Avalanche: Overwhelming input voltage sets off cascade of process-controller failures
Tales From The Cube: Semiconductor junctions have a “reverse breakdown voltage” at which a reverse-biased junction begins to conduct. This situation is unhealthy for sensitive op-amp input transistors.
By Walter Lindenbach -- EDN, July 19, 2007
Customers often returned process controllers to us for repair, because the field operators could not adjust them correctly. For example, consider a gas pipeline that is to operate at 3000 psi. When measured by a pressure transducer calibrated for 0 to 5000 psi (with an output of 1 to 5V dc), 3000 psi produces an output of 3.4V, which is the process variable. The operator will have adjusted the setpoint to 3.4V also for operation at 3000 psi. Now the process variable is equal to the setpoint, and the control loop should “sit still”—that is, produce no output change.
Often, the field operator could adjust the controllers for constant output when the process variable and setpoint were both equal to 1V but not when they were both equal to 5V. In this situation, the CMRR (common-mode rejection ratio) of the input differential amplifier was unsatisfactory.
Consider a typical process-control loop, which contains a 45V power supply, a wire-line pair (or equivalent) from the control room to the measurement site, a current transducer, and a 250Ω “range resistor,” all in series. The current transducer, which measures the process variable, produces a current of 4 to 20 mA in the wire-line pair, representing 0 to 100% of the value of the process variable. This current in the 250Ω-range resistor produces an input for the controller of 1 to 5V.
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But sometimes a 1 to 5V input signal is already available and the range resistor is unnecessary. Then the installer does not plug it in at the controller. However, if the controller is to work with a 4- to 20-mA signal, and the installer forgets the range resistor, the input to the controller is about 45V. Not good!
The process-controller circuits included resistors between the input connector and the inputs of the first operational amplifier (configured as a differential amplifier) that were greater than 100 kΩ, so an input voltage that exceeded the amplifier’s maximum common-mode rating would not cause damage, would it? Could a current great enough to change the characteristics of the op amp come from a 45V source through a 100-kΩ resistor?
I asked an engineer leading a Motorola seminar whether applying 45V to a 100-kΩ resistor connected to an op-amp input, operating with a ±15V power supply, could harm the amp.
“Yes,” he said. “The op-amp input devices will avalanche. The amp may not fail, but the operating characteristics will change.”
Avalanche? What did that mean?
Semiconductor junctions have a “reverse breakdown voltage” at which a reverse-biased junction begins to conduct. Zener diodes behave in this way, which makes them useful as voltage regulators. But this situation is unhealthy for sensitive op-amp input transistors. With a circuit like the one described above, this phenomenon is not usually fatal, but it will change the behavior of the op amp. It will spoil the balance of the input differential pair.
So, we had to keep the input voltage from exceeding the supply voltages (±15V). We placed diodes at the controller inputs (cathodes to the 15V supply; anodes to the connector-input pins) that would conduct if the controller-input voltage exceeded the positive supply voltage. To protect against input voltages more negative than –15V (in case of a wiring error), we also placed diodes between the controller inputs and the –15V supply (anodes to the –15V supply; cathodes to the connector-input pins).
These diodes had to be good; leakage had to be low at the highest operating temperature. At a 0% reading (4 mA, 1V), 40 µA represents an error of 1%.
After we added the diodes, customers no longer returned the modified controllers.
