[SOLVED] 4-20mA to 0-3V3 Converter

[yolco]

Hi,


for designing a 4-20mA to 0-3V3 converter I'm thinking on two circuit configurations (Please, forgive me about the pictures, they were not done with proper software).


Powered at +10V. This configration has three steps:

1. First. Differential step. To subtract 1 to input signal, and reduce to 0 when the measured signal is 4mA. Output1: 0-4V
2. Second. Apply 1.25 gain to get 5V at 20mA. Output2: 0-5V
3. Third. Voltage Divider Resistor. Convert the output signal from 0-5V to 0-3V3. Output3: 0-3V3

Powered at +24V. This configration has three steps:

1. First. Differential step. To subtract to input signal, and reduce to 0 when the measured signal is 4mA. Output1:
2. Second. Apply gain to get 3V3 at 20mA. Output2: 0-3V3

• Which of those configurations are better for this application?
• In case of designing the first option, are the final voltage divider resistors (R10 & R11) values correct? Or maybe, it would be better to reduce them as there is no high current through them.
• Any additional info will be helpful.

Regards.

* EDIT:

• Talking about power supply voltage, which would be the better one, +10V or +24V?

 

[schmitt trigger]

By far, and I mean by far, the best configuration is the simplest one, the one that converts from 4-> 20 mA to 0 -> 3v3

There is an even simpler option. If you are feeding the voltage to a microcontroller's ADC port, use only a resistor for a 0v66 -> 3v3 voltage, and perform the conversion in software.

As a matter of fact, I just finished an anemometer project, which has an output of 0v4 -> 2v0 for a windspeed of 0 -> 100km/h, on which I'm doing exactly that.

 

[yolco]

Hi Schmitt trigger,

thanks for your answer!

you're right the simplest option will be the best configuration for my requirements.
Although I was looking for understanding a bit more both configurations to know their pros & contras.

About your design, I think you left that 0V4 for being advised of when an error becomes (wire broken, malfunction...), and as you said before, by sotware you introduce those values and alerts, isn't it?

One more doubt I have, is the power supply, +10V or +24V. We can talk about +24V is the standard power supply, although this is not always true. So, which of them will be better to source the circuit?
I'm going to need a DC boost, so maybe it will be better to add a +24V than a +10V (with all the risk it has, power consumption...). What do you think?

 

[schmitt trigger]

In software, you would test the condition for V< 0v4, and send an error if true. That way broken wires can be detected.

For the source voltage, use what is commonly available, and that properly powers the remote circuit.

For my project I could have used anything from 7 to 24 volts, but I used 12 volts because it was available.

 

[yolco]

Thanks a lot Schmitt Trigger!

I go on with the design!

 

[schmitt trigger]

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