Simple Circuit Diagram for Detecting Loss of 4-20 mA Signal


Four-to-twenty milliampere current loops are commonly used in the process control industry. They take advantage of the fact that a remote amplifier can be powered by the same 4-20 mA current that it controls as its output signal, thus using a single pair of wires for signal and power. Circuits for making 4-20 mA transmitters are found in the LM10, LM163, and LH0045 data sheets.

In general, an expensive isolation amplifier would be required to detect the case of a 4 mAsignal falling out of spec (e.g., 3.7 mA) without degrading the isolation of the 4-20 mA current loop.

But this new circuit (Figure 1) can detect a loss or degradation of signal below 4 mA, with simplicity and low cost. The LM10 contains a stable reference at pins 1 and 8, 200 mV positive referred to pin 4. As long as the loop current is larger than 4 mA, the I x R drop across the 47.6W resistor, R4, is sufficient to pull the LM10’s amplifier input (pin 2) below pin 3 and keep its output (pin 6) turned OFF. The 4-20 mA current will flow through the LED in the optoisoBut this new circuit (Figure 1) can detect a loss or degradation of signal below 4 mA, with simplicity and low cost. The LM10 contains a stable reference at pins 1 and 8, 200 mV positive referred to pin 4. As long as the loop current is larger than 4 mA, the I x R drop across the 47.6W resistor, R4, is sufficient to pull the LM10’s amplifier input (pin 2) below pin 3 and keep its output (pin 6) turned OFF. The 4-20 mA current will flow through the LED in the optoisolator and provide a LOW output at pin 5 of the optoisolator. – Article quote

Current Loop fault detector circuit

Current Loop fault detector circuit

The topics discussed inside the application note including the circuit of current loop fault detector, test circuit for threshold detector, fault detector with low-cost trim scheme, life support policy, and many more.

Source : Robert A Pease (National Semiconductor)

Find more details about the loss detection from the National Semiconductor by downloading the pdf document from the following source (national.com) :

Simple Circuit Diagram for Detecting Loss of 4-20 mA Signal DOWNLOAD


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