Microcontroller-controlled current loop AFE protection

Summary: Current shunting in an isolated 0-20mA current loop with a precision resistor analog front end can quickly protect a fuse or automatic current-limiting switch. The circuit shown, unlike the switch, offers an option to record fault events under microprocessor control. The circuit includes a comparator (MAX9910), a watchdog timer (MAX6706), a microprocessor supervisor, a D-type flip-flop, a NOR gate, an OR gate, and a solid-state relay.

The 0-20mA current loop remains a reliable means of communication for industrial applications. For systems that include a high-precision analog front end (AFE), the current shunt resistor must also be a stable and high-precision device. Precision shunts can be damaged by an accidental direct connection to the current loop power supply. You must then replace the shunt and recalibrate the system. To avoid this sacrifice, consider the microcontroller-controlled protection circuit in Figure 1.

Figure 1. This circuit protects the isolated analog current loop front end (AFE). For simplicity, we will not show the power and ground connections (5Viso and GNDiso) for IC2, IC4, IC5, and IC6.

A shunt can be protected with a fast-acting fuse, either by limiting or closing the switch automatically with the cycle, then opening after a specified time to recover, i.e. The Figure 1 method offers several advantages over these methods.

This is faster than fuse protection. The slowest device in Figure 1 (IC1) requires less than 500μs to switch off. It provides a higher accuracy by not switching the fuse below the threshold, and of course there is no fuse replacement. Unlike "switch auto-retry" protection, it provides microcontroller control of the protection circuit, plus an option to log the event, thus providing a record of the protection circuit being invoked.

In addition, the protection of Figure 1 has virtually no effect on the exchange: input current is less than 30pA, thanks to IC2's buffer, and the resistance on IC1 is less than 2Ω. No additional isolation of the data channels or microcontroller I/O ports is necessary. To avoid damage during system installation or repair, the loop is turned off after power-up also when power is not applied. Implementing the protection algorithm is a power-fail comparator and watchdog circuit, such as that in IC3 (MAX6706), with independent outputs, together with a D-type flip-flop (IC6). At power-up the flip-flop is in the reset state, the current loop is open, and the high-level reset signal from driving the NOR gate (IC4) of IC3 is the cause. At the first low-to-high transition of the SCK line, the rising edge of the WDO output by IC3 sets the flip-flop and pulls current through the solid-state relay (IC1), thereby connecting the input to the loop.

In the event of a loop current overload (i.e., greater than approximately 27 mA), a high level from the power-fail comparator (PFO on IC3) resets the flip-flop and turns off IC1. With IC5 gated down, the microcontroller sees only a "1" on the MISO output, which indicates overcurrent protection. To switch the loop again, the microcontroller must pause the SCK line for at least 2.4 seconds. Then, the next low-to-high transition on SCK reconnects the current loop.