TRANSISTORS AND JUNCTION DIODES 



841 



A shunt regulator is a variable current device, connected in parallel 

 with the load. Both the load and th(> shunt re<i;ulator draw current iVom 

 the source of power throuj^h a common impedance, 'i'he operating ic- 

 ([uirement for a good shunt regulator is that the voltage drop across it 

 must remain constant over a certain range of current. Certain types of 

 cold cathode tubes such as the \'R-150-30(0D3) exhibit this effect. It 

 has been determined that certain semiconductor junction diodes exhibit 

 the same efTect. Note that diode No. 1 that is shown in Fig. 3 has a re- 

 verse ^'oltage drop of about 2-1 volts over a range of reverse current 

 from less than 1 milliampere to almost 10 milliamperes. If such a device 

 is connected in parallel with a load as in Fig. 11, shunt regulating action 

 will take place. Consider the operation of the circuit in Fig. 11, first 

 assuming that the load impedance is constant and that the potential of 

 the power source increases. Additional current tends to flow from the 

 source, but since the potential of the reference voltage diode designated 

 "s" in Fig. 11 is fixed, this additional current develops an increased 

 voltage drop across the regulating resistor, and the load voltage does not 

 change. Similar reasoning can be applied to the case of a decrease in 

 source voltage. Next assume constant source voltage and an increase 

 or decrease in load resistance. This would normally tend to cause a 

 change in the voltage drop across the load, but the shunt element draws 

 respectively more or less current than normal and the load voltage again 

 does not change. 



The value of the regulated load potential in Fig. 1 1 is controlled by the 

 saturation voltage of the diode and it cannot be adjusted to any other 

 value. The accuracy of regulation is controlled by the slope of the reverse 

 characteristics shown in Fig. 6. An additional limitation is that the 

 usefulness of this tj^pe of regulator is controlled by the power handling 

 capacity of the diode. The next section shows how these limitations can 

 be circumvented by the addition of transistors. 



REGULATING 

 RESISTANCE 



— ''V^A 



FROM 

 UNREGULATED 

 DC VOLTAGE 



REFERENCE 



VOLTAGE 



DIODE 



I 



I 



REGULATED 



OUTPUT 



VOLTS 



I 



I 



P. 



Fig. 11 — Simple shunt regulator. 



