I know this is going to be PAINFULLY simple to some of you. However I haven't used a 4-20mA sensor in +10 years. I am a hobbyist who is looking to build a sensor system that I will use at work.

I have my controller figured out, which will read the 1-5V signal derived from the voltage drop across a 250 ohm resistor. What I can't remember is if I have to send a set current of 20mA to the sensor, or will it automatically output 20mA (or less) even if I send lets say 200mA....

Do I require a power supply for this sensor that supplies exactly 20mA? Are there power supply conditioners that will do this for me?

I truly appreciate any response.

Best regards,
Joe

 

Yes, AC in, DC out, it's called a DC power supply. There are hundreds of vendors. Google will find them.

A 4-20mA sensor' transmitter outputs a signal proportional to whatever it senses. It needs DC voltage to operate.

The controller does not 'send' the sensor/transmitter electrical current; the transmitter generates (automatically outputs, as you put it) the correct current proportional to the sensed variable.

There are 2 wire loop-powered transmitters, 3 wire and 4 wire transmitters, each of which gets wired differently to the power supply. You need to consult the documentation that comes with the sensor/transmitter.

 

Sensor dependent. If it a simple sensor, supplying 24VDC (or just above the minimum operating voltage) will enable the sensor to operate as a 4 - 20 mA source. Power supply (+) to sensor (+), sensor (-) to resistor (+), resistor (-) to power supply common. Resistor connected in parallel across your input.

If you have a more complex <i>transmitter</i> it might require a separate supply. This type of transmitter requires 4 wires -- 2 for power, and 2 for the 4-20 mA signal.

 

See this manual for wiring examples, simply substitute the meter with your controller.
https://www.predig.com/sites/default/files/documents/LIM765QS.pdf

Best regards,
JUmana