rjvh

Hi can sombody explane me a bit about this subject please

as i work with a lot of sensors and control equipment i mainly use the voltage controled ones
standard is 0-10V and 0-5V so far no problems working with them and undesrstand how to manipulate and use the signal

now we do also have a 0-20mA and a 4-20mA sensors

and personaly i don't get the theory completly and do have problems how to handel these signals

these mA sensors are more used where long distance of cable is involved but you will still have a voltage drop over the cable

at the end of the line i think that the current is measured by the voltage over a resistor with a low tolerance and then handled by opamps again as a voltage signal (please educate me if it is not like this)

do I see it right that the power supply for these sensors are much more critical on the occasion of usage than the voltage sensors??

because i don't see differents in power reqirements in the sensor manuals??

while if you take the cable resistance and a given current the power suply have to vary in voltage for every different sensor conected on it unless they have all the same cable resistance

I do see circuits conected with several sensors but i don't see and don't know how they are uncoupled from each other

Thanks for any replies

Robert-Jan

ericgibbs

Hi Robert Jan
Hope this helps, please feel free to ask for more detail, if required...

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rjvh

Hi eric thanks for the info

is there a sertain standarised resistance in the sensors (like you mentioned 250hmor not
and if there is does the sensor have a voltage regulation build in as that majority of the sensors are feed with 12V to 24V?

Robert-Jan

ericgibbs

hi,
The resistor value of 250R is chosen to give +5V at 20mA, so it can be connected directly to a PIC.
For PIC working with a Vss of 3.3V you would chose a 165R resistor.
Remember the ZERO will be +1V thats 4mA thru the 250R.

If you look at the sensor spec sheet it will say what the Vmin for the supply voltage is, mine have always been 12Vdc.
I used a 24Vdc psu to drive the sensor and the cable, so 12V + 5V across the resistor = 17V... that leaves a permitted drop in the cable of 7Volts.

Knowing the resistance/meter of the cable you can calculate the max length of the cable to give 7V maximum drop.
If required you can increase the psu to say 28V. BUT you must also consider the Vmax the sensor spec states. about 28/30Vdc in my experience.

Does this help.?

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rjvh

if you have as in your exsample a permited voltage drop of 7 volt by max sensor output but your voltage drop is max 5V over the cable so there's still 2V left open
you have to adjust this on your power supply or put another resistor so that there is a voltage drop over the "compensation" resistor from 2V

do i understand that good?

Robert-Jan

ericgibbs

hi,
No, another resistor is not required.
Consider the 'sensor' has a quoted voltage operating specification of 12Vdc thru 24Vdc.
This means the 'sensor' has an inbuilt voltage regulator that can handle 12Vdc thru 24Vdc.

So any 'excess' voltage appearing at the 'sensor' is taken care of by the internal regulator.

Think of the 'sensor' as a LM317 connected as a Constant Current source,say set for 20mA.
If you now connected the LM317 to a variable psu, the constant current would remain
the same over a wide range of input voltages.

Consider now that the resistor thats sets the constant current in the LM317 is able to change, acting as the 'sensor element' then the constant current would vary.

You could measure this change in current by adding a series resistor, say 250R and measuring the voltage across the resistor.

Another important point about 4 to 20mA loops, is the current will still remain the same even if the 250R is now 0R.

If its open circuit, the sensor will not work.

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rjvh

it's clear for me now

i will be soon playing with these sensors

Thanks
Robert-Jan

ericgibbs

Hi,
Which type of sensor and what are you going to measure.?

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rjvh

water pressure that have to comand VSD and comparators

pressure sensors i use are from DS europe

they have them in all the standard ranges and outputs but
i am constanly looking for other sources as i don't want to rely to much on one suplier(price and delivery reasons)

Robert-Jan

ericgibbs

hi,
My company product range was Hydrographic surveying electronics, tidal water gauges [4 to 20mA] , range finder lasers,
depth echo sounders, digitisers,data loggers, radio telemetry..etc.

If I can help, please ask..

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PIC tutorials:
Gramo's: www.digital-diy.net/
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rjvh

i will sent you a PM


Robert-Jan

Krumlink

As for sensor outputs that have possibly low power/yield, couldn't you put them into a operational
amplifier with low distortion? Seems like that would work.

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rjvh

Hi krumling

sensors in the industry envoirement do use only standarize output so you they are exchange able with other brands and types without that you have to adapt the equipment that it is conected

if you want to use the signal (and in the conected equipment 99% is) i always put it in a opamp just for buffering if i want to swich on the signal or re distribute it to more devices

Robert-Jan

Krumlink

Ok thanks a lot, that helped solve a few conundrums.

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Leftyretro

The 4-20ma analog instrumentation standard was developed for the petro-chemical industry many decades ago.

The thing to remember in these kind of instrumentation 'loops' is that the sensor is an active device that also knows it's output current value and constantly dynamically adjusts it's output to the value it's sensor is measuring. This allow it to be used in wire loops from any distance up to several thousand feet without having to be concerned about the voltage drop in the wire run. The typical measurement resistor at the receiving end is 250 ohms, resulting in a 1 to 5 volt measurement range, however any resistor load can be used to taylor the voltage measurement range desired up to the limit of what the loop power supply voltage can deliver.

As I said the typical 24 vdc loop power can power a loop with several thousands of feet or wire and the 250 ohm load resistor. The reason for the 4 ma 'zero' value (called live zero) is the power generated in the 0-4ma range powers the active circuitry in the sensor (generally called transmitters) and also allows the receiving circuits the ablity to detect a bad measurement value if there is less then 4ma flowing in the loop, say as a result of a broken wire, otherwise it would be impossible to determine if a 0ma signal was an valid active measurement or a open wire. Also a shorted wire loop would result in a larger out of range value of up to 96ma (assuming a 24vdc loop and a 250 ohm load resistor and ignoring loop resistance) that the recieving circuitry could also ignore as being a bad out of range measurement value.

The 4-20ma standard is a very robust, reliable, accurate and well known method of measureing remote sensors. The main disadvantages are that each measurement loop requires it's own dedicaded wire pair and that the sensor requires active electronics devices at the sensor end of the loop. These kinds of sensors (industrical process transmitters) typically cost many hundreds of dollars (and up) but were designed for continous operation in very harsh enviroments.

Lefty

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