# Using 4-20mA analog as an input to the electric imp?

We have a device that gives out an output of 4-20mA analog output. Can this be somehow put through the electric imp to get the readings online?

Yes, with a precision resistor placed inline with the loop. Read with analog input.

Actually I am not sure you would care about precision if you are going to do a calibration anyway. You might care more about the temperature coefficient to reduce the amount the reading would change when the resistor changes temperature. All depends on the accuracy you desire.

http://en.wikipedia.org/wiki/Current_loop

You didn’t say whether or not your 4-20ma device was “source powered” or “loop powered”. I’m assuming it’s “source-powered” with an external 24VDC supply.

Putting a 250 ohm resistor across the load and converts your 4-20ma to 1-5VDC. It might be easier to convert that to something the imp can read.

If you put a 165 ohm resistor across the load, 20ma would be 3.3VDC

I’ve never tried that with an Imp, so don’t hook anything up until someone here verifies that might be OK to do.

You can do that hookup on a breadboard without the Imp and take measurements with a voltmeter. Also realize that some 4-20ma transmitters (like a Rosemount temp transmitter, pressure transmitter, level transmitter) can output 23-24ma as a full-scale value when faulted. That is done purposely to give the PLC a value that it knows there is a problem.

If your resistor opens or a wire falls off your breadboard, you will be giving your Imp pin 24VDC … that’s why I mention there might be a circuit to use that protects things. There are some isolation breakout boards like these:

You would think there would be a 4-20ma --> SPI breakout board?

EDIT:
I see a lot of boards going the other direction: digital --> 4-20ma output