4-20mA Pressure Project with Splitboard

[barkingsheltie]

@jim-p

I see you are now using a different protection circuit with a 120Ω loop resistor and a 10KΩ resistor in series. That arrangement will decrease your resolution and reduce the bandwidth. However you could use the internal reference but it is not accurate at all with a -6%/10% error range, hardly useful if you are trying to make precise measurements.

I recommend that you use an accurate external reference.

Hi Jim,

I am familiar with Fritzing app - before I stumbled upon DIYLC, this is what I started with. However, the stripboard which is available has two columns which are cut in the middle rows 5 and 15 - and there are components, such as the 2-pin JST-XH that don't exist - found a couple libraries, but there are challenges - the JST-XH library is visual only and doesnt work with simulation or schematic.

For now (next few days), I don't want to sidestep to learn how to make my own Fritzing parts - DIYLC is easier to use. Making the Illustrator schematic was fast & simple. If these are not acceptable to post here, I understand, and won't post my DIYLC/Illustrator again until I can produce a suitable Fritzing diagram. Is that the message I should take here?

Reference Voltage
You gave me more to think about - thanks.
I understand your point about the 6-10% error - I was thinking of calibrating for THIS box, and calculating the offset (slope) and storing in EEPROM.

The only sensor data that is dynamic is hydraulic - air is very steady and a little noise is ok. The hydraulic is at most 100hz. The air signal jumps around here, but was due to compressor issues.

I need to ensure I have the correct layout the Schottsky clamp, & that I understand the layout ins/outs with the splitboard - once this is complete, today or tomorrow, then will work on the final bit with reference voltage.

[Wawa]

@jim-p

I recommend that you use an accurate external reference.

That's why I recommended to use a 120 Ohm resistor
and switching to the stable 2.56volt internal reference of the Mega.
Leo..

void setup() {
analogReference(INTERNAL2V56; // switch Aref to 2.56volt
}

Edit: calibration is always needed, because the resistor might not be exactly 120 Ohm and 2.56V Aref is not factory calibrated.
Please post the code that you're currently using.

[jim-p]

@barkingsheltie

I understand your point about the 6-10% error

That is the acuracy of the internal reference the @Wawa suggested you use. Your measurements can never be any more accurace then the reference you use.

[jim-p]

@barkingsheltie

If these are not acceptable to post here, I understand, and won't post my DIYLC/Illustrator again

No they are OK just make sure they are readable. For instance I missed those little green squiggles you had that conneced the grounds. However don't post a schematic if it does not reflect what you are actually doing, it just makes things confusing.

[Wawa]

@jim-p

you could use the internal reference but it is not accurate at all with a -6%/10% error range

Which is not important. You can calibrate that completely out, at which point stability becomes more important. Then the internal Aref wins over the potentially less stable default Aref, which is the 5volt supply.
Leo..

[jim-p]

@Wawa

Then the internal Aref wins over the potentially less stable default Aref

but not an external reference as I suggested.

[tigger]

4 to 20-mA devices in industrial settings generally use 24-V DC as the power source.

You put a 500 or 250 Ohm precision resistor in the DC circuit and measure the DC voltage across the resistor. A 500 Ohm resistor gives you 10-volts at 20-mA.

24-V DC is chosen as it allows several measuring devices to be on the same circuit, for example, a chart recorder and a SCADA analogue input module.

If your pressure sensor will work off a lower voltage, you can scale down things.
A 125 Ohm resistor at 20-mA will give you a full scale output voltage of 0.5 at 4-mA and 2.5-V at 20-mA. You have to offset the 0.5-V to make it sensible.

The specification of the 4 to 20 device will determine the working voltage you need to use.