I'm trying to design the electronics to read a 4-20mA output from a 5V pressure sensor. Initially I was using 150ohm shunt resistor with the theory of creating a .6V to 3V range for the 4-20mA and a 1kohm resistor in series with the ADC pin to protect it. But I'm having some doubts about this working, a friend who is kind of an expert mentioned needing a voltage divider to protect the ADC from the 5V as its only meant to work with 3.3V but from what I've seen on this forum no one really mentions this so maybe I'm missing something. Any advice would be greatly appreciated. Rough schematic of my solution included. (Sensor link: Amazon.com)
A link to the exact sensor would be required.
Your design should be OK, but I would increase the 1K to 10K, just in case.
If the sensor is working correctly and all grounds are connected, the ADC voltage should never be above 3V.
I'm waiting on the link currently (I didn't choose the sensor), but will edit the post when I receive it.
Signals in a conventional diagram go from left to right.
Then it shows the diagram is not a voltage divider.
The value of the shunt resistor might not be optimal for the MCU that you're using.
So which processor/board.
Leo..
I can bump up the resistor, so whats a general explanation of not needing the voltage divider and why my friend thinks I need one. Just trying to learn, thank you for the feedback.
I'll remember the diagram feedback for next time. Its an ESP32 but I'm using an external I2C ADC, specifically the ADS7830, why would the shunt resistors resistance matter for the ADC?
One of the hazards of 4-20 is, if you remove the burden resistor while the circuit is powered, your 3.3V processor will most likely see a 5V signal, as the current source is unable to deliver it's current. That's what I was looking for in the sensor spec's - what the output might rise to open-loop.
In this case, it's relatively trivial (5V max), so 10 k should protect your processor. I'm familiar with 36V systems, wherein removing that resistor while powered can be catastrophic.
Under normal operating conditions, with 20 mA maximum sensor output, the ADC input voltage is 3 V, so there is no need for a voltage divider. Your friend is not correct.
If there is a sensor fault then the ADC may be exposed to 5V. In that case a 10K resistor should limit the current through the ADC input protection diode to a safe value.
The shunt has to match the reference voltage of the A/D.
In your case (ADS7830) it seems to be 2.5volt, not 3.3volt.
120 ohm seems the optimum value for 2.5V Aref.
Leo..
Okay that makes sense, thankfully its all soldered to a protoboard so I don't think it can realistically be removed. I've added the link anyway, its just kind of a random Chinese sensor that someone else selected so not much of a datasheet.
The reference pin is connected to 3.3V in the design
Okay thank you, maybe they misunderstood what the operating voltage of the sensor but I appreciate the additional info and will proceed with the design but with a 10k series resistor.
The 3.3volt supply of the ESP is a poor (noisy) reference.
The ADS7830 seems to have it's own 2V5reference.
Why not use that.
Leo..
Its powered by an external 3.3V power supply, not the ESP32. Honestly, just didn't consider the 2.5V reference voltage and its not currently a concern.
That sensor does not output 4-20mA
Flow Sensors
parameter
Measuring range: 300-3500psi gauge pressure
Accuracy: Β±1.0%FS
Output signal: 0.5-4.5V
Supply voltage: 5VDC or 12VDC
Working temperature: -40~105β
Compensation temperature: -20βο½80β
Working current: 3mA
Temperature drift (zero point & sensitivity Β±0.03%FS/
Long-term stability: Β±0.2%FS/year
Response time: 3ms
Overload pressure: 150% FS
Burst pressure: 300%FS
Cycle life: 500,000 times
Sensor core: Al2O3-96% ceramic sensor
Pressure medium: non-corrosive liquid or gas
Pressure Connection: 1/4" NPT
Connection: Direct plastic cable
Shell material: 304 stainless steel
Protection level: IP65
Explosion-proof grade: ExiaCT6
You will need an op-amp buffer in order to interface it to an Arduino
As @jim-p pointed out, either that's the wrong link, or you've misunderstood the sensor - or been mislead.
I think it might be listed in the wrong department
| Package Dimensions | 1.18 x 0.79 x 0.39 inches |
|---|---|
| Item Weight | 1.76 ounces |
| Department | Unisex Adult |
| Manufacturer | YIOGEQKWG |
| ASIN | B0DJHVMT43 |
| Item model number | YIOGEQKWG |
My comments assumed that this description is correct, but as pointed out, the link you gave is instead to sensor with 0.5 - 4.5V output.
If that is the case, a voltage divider is required for input to a 3.3V ADC.
Yeah we've had problems with this sensor, it says its a voltage output but the pigtail coming out of the sensor only has two wires coming out of it and in my experience that indicates that its a 4-20mA sensor. We're gonna test it and find out.