# Reading a 4ma-20ma sensor with ADS1115 ADC - resistors and voltage range

Thread Starter

#### zirconx

Joined Mar 10, 2010
130
Hello. I'm working on a project to monitor the water level in my sump pit using a raspberry pi. I've ordered a submersible transducer sensor, which outputs 4-20ma of current to indicate 0 to 5 meters of water depth. I plan to convert this current into a voltage using a resistor, then read that voltage using an ADS1115 analog-to-digital converter. I have questions about what resistor to use and the resulting voltage range that will be measured.

My pit is only 1M deep. I think it would good to calibrate the resistor/voltage circuit so that I get more "range of motion" in the area I care about. I've read that it's common to use either a

100R resistor - resulting in a voltage range of .4v-2v
250R resistor - resulting in a voltage range of 1v-5v

Since my pit is only 1M deep, that means for a 100R resistor I'd probably only be reading a range of .4v - .7v. It seems like it would be harder to get an accurate measurement of water level when translating from such a small voltage range of (.7-.4)= .3 volts (considering about .3v per meter).

So I am thinking of using something like a 470R resistor, which gives a theoretical voltage range of 1.8 - 9.4v, but since my sensor will never be more than 1M deep, I would get voltages between 1.8 and 3.3. Which leaves me with a range of 1.5v to indicate the depth 0-1 meter.

Does this seem like a good idea? I would of course test the circuit with a voltmeter first to make sure even at full depth it does not exceed 3.3v. Reading the specs on the ADS1115, the absolute max voltage it can handle is vcc+.3. I will be powering it from a 3.3v pin on the raspberry pi.

Thanks for your help.

#### AlbertHall

Joined Jun 4, 2014
11,500
You might want to include some protection for over-voltage for the ADC. If the sensor ground wire is disconnected there would be up to 24V to the ADC.

#### dendad

Joined Feb 20, 2016
3,909
Firstly, you may find the sensor will run quite well on a lower supply voltage, so that will be a bit safer.
Usually, I add a 50mA Polyswitch (resettable fuse) in line with the 4-20mA input, a 10K series resistor between the termination resistor and ADC input to limit ADC input current, and clamp diodes on the ADC input to protect from over voltage. A Tranzorb is a good option too.
To work out the resistor, find the full scale input voltage of your ADC.
Then, drop the sensor in the well, or a similar water depth. Measure the current and calculate the resistor to give a little less voltage. Do not go to full volts under normal use as then you can read a fault condition if the current goes over the "full" reading.
But, as @AlbertHall notes, the input needs to be able to take the full supply volts being applied to it.

That link on Ebay for the ADC sounds like it is a interesting device! It has this in the description

Features:
Maximum Voltage Output :300V
Maximum Current Output:1000A
Number of Channels/Outputs :1
Adjustable Power Supply Type:Linear Power Supply

Thread Starter

#### zirconx

Joined Mar 10, 2010
130
.... and clamp diodes on the ADC input to protect from over voltage. ...
Like a 3.3v zener from the ADC input to ground?

#### dendad

Joined Feb 20, 2016
3,909
A Zener can be ok but they can introduce non linearity. I usually have a pair of reverse diodes, Cathode to +V, anode to ADC input. Cathode of second diode to ADC input, Anode to gnd.

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