4-20mA Receiver - How To Also Have Option For 0-10VDC Analog Input?

Thread Starter

Mahonroy

Joined Oct 21, 2014
312
Hello, I have 2 terminals where you can connect up a 4-20mA transmitting device (such as a sensor). This 4-20mA receiver also has integrated 24VDC power for the current loop. The circuit reads the current draw and converts this to an analog (0-3.3VDC) signal that I can read using a micro-controller and an analog-to-digital converter (ADC).
4-20ma_question1.jpg

My question is, how do I also provide the option for a 0-10VDC Analog Input in place of the 4-20mA? I could connect up a spare ADC pin to pin 1 of the "TB4", along with a voltage divider to translate it from 0-10VDC to 0-3.3VDC. This would allow someone to connect up a ground and a 0-10VDC sensor instead of a 4-20mA sensor. The problem with doing that is the 24VDC and the resistor R81 would interfere with this reading. So I somehow need to disconnect it if I want to do that.

Instead of using a jumper/shunt/switch here (e.g. jumper for selection of "4-20mA" or "0-10V), is there a way to accomplish this by using a mosfet or something? This way the microcontroller could switch between the 2, instead of the user having to mess with jumpers/shunts/switches?

I have seen this a few times and have always been curious how its been done. For example I see devices that have inputs that can be configured to be 4-20mA receiver, 0-10VDC analog input, 0-10VDC analog output, digital input/output, etc. Basically a universal port. I am curious how this is typically done?

Thanks again and any help or advice is greatly appreciated!
 

crutschow

Joined Mar 14, 2008
24,730
For example I see devices that have inputs that can be configured to be 4-20mA receiver, 0-10VDC analog input, 0-10VDC analog output, digital input/output, etc. Basically a universal port. I am curious how this is typically done?
Don't know for sure, but you could likely do it with analog switches to go from one configuration to the other.
Are you familiar with those (such as the CD4066)?
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
312
Don't know for sure, but you could likely do it with analog switches to go from one configuration to the other.
Are you familiar with those (such as the CD4066)?
Thanks for that suggestion! I have never looked into these parts before but they look pretty cool. The only problem I see is they seem to be limited to 18V max, and I have to continue to use the 24V for the current loop (a few sensors have to use this higher voltage).

Would a solid state relay work, such as this one? "CPC1017 "
https://www.digikey.com/product-detail/en/ixys-integrated-circuits-division/CPC1017NTR/CLA233CT-ND/1212844

It mentions that the on-state resistance is 16 ohms, and the off-state leakage current is 1 uA. Would this cause any problems if I put this SSR in-between pin 1 of TB4 and the resistor R81? This way if I'm using "4-20mA mode", I would keep the SSR activated, and the ADC won't interfere since its an input. Then when using "Analog mode" I would deactivate the SSR, and could then take analog readings from the ADC?
 

dendad

Joined Feb 20, 2016
3,428
This is the circuit I use on a couple of my products. It works well and is simple.
4-20:0-10.jpg
With the switch open, the 0-10V is fed through the voltage divider 22K and (10K +150R) to the ADC input.
With the switch closed, the 4-20mA develops a voltage across the 150R and fed to the ADC input via the 22K and 10K in parallel.
As I said above, it is pretty simple but it actually has worked well for years.
The max ADC reading is a bit higher that 10V or 20ma, and that is necessary to be able to detect sensor faults.
Also, a 50mA Polyswitch along with the 6V Zener limits the input voltage, as does the BAV99 dual diode.
Note that this only works for the termination to 0V, not floating.
I know it does not answer your questions, but thought it may be of interest.
 
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