need help 4-20ma circuit

[jules28]

Hi
I have to convert an existing circuit to run a more modern gauge due to obsolete parts.
I have a 28.8volt power supply a variable resistor 10-180ohms which measures oil pressure, of which at 10 ohms i wish to output 4ma and at 180 ohms i wish to output 20ma to drive a gauge which has 106 ohms across the terminals.
My knowledge is basic and would be grateful for help

Jules28

 

[ericgibbs]

Hi
I have to convert an existing circuit to run a more modern gauge due to obsolete parts.
I have a 28.8volt power supply a variable resistor 10-180ohms which measures oil pressure, of which at 10 ohms i wish to output 4ma and at 180 ohms i wish to output 20ma to drive a gauge which has 106 ohms across the terminals.
My knowledge is basic and would be grateful for help

Jules28

hi,
Look at this option.
Is the 28V supply and/or 106R load floating.??

 

[jules28]

Hi
Thanks for reply,i am not quite sure what you mean by floating but the 28 volt supply shares a common ground/neutral with the variable resistor and comes from a battery bank with charger regulated to 28.8V,the gauge has just the two terminals and i believe is just a straight ammeter type measuring 4-20ma.
In the design drawn how accurate would the 12 volt supply need to be and what device is U2 ?.
Sorry for my ignorance
Jules28

 

[ericgibbs]

Hi
Thanks for reply,i am not quite sure what you mean by floating but the 28 volt supply shares a common ground/neutral with the variable resistor and comes from a battery bank with charger regulated to 28.8V,the gauge has just the two terminals and i believe is just a straight ammeter type measuring 4-20ma.
In the design drawn how accurate would the 12 volt supply need to be and what device is U2 ?.
Sorry for my ignorance
Jules28

hi,
You could get a operational amplifier that would work from 28V

If the ammeter is not connected to ground in anyway, that is the both leads are free, then connect the ammeter in place of the 106R resistor.

A simple 7812 voltage regulator would give a steady 12V from the 28V, added a couple of decoupling capacitors across the 12V output, say 10uF and 100nF

 

[ericgibbs]

hi Jules,
This circuit will provide more current than a basic OPA, again I have shown a 12V supply.

 

[jules28]

Hi
Thanks again, i will build over the winter but probably wont use until April, will let you know the outcome.
One last question i notice two trimmers how accurate would it be without these? +- 10% is fine.
Regards
Jules

 

[ericgibbs]

Hi
Thanks again, i will build over the winter but probably wont use until April, will let you know the outcome.
One last question i notice two trimmers how accurate would it be without these? +- 10% is fine.
Regards
Jules

hi,
If you use 1% resistors, it should be better than 10%.
Its important that you use the version with transistor because of the 20mA thru the meter.

 

[KeepItSimpleStupid]

Why not use an off the shelf part?

See companies like Action instruments, Acromag and m-systems M-System - Your I/O Superstore

It seems as though you want to convert a resistance to 4-20 mA and you can provide a regulated voltage source, so that you can convert the resistance to a voltage?

Depending on your system, you may ormay not require an isolated output.

 

[jules28]

Hi
Thanks for reply,however cost is too high for me!
Jules