4-20 mA conversion
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Posts: 46,084
Techno Masai wrote:
>Hi Gerry,
>Nice post, I've been interested in current loop stuff since it seems to be
>the way to go with sensors as far as noise reduction, false signals,etc.
>Trouble is I don't know how to get sensor mV output translated into the
>current loop and then get it back out again.
>I would be interested your circuits, can you post them?
Thank you for the kind words. The circuit I have should work with milli volt
signals as the gain just needs to be turned up but extra filtering may be
required to take out any noise superimposed on the signal before it hits the
op amp; that will take a little more brainpower but a cascaded RC network
should do the trick.
A cautionary note on millivolt signals!!! I primarily see these used on load
cells and thermocouples. One way to keep the noise down is to use shielded
cable and ground the shield at one end only. Another problem with
thermocouple wire is the physical interface. It has to be of similar
material; otherwise there is a potential problem of two dissimilar metals
creating a galvanic effect that can induce noise onto the signal and
possibly produce an offset voltage that will skew the results. If you can
mitigate these problems, you are more than halfway to resolving any issues
with these signals. I find them fairly finicky to work with but since
thermocouples are cheap, compared to devices like RTD's, they are still
popular. Also once the millivolt signal is converted into current, I would
just leave it in that format until you get to the ADC chip. Then just use a
voltage dropping resistor to turn the current into a 1-5 VDC or a 2-10 VDC
signal. From there the Basic Stamp can do all the rest in terms of
displaying the results, alarms, control, etc.
If you need any further assistance please let me know. As for the circuit,
this is going to be tricky to post because the originals are hand drafted
and I do not have a full blown version of Adobe to make the PDF conversion
happen. What I may have to do is one of two things:
a. send you a copy via snail mail; or
b. re-draft in AutoCAD and hope you have a program that can read the file. I
can save it in a bitmap format but this does not always work when you try
and open it up.
If anyone has any ideas or suggestions, please let me know. As an
alternative, I can make up this circuit into a board using my TraxMaker
program and just flog them complete and ready to go. If anyone wants price
and delivery on this, feel free to contact me.
Thanks and regards,
Gerry Shand
gwshand@c...
(780) 471-0470
>Hi Gerry,
>Nice post, I've been interested in current loop stuff since it seems to be
>the way to go with sensors as far as noise reduction, false signals,etc.
>Trouble is I don't know how to get sensor mV output translated into the
>current loop and then get it back out again.
>I would be interested your circuits, can you post them?
Thank you for the kind words. The circuit I have should work with milli volt
signals as the gain just needs to be turned up but extra filtering may be
required to take out any noise superimposed on the signal before it hits the
op amp; that will take a little more brainpower but a cascaded RC network
should do the trick.
A cautionary note on millivolt signals!!! I primarily see these used on load
cells and thermocouples. One way to keep the noise down is to use shielded
cable and ground the shield at one end only. Another problem with
thermocouple wire is the physical interface. It has to be of similar
material; otherwise there is a potential problem of two dissimilar metals
creating a galvanic effect that can induce noise onto the signal and
possibly produce an offset voltage that will skew the results. If you can
mitigate these problems, you are more than halfway to resolving any issues
with these signals. I find them fairly finicky to work with but since
thermocouples are cheap, compared to devices like RTD's, they are still
popular. Also once the millivolt signal is converted into current, I would
just leave it in that format until you get to the ADC chip. Then just use a
voltage dropping resistor to turn the current into a 1-5 VDC or a 2-10 VDC
signal. From there the Basic Stamp can do all the rest in terms of
displaying the results, alarms, control, etc.
If you need any further assistance please let me know. As for the circuit,
this is going to be tricky to post because the originals are hand drafted
and I do not have a full blown version of Adobe to make the PDF conversion
happen. What I may have to do is one of two things:
a. send you a copy via snail mail; or
b. re-draft in AutoCAD and hope you have a program that can read the file. I
can save it in a bitmap format but this does not always work when you try
and open it up.
If anyone has any ideas or suggestions, please let me know. As an
alternative, I can make up this circuit into a board using my TraxMaker
program and just flog them complete and ready to go. If anyone wants price
and delivery on this, feel free to contact me.
Thanks and regards,
Gerry Shand
gwshand@c...
(780) 471-0470