DE2808929A1 - Self-balancing impedance bridge for alarm circuits - uses differential amplifier with regulating stage to provide balancing - Google Patents

Abstract

An impedance bridge for use with alarm circuit lines includes a f.e.t. stage to provide a self balancing facility. the impedance bridge is coupled between the supply (A) and ground with the bridge diagonal(BC) connected to a differential amplifier(5). Any voltage output from the amplifier is coupled to a regulating stage, consisting of a f.e.t. (3) and capacitor (4). When the switch(6) is closed a fast balancing of the bridge takes place. A tuning stage operating over a delay avoids problems due to transient oscillation.

Description

Richard Hirschmanr.

Radio engineering plant Rich. Hirschmann-Str. 19 7300 Esslingen a.Neckar Self-balancing impedance bridge circuit for monitoring alarm system detection lines The invention relates to a self-balancing impedance bridge circuit with a to monitor impedance changes in an alarm system signal line switched on in a bridge branch and a balancing control circuit, in which a lying in another O-back branch Actuator preferably via a differential amplifier connected to the bridge diagonal and a retarding matching circuit can be controlled.

Such, for the detection and reporting of errors or interventions in Alarm systems - signal lines used bridge circuits are known from practice. The alarm lines have a number that are switched on in series or in parallel Secondary alarms that when responding, i.e. in the event of an alarm, an impedance change the reporting line and thus also the corresponding bridge branch. Through this a voltage is generated at the terminals of the bridge diagonal, which is necessary for evaluation (e.g. 6. as an alarm signal) is forwarded to the control center. For safe detection and Utilization of even small changes in impedance is to the terminals of the bridge diagonal A differential amplifier is expediently connected.

Adjust the bridge required when the alarm system is commissioned (Zero voltage on the bridge diagonal) takes place automatically by regulating a as an actuator (e.g. transistor with ohmic resistance in the emitter branch) -formed branch impedance with the help of the control signal supplied by the differential amplifier.

The actuator and the differential amplifier can in principle be directly connected to one another if their properties match one another are.

In the case of the known impedance bridges of the type mentioned, the control signal however, performed via a delayed matching circuit, which makes it easy Way, an adaptation largely independent of the device properties is achieved, the risk of vibrations in the balancing circuit (caused by a too fast Adjustment) effectively avoided and the control signal that arises in the event of an alarm in the adjustment circuit is so delayed that the bridge adjustment will certainly only take place after the alarm has been triggered occurs and cannot prevent it. The delay time of this matching circuit is dimensioned as small as possible so that the bridge adjustment can be commissioned the system is not unnecessarily extended.

The automatic one, for example by closing a single switch Alignment that can be set in motion for all reporting lines is compared to a means an adjustable branch resistance (potentiometer) for each impedance bridge manually carried out comparison, especially in systems with a large number of detection lines more cost-effective and time-saving, but has because of the short delay time of the matching circuit (or the direct connection of differential amplifier and actuator) a series serious disadvantages to 1. After the elimination of the reporting line malfunction, which the Caused the alarm £ e.g. Closing an open window contact), the Bridge to be adjusted again to the target impedance of the detection line, because in the In the meantime, an automatic adjustment to the signal line impedance present during the fault has taken place.

2. All changes in the impedance of the zone, the duration of which is the time constant exceed the adjustment circuit are automatically compensated so that "creeping Error "(e.g. due to a slow change in a resistance value, salt water ingress in junction boxes or formation of Condensation that i over the Time is governing), but above all corresponding manipulations are not detected and 3. The one in the event of a malfunction, e.g. during the day, that cannot be overgrown and therefore discarded the alarm line that was switched off by preventing the forwarding of alarms Although it is close to the bridge, it is automatically restored when it is switched on again, however, a change in impedance that has taken place in the meantime is not recognized will.

The last two disadvantages in particular are more fundamental Art and in many cases prevent a meaningful practical use of the known Bridge circuits.

The object of the invention is to address these disadvantages as possible avoid simple and cost-saving means without the advantages mentioned to lose an automatic impedance adjustment.

This object is achieved in that a Circuit for long-term storage of the target control level is provided and the adaptation circuit can be switched off.

When the setpoint control level is applied, the actuator has that impedance on, in which the bridge is exactly balanced if the II, eld line impedance their Has setpoint.

To commission the entire alarm system by the authorized person Person, the adaptation circuit is initially switched into the adjustment control circuit. This results in a quick adjustment of the bridge and at the same time the storage of the Target control levels in the memory circuit.

Then the adaptation circuit is manual or automatic switched off, so that from then on the impedance of the actuator is fixed and thus any change in the signal line impedance, even over a long period of time leads to an alarm beyond an approved limit and not to an undesired one Brunch comparison.

Motor-adjustable potentiometers, for example, can be used as memory or long-term storage binary circuits with adapted to the desired resolution Levels numbers are used. You are able to share your information (target control level) Preserved for years, so compared to the known circuit arrangement to simple as well as effective, quietly and while maintaining the advantageous automatic adjustment when switching on the entire system, even the slowest Impedance changes that can be imagined in practice due to errors or manipulation are reported will.

In addition, the advantage is achieved that, even when switched on again an alarm system that has been switched off or disarmed for a long period of time, no recalibration The bridges are also required in the meantime, e.g. through manipulation Impedance changes that have occurred are recognized by the circuit and after switching on again reported by alarm.

The security against sabotage can still be achieved in a simple manner can be increased further, so that different target impedances for the individual detection lines are provided.

The subclaims relate to advantageous configurations and additions the push switch according to the invention.

For many applications, the use of the above is motorized adjustable potentiometer or high-quality binary memory currently still too expensive; a use of simpler and cheaper for the stated purpose more suitable Binary circuits, however, have the disadvantage that to maintain the Information a minimum power required when switching off the central Power supply must be applied from additional batteries to be provided For this reason, in an advantageous embodiment of the bridge circuit according to the invention according to claim 2 an analog circuit is provided as a memory, as for example is known from the contrast and brightness setting in television receivers. This is a film capacitor with low leakage current, its Voltage with a field effect transistor is sensed. Such Storage a @ extremely inexpensive. They lose their information over time, however, the time constant (decrease of the stored level to the 1 / e - times) large enough for most applications1 cheaper bipolar field effect transistors (e.g. BF 245) and reached usable values when using field effect transistors of the 21OS type without protective diodes with inexpensive polycarbonate film capacitors approx. 2 x 107 sec (approx. 230 days); this means that with an allowed alarm criterion (impedance change) of e.g. 10% after about 20 days the alarm occurs. To avoid this alarm is it is necessary with such bridge circuits, about every three weeks by switching on the delayed adjustment circuit in the adjustment control circuit the setpoint control level to be saved again.

By supplementing the bridge circuit according to the invention There is no need for this to be replicated in an elegant and effective manner made because the refreshing circuit equalizes the bridge by feeding in a current corresponding to the losses continuously or at intervals readjusts; thus practically an infinitely large time constant is reached, whereby the bridge equilibrium is always maintained and false alarms are effectively avoided are.

A very extensive simplification of the back circuit according to the invention with saving of circuit means is achieved by a circuit structure according to claim 4 reached, in which the storage tank sensor also forms the actuator.

The field effect transistor used for this purpose is expediently according to Claim 5 switched on in that bridge branch that is not from the Moldelinienstrom is traversed because it is there where it is best protected from interference from the signal line is.

The figure shows the basic circuit diagram of an exemplary embodiment the self-balancing bridge circuit according to the invention.

The impedance bridge is created by a between terminal A and ground applied operating voltage UB supplied. In the two branches of the bridge between the clamp A, as well as terminals B and C of the bridge diagonal, are each an ohmic fixed resistor 1, 2 switched on.

The input impedance is in the test branch between terminal a and ground the reporting line to be connected to terminals D and E.

In the fourth branch of the The impedance bridge is a field effect transistor 3 with its drain-source connections inserted directly; the gate connection is via a highly insulating film capacitor 4 connected to terminal C. This analog circuit 3, 4 also acts as an actuator and as a memory circuit with the capacitor 4 as the actual memory element and the field effect transistor 3 as a memory sensor, the licgende on the capacitor 4 Senses tension without noticeably influencing it.

When commissioning the entire alarm system, the impedance bridge is in imbalance, so that an equalization voltage arises at terminals 5, C, which is amplified by means of a differential amplifier 5. By sled one Switch in the adjustment control circuit, this voltage controls the actuator 3, 4 and thus causes a quick adjustment of the impedance bridge. To adapt and avoid the risk of oscillations of the balancing circuit is in a known manner between the output line of the differential amplifier 5 and the switch 6 delaying one effective matching circuit 7 switched on.

With the adjustment, the target control level is saved at the same time into the analog circuit 3, 4. Thereafter, the matching circuit 7 is opened by opening of switch 6 disconnected from the adjustment control circuit. As long as the memory has the target control level brightens, the actuator 3, 4 has an impedance at which the bridge is exactly in equilibrium is when the teld line shows its target impedance1 The bridge diagonal between terminals 5 and C are then de-energized, changes in the event of an alarm, i.e. in the event of errors or manipulation of the detection line whose impedance exceeds a tolerance limit, this creates an equalizing voltage at terminals 8 and C, which is generated via the differential amplifier: rkcr 5 and the terminal F is forwarded to the control center and there, for example, a Triggers an alarm.

The analog memory has very little circuit complexity Inexpensive and due to its arrangement in an area not traversed by the signal line current Bridge branch optimally protected from interference pulses from the alarm line. His film capacitor 4 loses, even if only slowly, the stored information over time, sodab - without additional measures - at certain time intervals Cz.B. about all three weeks in the event of an alarm criterion from losO and a drop in the stored Level to 1 / e - inert in approx. 2 x 107 sec) to adjust the bridge again To avoid such Nachabgleiehe would be between the output of the differential amplifier 5 and the analog circuit 3, 4 a refreshing circuit 6 switched on, depending on the structure either continuously or at intervals of the memory circuit 3, 4 supplies a current within the limits of the alarm criterion that is somewhat larger is than the leakage current of the film capacitor 4.

The bridge circuit described is not only particularly simple and appropriately constructed, it also offers optimum operational reliability with minimal expenditure of time for the adjustment, moreover that of little trained staff can be made.

Claims (5)

Patent claims 9 self-balancing impedance bridge circuit with one switched on in a bridge branch to monitor changes in impedance Alarm system alarm line and a balancing control circuit in which one in another Bridge branched actuator preferably via one on the bridge diayonale connected differential amplifier and a delayed matching circuit is controllable, characterized in that a circuit in the adjustment control circuit provided for long-term storage of the target control level and the adaptation circuit (7) can be switched off. 2. Self-balancing impedance bridge circuit according to claim 1, characterized characterized in that a storage circuit consisting of a storage capacitor (4) and an analog circuit constructed as a sensor field effect transistor (a) is provided. 3. Self-balancing impedance bridge circuit according to claim 1 and 2, characterized in that in the balancing control circuit, in parallel with the matching circuit () A regeneration circuit (8) known per se is provided, which is connected to the storage capacitor (4) continuously or at intervals, a current that is slightly larger is than the leakage current of the storage capacitor (4). 4. Self-balancing impedance bridge circuit according to claim 2 and 3, characterized in that the field effect transistor (3) with the drain-source connections is switched on directly as a bridge branch in the impedance bridge. 5. Self-balancing impedance bridge circuit according to claim 2 to 4, characterized in that the field effect transistor (3) is in a bridge branch is switched on, which is not traversed by the signal line current.