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Bell & Howell Information and Learning: Formula Omitted ...
The current limiting diode (CLD) or current regulating diode (CRD) has been available since the early 1960's. Unfortunately, despite its simplicity and distinct advantages over conventional transistorized applications, it has seen only limited use. One reason may be designers lack of familiarity with practical circuit design techniques involved with its use. Another reason may be that although many papers have been published on the device, most have dealt primarily with solid-state theory, rather than with practical applications. Therefore, it is the purpose of this paper to focus on how this device is used, rather than on what it is.
Conventional Constant Current Source vs. CLD
From basic circuit theory, an ideal current source is one with infinite output impedance. The term constant current source usually applies to a circuit that supplies a DC current whose amplitude is independent of a change in either load or supply voltage.
Basic Constant Current Circuit
The simplest circuit is a voltage source in series with a resistor as shown on Fig.1. The current is (V^sub S^-V^sub L^)/R^sub S^. The current would change very little if the load voltage, V^sub L^, is small compared with the supply voltage, V^sub S^, and the source resistance, R^sub S^, is much larger than the load resistance, R^sub L^. When the load voltage is in the order of several volts and accuracy within a few percent is required, the circuit in Fig.1 can be achieved only if V^sub S^ has a magnitude of several hundred volts. This may be feasible, but is impractical.
Transistor Constant Current Source
For a constant current source, the use of a transistor as snown in Fig.2 would eliminate the need for a high voltage source. This circuit provides a constant current of approximately 10 mA, which is determined by the current through Rl, and in turn on the voltage across R^sub 2^, i.e. V^sub S^*R^sub 2^ [(R^sub 2^ + R^sub 3^)R^sub 1^]. Since I^sub R1^=I^sub E^=I^sub C^=I^sub L^, The load current, I^sub L^, is also V^sub S^*R^sub 2^/[(R^sub 2^ + R^sub 3^) R^sub 1^]. This current will maintain its constant amplitude provided the transistor is not saturated, i.e. V^sub L^ < S*R^sub 3^/(R^sub 2^+R^sub 3^).
As the load, R^sub...