Voltage comparator and monitor

Details Voltage comparator and monitor Voltage comparator and monitor

1997-06-09

1999-08-10

Callahan, Timothy P.

Miscellaneous active electrical nonlinear devices, circuits, and

Specific signal discriminating without subsequent control

By amplitude

327 77, 327361, H03K 5153

Patent

active

059364351

DESCRIPTION:

BRIEF SUMMARY
The more automated the individual processes in modern systems become, the greater a priority it is to design these processes to be as safe as conceivable. Safety cannot be compromised by disturbances of individual components. Likewise, if such components break down, these problems must be detected before they lead to dangerous situations.
One critical point in this matter is the supply voltage of electronic controls that are used for process management and supervision. Today, these types of controls are generally digital controls that only operate within a relatively small supply voltage range per their specifications. If the supply voltage becomes too high, the components face the threat of a breakdown due to excessive voltage or too much dissipated heat. These damages are irreversible and cause major repair expenditures.
In contrast, if the supply voltage falls below critical values, the signal-to-interference ratio in the circuit is decreased, i.e., the switching thresholds can be accidentally exceeded or not met due to outside interfering signals, which transgressions in turn cause control errors. Further, the signal differentiation can become unsafe with a supply voltage that is too low. An unsafe differentiation could lead to an erroneous switching of the two digital signals zero and one.
Window comparators are used, for example, for checking whether a voltage is within a specified tolerance window. In this type of comparison, a reference voltage that is essentially independent of the supply voltage is fed into a comparator input. The voltage to be compared to this voltage is rectified, if it is not already present as a DC voltage, and then supplied to the other comparator input. The comparator output signal is a function of the comparison, and assumes, for example, the value of one if the monitored signal is within the tolerance window, and changes to the zero state as soon as the monitored signal goes out of the tolerance window.
This type of circuit is, however, insufficiently safe because errors in the comparator itself can lead to a constant voltage at the comparator's output, which simulates one of the two possible comparison statuses.
Based on this, it is the object of the invention to create an apparatus for comparing two voltages to one another, with the output signal of the device having a very high degree of reliability.
The novel apparatus does not compare the two DC voltages directly with the aid of a comparator; instead, one of the two voltages to be compared enters a voltage-conversion device, which creates a voltage from the DC voltage supplied, which voltage is superinposed by an AC voltage. The signal present at the output of the voltage-conversion device is thus a DC-voltage signal having an AC component, i.e., an AC-voltage signal that is shifted with respect to DC-voltage level. The signal at the output of the voltage-conversion device, therefore, fluctuates back and forth periodically between an upper amplitude value and a lower amplitude value, with the distance between the two amplitude values being a predetermined differential value. This differential value can be made to equal the window within which one of the voltages to be monitored should move.
The signal modified in this manner enters the other comparator input, which then and only then emits an AC signal at its output if the AC signal fed in is periodically larger or smaller than the continuous signal fed into the other input. For example, if the DC voltage supplied to the voltage-conversion device becomes too large, then, with corresponding dimensioning, the lower amplitude value increases to a value greater than the comparison voltage, whereupon the comparator emits a continuous-wave signal at the output. The same applies if the voltage supplied to the voltage-conversion device is too small, because then the output signal of the device cannot become larger than the other signal fed into the comparator. Thus, the occurrence of an AC signal at the comparator output is a sure sign that the two voltages to be compared to

REFERENCES:
patent: 3612975 (1971-10-01), Keefe
patent: 5491445 (1996-02-01), Moller et al.
patent: 5666075 (1997-09-01), Schinzel

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