Microcontroller supervises 0- to 20-mA protection circuit
A microcontroller replaces fuses and trip circuits.
Anatoly Andrusevich, Maxim Integrated Products; Edited by Martin Rowe and Fran Granville -- EDN, August 26, 2010
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The 0- to 20-mA current loop is a reliable means of data communication in industrial applications. These circuits use a precision shunt in the receiver to convert the current signal into a voltage signal. Accidentally connecting the precision shunts to the current-loop power supply can cause damage, after which you must replace the shunt and recalibrate the system. To avoid that expense, you can use a microcontroller-controlled protection circuit (Figure 1).
With conventional techniques, you
protect the shunt with a fast fuse or
by turning off the loop with an automatic
switch, which then turns back
on after a specified period. The circuit
in Figure 1 provides protection that is
much faster than a fuse. IC1, the slowest
device in the circuit, switches off
in less than 500 μsec. It offers a higher-precision switching threshold than
a fuse, and, of course, there’s no fuse to
replace. Rather than making you cycle
power to restore the loop, the microcontroller
provides control of the protection
circuit. The microcontroller
also logs the event, thereby providing
a record that the system invoked the
protection circuit.
The protection circuit has virtually no effect on the analog front end. The IC2 buffer ensures an input current of less than 30 pA. The on-resistance of IC1 is less than 2Ω. The circuit needs no additional isolated data channels or microcontroller-I/O ports, and it prevents damage during system installation or repair. It also turns off the loop after power-up and when no power is available.
You implement the protection algorithm with a power-fail comparator and a watchdog circuit, available as separate outputs on IC3, together with IC6, a D-type flip-flop.
At power-up, the flip-flop is in the
reset state, and the current loop is open,
due to a high-level reset signal from IC3
driving IC4, a NOR gate. After the first
low-to-high transition on the SCK
(clock-signal) line, a rising edge from
IC3’s 
(watchdog output) sets the
flip-flop and pulls current through the
solid-state relay, IC1, thus connecting
the input to the loop.
(watchdog output) sets the
flip-flop and pulls current through the
solid-state relay, IC1, thus connecting
the input to the loop.In the event of a loop-current overload
greater than 27 mA, a high level
from the 
(power-fail-output) comparator
on IC3 resets the flip-flop and
switches off IC1. Thanks to the IC5
gate, the microcontroller inputs ones
at the MISO (master input/slave output),
meaning overcurrent.
(power-fail-output) comparator
on IC3 resets the flip-flop and
switches off IC1. Thanks to the IC5
gate, the microcontroller inputs ones
at the MISO (master input/slave output),
meaning overcurrent.To again switch on the loop, the micro controller must stop the SCK line for at least 2.4 sec. The next low-to-high transition on SCK then reconnects the current loop.
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