A Simple, Inexpensive, & Quick Solution Saves the Day!

We recently were asked to solve an unexpected issue for one of our PLC customers.

They use the high-performance Mitsubishi Electric iQ−F PLC, also known as the FX5U, which is equipped with 2 Analog Inputs and 1 Analog Output. The Analog Inputs are rated for 0 to 10 Volts with a 12-bit Analog to Digital converter allowing for a resolution of 2.5 mVolts. The digital number seen in the PLC will range from 0 to 4000.

The Analog to Digital converter has a very fast conversion speed of 30 microseconds per channel, ensuring an updated Analog Input reading every PLC scan.

These Analog Inputs are easily used for many applications, such as Potentiometers, Level Meters, Light Meters, Height Sensors, etc.  If your device outputs 0 to 10 Volts, you can connect it to the off-the-shelf iQ−F PLC.

Well, this is where the problem came in.  They had already purchased a laser height sensor for their project, but it was not until they added it to their machine that they realized they had ordered the wrong sensor, one with a 4-20 mA output which their iQ-F PLC could not read.

They were in a real bind because they had to get the line running as soon as possible, as in minutes, not days or weeks.

Had they ordered the iQ-F PLC with the FX5−4AD−ADP module, they could have used either voltage, or current inputs, but that is not what they were using.

The “5-cent solution” we recommended, quickly saved the day.  By adding a simple 250Ω resistor (¼ watt, 5% tolerance) across the Analog Input terminals, we instantly converted the Voltage Input to a Current Input!

This is the normal wiring for a 0 to 10 Volt Analog Input…

And this is what the wiring looks like after adding the resistor…

This resistor converted the 4 to 20 mA signal into a digital value of 0 to 2000 in the PLC. Now our customer was able to use their Current type of Analog device.

And that is all there is to it!