Reading a 4 - 20mA Signal from a PLC

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gfcwfzkm
 
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Reading a 4 - 20mA Signal from a PLC

Post by gfcwfzkm »

Hi @ all

I got a problem: I want to read a 4 - 20mA signal that comes from a programmable logic controller. I already have an idea using a Shunt-resistor, but i still have some Questions...

How high can the Resistor be?
How high could the Voltage on the Line be? Should i take a OP Amp to bring it to a nice 0-5V Signal and then to the microcontroller?

My Idea is to measure the Voltage on the Shunt-resistor, which is connected to the PLC and to Ground. Do i have to take care about higher Voltages (higher than 5V)?

~Pascal

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adafruit_support_bill
 
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Re: Reading a 4 - 20mA Signal from a PLC

Post by adafruit_support_bill »

A 250 ohm resistor will give you 5v@20mA (V=IR). You can read that directly with an Arduino. 4mA gives you 1v. That is considered a 'live zero'. If the input drops below that, you know there is something wrong with the current loop.

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