Peter wrote:
We are using a MMi202 controller in conjunction with an instrument which has a 4mA-20mA signal output for control. Does the voltage of this signal have any significance? We have measured the signal at 15Vdc.
The MMi202 has a 250Ohm "burden" resistor in milliamp mode. 20mA will develop 5V across this resistor. If you are measuring 15V out of the instrument I expect that is when it is not connected to the MMi202, or you have forgotten to set the input mode jumper to "I".
The assumption is that the instrument output terminal is positive with respect to the instrument's common terminal, or that the instrument has a floating pair of wires for its output. The negative (or common or ground) terminal of the instrument goes to MMi202 0V, while the positive output terminal goes to the MMi202 analog input. More on this in the SPLat Knowledge Base.
If you are still unsure you can email me or point me to the the manual for the instrument.
Peter replied:
Thank you for your reply. I understand what you said, but my query is whether the 15v signal (4 - 20mA) from the analyzer to the SPLat can damage the SPLat, and do we need to reduce this signal voltage or is it OK.
Assuming it maxes out at 20mA it is fine. Forcing the input voltage to 15V in current mode for any length of time would damage the 250Ohm current sensing resistor. You can always connect it and quickly measure the voltage. In the 5 seconds it takes to make a measurement no harm will be done.
A current source, like your instrument presumably has, is a circuit that will force the required current into the external load. A ideal current source will generate an infinite voltage, if that's what it takes. In reality the maximum voltage will of course be finite ... in your case 15V. 4-20mA is a popular industrial standard because the same current arrives at the other end irrespective of wire resistance, so it works for long (hundreds of meters) wire runs on plant sites.
