Electricity: electrical systems and devices – Safety and protection of systems and devices – Load shunting by fault responsive means
Patent
1997-04-15
1998-09-29
Gaffin, Jeffrey A.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Load shunting by fault responsive means
361 58, 361 91, 361111, H02H 900
Patent
active
058153535
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
For every electrical connection between an output and an input, voltages which are higher than the voltage for which the particular component's input or output is rated can be generated at the output or input under consideration due to component faults or parasitic voltages. This risk is present especially for controllers where a controller operating with semiconductors is interconnected with a controlled device via lines which are relatively excessive in length. The hazard for the components cannot be prevented solely, for example, by connecting the output to the input in a potential-free manner via an optocoupler. Here too parasitic voltages due to ground faults or overvoltages caused by electrostatic charges can exceed the permissible voltage between the input and output of the optocoupler and destroy it.
To prevent this, overvoltage protectors are used which are intended to clip overvoltage peaks by diverting an appropriate current to ground.
Nevertheless, it is difficult to assure the functionality of the protector over a long period of time, because a precondition of existing tests is a suitably high overvoltage. If the overvoltage arrester fails to function, the test will have resulted in the destruction of the relevant controller input or output.
SUMMARY OF THE INVENTION
Based on this situation, it is the objective of the invention to create an overvoltage protector which can be tested for functionality without the use of overvoltages.
According to this invention the problem is solved by the overvoltage protector disclosed herein.
Because the new overvoltage protector contains at least two limiter means connected in series between the two power terminals, it is possible to test the limiter means below the clamp voltage of the overall configuration. For this purpose a series of switching devices is connected in parallel to the series of limiter means, with each node of the one series interconnected via a current sensor to a node of the other series. If in the process of testing all switching devices except one switching device are placed in a low-resistance state, then all of the limiter means except one limiter means are essentially short circuited, so that the particular limiter means connected in parallel to the switching device that is not in the low-resistance state, will receive directly the voltage present at the power terminals. Any current flowing is a measure of the functionality of the particular limiter means.
Several different circuit configurations are feasible for implementing this evaluation. For example, the series of limiter means and the series of switching devices can be interconnected in parallel via 0-Ohm electrical connections, i.e. there is one switching device connected directly in parallel to every limiter means. To prevent a current overload of the limiter means during measurement, the series of switching devices contains at least one current limiting element which can if necessary--simultaneously serve as a current sensor. Another possibility is to provide current sensors in the cross links between the two series circuits.
Since the operating voltage and the maximum permissible overvoltage generally differ from one another rather significantly, it is beneficial to have the limiter means consist of several limiter elements connected in series. Furthermore, in the event of triggering this provides a better distribution of the power dissipation.
Candidates for limiter elements are all those components capable of switching from a state of low resistance to a state of high resistance and vice versa, as a function of voltage. Examples of this are zener diodes, diodes, or series circuits made of these, varistors, VDRs example a circuit consisting of a bipolar transistor and a zener diode connected between the base and emitter of the transistor. This assembly then behaves like a zener diode but with a greater level of power dissipation.
Similarly, there are a number of components which are candidates for switching devices, for example electronic se
REFERENCES:
patent: 3601632 (1971-08-01), Frazier
patent: 3731154 (1973-05-01), Torosian
patent: 3943427 (1976-03-01), Tolstov et al.
patent: 4437133 (1984-03-01), Rueckert
patent: 5379022 (1995-01-01), Bacon et al.
patent: 5644461 (1997-07-01), Miller et al.
patent: 5650904 (1997-07-01), Gilley et al.
Siemens, "Technische Mitteilung aus dem Bereich Bauelemente," p. 10, with 3 pages of translation.
Gaffin Jeffrey A.
Pilz GmbH & Co.
Sherry Michael J.
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