Internet PLC Forum
General => Technical support => Topic started by: karen on March 08, 2002, 01:22:43 PM
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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.
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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
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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 :)
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thanks you are right Leon this is what i was looking for can you recommend an amplifier that i can use
thank you
???
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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