reading current

[karen]

i have the 888+ is there any way to read current and display on the lcd the range is 0-100ma how or is it possible.  

[Ken Talley]

Check with Leon, but I am pretty sure all you would have to do is some math to figure out what resistance you would need to convert the amperage signal to a voltage.  For example to convert the signal from 0-100mA to 0-5 volt you would put a 50 ohm resistor across the circuit in parallel.  You would then sample the voltage drop across the resistor with the 0-5 volt input on the plc.  This is making the assumption that I did my math right and the amperage you are measuring is in a current loop which should compensate for the load applied to the circuit to read it.  Again, check with Leon, but I think this will work.
Ken

[support]

Ken would be right if the current that you are measuring is just a current loop signal that represent the numerical dimension of the measurand, and is not needed to do other work.  The 50 ohm resistor will become a "load" to the current loop signal that turns 100mA into 5V.

However, if your purpose is to measure the size of the current in a working circuit without introducing significant loading effect on the circuit, then an additional 50 ohm added to a circuit is probably unacceptable.  In this case, you will have to add a much smaller load, say a 0.5 Ohm resistor in series to the circuit. A 100mA current will generate 0.05V across the resistor and you will have to measure this small voltage drop using a differential amplifier that magify it by 100 times to get a 0-5V signal to the T100MD888+'s analog input.  - Leon  

[karen]

thanks you are right Leon this is what i was looking for  can you recommend an amplifier that i can use

thank you

 

[support]

I am not an expert on OpAmp. But I think a low cost 741 will probably do the job. I would like to refer you to a document found on the following weblink:

http://www.oldradio.de/service/uel/reports/anel/opamp741.pdf

- Leon