Voltage Regulator and Power Supplies


The circuit below contains the three basic elements of regulated power supply. They are an input (15V) considerably higher than the desired output, a reference voltage from a 7.4V Zener diode, and a transistor amplifier (2N4231A).  If load increases (resistance Rload decreases) any decrease in voltage at the emitter of the transistor will increase the transistor forward bias stabilizing the output voltage. 




Learn More and Test Understanding of above circuit.




Observe Animation Below: The Dynamic Load Simulator* produces a variable direct current load initially, and generates a AC load in the lather part of the Gif animation.  Dynamic Load Simulator sole purpose is to simulate loads that are not constant.  You should observe how the feedback increases the base emitter bias across T1 as the current increases.  The current meter displays both DC and AC readings.  The signal generator displays frequency during AC measurement phase of animation.  DC current was held at 3 Amps during AC measurement phase.



Above voltage regulator simulation uses a battery rather than a rectifier as a power source.  Since batteries do not generate ripple voltage, I provided a dynamic load to demonstrate that voltage regulators not only adjust for slow changes in current but can attenuate AC and ripple voltages as well.  The frequencies I used were 50, 60, 400, and 1200 hertz corresponding to European, US, Military, and three phase 400 ripple hertz ripple frequencies.  Thus, it shows effect of regulator on AC current loads and illustrates the effect the regulator would have on the ripple voltage of rectifier assembly.

Note that Zener diode reference is about 7.4 volts, the power supply output is 15.0 volts.  This is an improvement over the design of a simple regulator at top off page, in that the output voltage level can be adjusted for a wide range of voltages. Only a fraction of the output voltage is compared to the Zener voltage.  The potentiometer determines the fraction of the 15.0 volts output that is feedback via the base emitter junction of the unlabeled transistor to the reference Zener diode.  The error signal or voltage is amplified by the three transistors.  The amplified voltage at the collector of the unlabeled transistor is current amplified by T1 and T2.  Thus, when the voltage output drifts higher, the voltage at the Base of T2 and consequentially the voltage at the Base T1 are forced lower.  This reduction of the forward bias of the T2 base emitter junction result in less current flow through T2 and to the load.  The reduction in current output results in a lowering of the voltage across the load in accordance with ohms law.  The loop described above is called a negative feedback loop.  Low frequency ripple voltage generated by the Dynamic Load Simulator* is also feedback via the same current loop and partially cancels itself out. At low frequency the feedback is essentially 180 degrees out of phase.  At higher frequencies the phase shifts from 180 degrees reducing the effectiveness of the feedback loop.          


*Dynamic Load Simulator -   The Dynamic Load Simulator shows the effect of a changing load on a regulated voltage.  If you were to imagine the source of the ripple voltage as the battery, then this simulation could be used as an illustration of ripple voltage from a rectifier being reduced by a voltage regulator.  Based on my decades of troubleshooting regulators with an oscilloscope, I think that this illustration is quite good. Note that this is a method of forcing my CAD software to generate a ripple voltage at the output of a battery.  I am not recommending this technique be used in the laboratory.