Hello,
I'm interested in connecting TOC-903-X5 gas transmitter to Arduino to log the gas levels over time.
I'm having trouble understanding how to wire the analog wire from the transmitter. In all the drawings I've seen there are only 2 wires and you need to connect a 250ohm resistor to change the 4-20ma signal to 0-5V the Arduino can read.
Is it the same case here? just connect a 250 ohm resistor to the analog wire and than straight to the Arduino?
Thanks for the help
Yeah, but your Arduino needs to share a common GND with the sensor.
So it'll be like this:
Instead of 250R I'd pick something a little smaller so that the Arduino will actually be able to read the full range. The effective supply voltage of an Arduino may be a little less than 5V in practice and this would make the upper part of the sensor's output potentially go out of range. A 220R resistor will give a 4.4V range and it's still within the permissible range as per the image you showed.
Thanks for the quick reply.
So basically I just need to connect the blue wire to a resistor, and from there one wire to an analog port in the Arduino and one wire to the Arduino gnd ?
And another question... if i have a DC/AC Current Sensor (ACS70331), can i connect it without using the blue analog wire of the detector?
You mean using the acs70331 to measure the 4...20mA sensor current? That wouldn't make sense. Just use a resistor. The acs isn't sensitive enough by a long shot to get meaningful data from a 4-20mA signal.
And this part is correct?
See the schematic. Your formulation is inconclusive and might be correct or it might be incorrect depending on how it's read.
Why not use this diagram?
Page 15 of manual linked below.
As far as I can tell the transmitter requires an external power supply of between 18 to 24 volts DC. I don’t comprehend the meaning of the little graph labeled “Allowable Operating Region”, but the specification says: power 18 - 30 volts DC on page 4 of this manual:
The two connection diagrams in the document both show “Typical Control Card Connection”. That block is your Arduino in your case.
I guess I sort of understand it. Depending on the sensing resistor there is a minimum supply voltage requirement. But the sensor will tolerate a supply range of 18 - 30 volts. With a 800 ohm sensing resistor a 30 volt supply is needed. With a 24 volt supply the max resistor will be 500 ohms.
24 volts is 50% (24 - 18)/(30 - 18) of voltage range so 50 % (((800 - 200) * 0.5) + 200) of resistance range is 500 ohms.
By similar calculations 250 ohms is 8.33% of resistance range so a minimum of 19 volts is required.
Looking at this diagram I question the connection between the PSU “0 volt “ terminal and the “Sense” terminal of the input card. It would make more sense if it was connected to the input card “0” volt terminal, but I don’t see why it is required at all.
If I were using this sensor I would have to inquire about that with the factory personnel.
Looks like not enough information, and I looked at the entire datasheet, too.
I would start with a voltmeter (or oscilloscope), try to run it over the full range and see what it does.
For a 250ohm sense resistor it might not matter so much that there is probably a few Kohm resistor inside the analog input, in series with the sampling capacitor.
If you decide to go much bigger, you might want to read it through a follower.
This might be a bias connection that ensures that the signal is biased at the PSU's 0V. IDK how the sensor design internally is done; maybe the analog output is DC-isolated in which case it basically floats unless/until it's biased.
With 4-20 mA instruments there are two categories. Rack powered and field powered. Field powered instruments are typically powered by 120 VAC (in N. America) or 220 VAC (in the rest of the world). Rack powered instruments are powered by the typically 24 VDC supplied by the input card of the DCS or PLC or SCADA system. This is a somewhat different example where the field power is 24 VDC.
Gas sensor systems are often needed by companies that don’t have any of these systems. In that case the vendor will sell a central data collection/monitoring system.
In any case I have never seen a setup that requires a connection as shown in the page 15 diagram for any instrument.
For a quick test I would supply the 24 volts as shown and put an amp meter and a 200+ ohm resistor in series to form the output loop without the extra connection from the PSU (-) and the output positive and see what happens when the calibration gasses are applied.
4-20 mA signals don’t require this sort of bias. In fact you have to be careful not to have more than one common or ground point to avoid creating a “ground loop” which is another way of saying multiple return paths for the signal. That extra wire looks to me like something we would normally avoid. I guess the ultimate authority would be to contact the manufacturer with this question.
