Linear Voltage Converter: 12–24 V to 5–10 V for PLC Inputs

Overview

The Linear Voltage Converter is a small board designed in 2015. It converts an input voltage of 12 to 24 V into an output of about 5 to 10 V, which suits a PLC input. The output changes linearly with the input, so by reading the output voltage the PLC also reads the input voltage.

How it works

The input voltage is connected to the board, where two op-amp stages (CA3140) scale it down by a small fixed factor. Because the factor is fixed, the output is always proportional to the input. According to the test table, the output is about 0.39 times the input. The op-amps are powered by a separate DC-DC module (9 to 36 V input, ±12 V output). The board also has a trimmer (P1), an input diode and three status LEDs.

Performance table

After the board is connected, the input voltage is checked first, and then the output is determined according to it. The table below shows how the circuit performs:

The chart below plots the same values. The points fall on a straight line.

Specifications

• Input supply: 12 to 24 V

• 5 to 10 V output, optimized for PLC

• The output is linearly proportional to the input

• Two CA3140 operational amplifiers

• Internal supply from a DC-DC module, ±12 V output

Hardware design

I designed the board, and it sits inside a plastic DIN-rail enclosure. The input and output screw terminals are on opposite sides of the board, so mounting and wiring inside a panel stay simple. The pictures below show the built board, the PCB layout and its 3D view.

Result

A small, simple board that maps 12 to 24 V linearly into the 5 to 10 V range, so that a PLC can read it.