Darlington circuit having means for detecting a line interruptio

Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage

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Details

361100, 327575, 327198, H03K 17615

Patent

active

053961129

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to a Darlington circuit having means for detecting a line interruption in its load circuit.


BACKGROUND OF THE INVENTION

In the case of power transistors, particularly Darlington circuits, one often needs to be able to locate an interruption in the connections and lines in the load circuit so that appropriate counter-measures or auxiliary measures can be introduced. It is generally known to use a measuring resistor, which is traversed by the load current flow or by a portion of the same, for this purpose. To keep the power loss dissipation generated across the measuring resistor as low as possible, the measuring resistor, as a rule, has a very small resistance value, so that in a proper state, the rated current produces only a slight drop in voltage and a small power loss. Also, for those cases in which a very small portion of the primary current flows through the measuring resistor, often only a slight voltage drop is nevertheless required to ensure that the conditions in the main circuit and in the measuring circuit do not differ too greatly.
A device for recognizing such a line interruption must also function reliably in response to current values that lie far below the nominal value, often even by several orders of magnitude below this value. In this manner, the voltages to be evaluated become extremely small and the necessary measurement techniques become very complicated and expensive.


SUMMARY OF THE INVENTION

The present invention provides a Darlington circuit including at least one prior transistor connected to a power transistor. A resistor is connected between the base and the emitter of the power transistor. When the voltage drop across the resistor decreases below a preselected threshold voltage, a line interruption in the load circuit of the Darlington circuit is indicated.
The advantage of the Darlington circuit according to the present invention is that a line interruption in the load circuit is able to be detected with very little circuit expenditure and, thus, expense, and that no measuring resistor is needed in the load circuit or in a secondary branch of the same, so that, also, no power loss occurs. In spite of the simple design employing inexpensive component parts, which can also be integrated monolithically in a simple manner, line interruptions are able to be detected in a very precise manner.
In the simplest case, another transistor is provided as a circuit component, whose base is connected to the base of the power transistor of the Darlington circuit. The entire measuring device for detecting a line interruption consists, therefore, of one single transistor and one single resistor which only has to be rated for very low loads.
A simple and effective way to wire this additional transistor, which is designed in particular as an npn transistor, consists in connecting its emitter to the emitter of the power transistor of the Darlington circuit. The collector of this further transistor is advantageously loaded, thereby by a voltage source, whereby, given a proper state, the collector potential is lowered to the low level of the emitter potential ("LOW") without the presence of a line interruption. Thus, in the simplest possible manner, a line interruption can be detected by circuit means which respond to high potentials ("HIGH") at the base of the Darlington circuit and, simultaneously, at the collector of the other transistor.
The conditions are particularly favorable when at least an three-step Darlington circuit is used. Since it works with very low base currents, the resistor situated parallel to the base-emitter section of the power transistor of the Darlington circuit can have a higher resistance value.
Often circuit conditions exist in which a Darlington circuit is fed with base current via an upstream emitter follower. In such a case, a considerable base current could flow via the base-emitter section of the input transistor of the Darlington circuit, which in some instances would not permit the load interruption to be

REFERENCES:
patent: 3210677 (1965-10-01), Hung Chang Lin et al.
patent: 3564281 (1971-02-01), Tokunaga et al.
patent: 4142113 (1979-02-01), Lundberg
patent: 4215279 (1980-07-01), Lataire et al.
patent: 4564771 (1986-01-01), Flohrs
patent: 4616144 (1986-10-01), Hideshima et al.
patent: 4764688 (1988-08-01), Matsumura

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