Patent Publication Number: US-4321524-A

Title: Transistorized voltage limiter

Description:
The invention relates to a transistorized voltage limiter constructed as a dipole and having in the conduction direction a voltage current characteristic similar to that of the Zener diode but with a stable voltage of about 1 V. 
     BACKGROUND OF THE INVENTION 
     It is known that Zener diodes of low nominal Zener voltages have a rounded elbow and a comparatively high dynamic resistance (ΔU/ΔI) in the working range of the characteristic. 
     This shortcoming, i.e., high dynamic resistance increases appreciably with the lower Zener voltages such that at low voltages such as 1 volt, Zener diodes can hardly or not at all be utilized in their basic function as voltage limiters. 
     SUMMARY OF THE INVENTION 
     The main advantage of the transistorized voltage limiter according to this invention is that it can, in various electronic circuits for the stabilization of voltage, current, etc., perform its function at a low voltage, e.g., 1 V, at which the Zener diode cannot operate effectively. 
     Another advantage of the invention is an exceptionally low dynamic resistance (ΔU/ΔI), of the order of 1 (Ω). In a certain interval, with stronger currents, the dynamic resistance is negative, and is also low in absolute value. Consequently, there appears in the characteristic U-I curve a comparatively large range in which the dynamic resistance can be considered as being zero. The elbow of the characteristic is sharp (right angle), but nevertheless with a small roundness. No noise appears in the curve, which also is an advantage. 
     The foregoing advantages are achieved in accordance with the present invention by connecting a diode across the base-collector junctions of a pair of complementary type transistors, cross coupled from collector to base. The diode is forward biased when the transistors are rendered conductive upon application of a predetermined voltage between their emitters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention will become more apparent from the following detailed description thereof, taken in conjunction with the appended drawings, in which: 
     FIG. 1 is a circuit diagram of the invention; 
     FIG. 2 is a plot of current versus voltage, illustrating the operation of the circuit of FIG. 1, and; 
     FIG. 3 is a circuit diagram of another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is shown on the diagram of FIG. 1. The circuit is a dipole with terminals at 1 and 2. The voltage between terminals 1 and 2 and the current through the circuit are indicated by U and I respectively. 
     The circuit comprises the transistors T 1  and T 2 , and the diode. The transistor T 1  is PNP, while T 2  is NPN. The types of the semiconducting elements should be suitably selected. Good results are achieved with silicon transistors and a germanium diode of low resistance in the conduction direction. 
     The emitter of transistor T 1  is one end of the dipole. Its collector is connected at 3 with the base of transistor T 2 . Its base is connected at 4 with the collector of transistor T 2 . The emitter of transistor T 2  forms at 2 the other end of the dipole. The diode D is connected with the anode at 3 and with the cathode at 4. 
     In operation, the DC voltage U to be regulated or limited is applied between the terminals 1 and 2, terminal 1 being positive. Assuming the applied voltage to be above the limiting value of the circuit, the transistors T 1  and T 2  will be in their conductive states, biasing diode D into its forward conducting state. 
     The current-voltage characteristic of FIG. 1 is shown in FIG. 2. As seen, the curve has a sharp knee at approximately 1 volt and rises substantially vertically from that point. From the characteristic curve it is apparent that the circuit of the invention not only provides voltage stabilization at a very low voltage value, but also has a very low dynamic resistance (ΔU/ΔI), as compared to devices such as the conventional Zener diode. 
     In the embodiment of FIG. 3, a resistor R is connected in parallel with the diode D. The primary role of the resistor R is to sharpen the curve of the characteristic when necessary. 
     The characteristic U-I of FIG. 2 was obtained by measurements on the circuit of FIG. 1, i.e., without resistor R, having the following elements: 
     T 1  --BC212A, manufacturers Ei 
     T 2  --BC182, manufacturers Ei 
     D--AAZ21 , manufacturers Ei , 
     It will be understood that the circuits of the invention can be fabricated using integrated circuit techniques, as well as with discrete components.