensors laying around here which are as you mention "loop powered" which I used quite a bit for pressure and temperature sensing.

Would you be sensing anything or is this just a learning exercise?
Thanks Reloadron, This is learning exercise at the moment however chances are i'll implement to a pressure sensing product in the future. I like the idea of calibrating the sensor and calibrating the output separately.

Cheers
Al
 

Reloadron

Joined Jan 15, 2015
6,712
Thanks Reloadron, This is learning exercise at the moment however chances are i'll implement to a pressure sensing product in the future. I like the idea of calibrating the sensor and calibrating the output separately.

Cheers
Al
Keep in mind when you calibrate a sensor, any sensor, you are looking at and calibrating the whole system such as sensor along with any display or indicating device. Some sensors actually allow calibration of the sensor/transmitter itself while some don't.

Have a good day and when or if you actually set something up return and ask questions. :)

Ron
 
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