Electronic Design - Voltage Regulator Models

PART I – Linear Series Regulator

Simulation Schematic 

Observation

DC Sweep:

Calculation

Let us calculate the Output voltage theoretically. Therefore,

          V _(Zener) = 4.7 V; (Taken from the Datasheet) and R₂ = R₃ = 1kΩ;

Operation of the circuit

 

                                     Circuit shown in the figure which we built is an ‘Op-Amp Shunt Regulator’. Same like in Series Regulator, the Op-Amp is connected here as a Comparator which compares the samples output and the reference Voltage taken from the Zener Diode. In contrast, the Control element ‘the transistor’ is connected in parallel here. So in here as the Output voltage increases, the comparator detects it via the feedback loop by comparing it to the reference voltage. As the feedback loop is connected to the positive terminal of the Op-Amp, the comparator detects a higher difference between the inputs incase of higher output Voltage. Due to this the Op-Amp output is increased which drives the transistor more. Hence the shunt current is increased and grounded via the transistor, reducing the output current, thus reducing the Output Voltage. 

Here the resistor R₄ is connected as a safety component for the Regulator. Because in case of Transistor being driven, the Regulator itself acts as the load for the incoming power supply. To avoid this ‘Drop resistor’ is added in addition.  

PART IV – Positive Voltage Regulator IC

Simulation Schematic

Input Voltage = 20 V (< 25V)

                                     Output Voltage = 5 V

 Calculations

 

1)      Let us bypass the diode and get the DC Sweep results.

DC Sweep result:

When we considering the diagram,

We can see the Output voltage starts to regulate at 5V when Input voltage = 6.8V (Value generated in the Excel Sheet)

Therefore,

Drop-out Voltage (When the Diode is bypassed) = V_in (min)- V_out

                                                                              = 6.8 V – 5 V

                                                                              = 1.8 V.

 

1)      Effect of Capacitors

 

                         When the Capacitors in parallel with the regulator input, output and ground, they show low impedance for AC current through the capacitor and ground, hence reducing ripple caused by noises. Because of that, these capacitors act as Decoupling Capacitors to filter the noise and give out a clean DC Output especially in input.

 

2)      Effect of Diodes

 

                         Diode stops and protects the power supply from any reverse currents that could occur due to the filter capacitor or by the Regulator itself.

PART V – LM317 Voltage Regulator

1)    1) V_DC = 10 V

 

        Simulation Schematic

Input Voltage vs Output Voltage

Output Voltage

Output Voltage = 3.78 V

 Voltage observed across R₁:

        Simulation Schematic

Input Voltage vs Output Voltage

Output Voltage

Output Voltage = 3.78 V

 Voltage observed across R₁:

 

Operation of the circuit

 

 

                              As per the data sheets, LM 317 is an Adjustable Voltage Regulator. In here we can adjust the Output voltage unlike in Series and Shunt regulators. The output voltage is adjusted by two external resistors used in the circuit designing. In the above simulation, they are R₁ and R₂. In here the voltage across the Output terminal and the adjust pin is maintained at a constant level of 1.25 V, in this case across R₁.

 

Therefore, the Output Voltage is equal to;

 

                       

As we can see the Output Voltage solely depends on the External Resistor values and the Reference Voltage only regardless of the input voltage.

From the above two simulations we could see that the Input voltages are fluctuating in the range of 10 V – 11 V for an input of 10V DC and 6 V – 7 V for an input of 6 V DC because of the AC voltage source at the input with 1V peak. But still the resulting output voltages at  V_DC = 10 V and V_DC = 6 V are the same with 3.78 Vat both instance as the external resistors R₁ and R₂ are kept constant at both the scenarios. But still we can see some fluctuations in the output voltage with an Input of 6V.

DISCUSSION

 

PART I and PART II

                  If we need a regulated voltage output when designing a circuit, we use voltage regulators. In the above simulations, Linear Series and Linear Shunt Regulators gives a similar DC Sweep output graph. This is because in both the cases the reference voltage is taken from the Zener diode even though the two circuit configurations are a bit different. Then the feedback is taken to be compared with the reference voltage even though they are connected to different terminals of the Op-Amp in the 2 cases.

DC sweep shown in the above figure is common for the both Series and Shunt Regulators. The light blue line represents the ‘Reference Voltage (Zener Voltage)’ while the dark blue line represents the ‘Feedback loop voltage’. Therefore it is apparent that the regulator gives out the regulated output once the difference between the Reference Voltage and the Feedback voltage achieves zero.  Until the Feedback loop Voltage acquires the reference voltage;

·         The transistor (Control element) in Linear Series regulator allows more current to pass through itself until the required Output voltage is received.(Op-Amp Output is shown in ‘Blue’ in the graph below to show the transistor drive)

·         The transistor (control element) in Linear Shunt regulator minimizes the shunt current passing through itself until the required output voltage is received. .(Op-Amp Output is shown in ‘Blue’ in the graph below to show the transistor drive)

    We also could see a slight deviation in the Simulation Output voltage and the theoretical output voltage in both cases. This is because of the variations shown by the reference voltage or the Zener voltage with itself with the data sheet value.          Even though the data sheet value represents a nominal value of Zener Voltage o be 4.7V, we could clearly see in the DC sweep that it averages around 4.66V. Therefore from that point where the output voltage is 9.32V, the Output voltage is regulated and maintained constant at 9.32V.

 

PART IV

LM 7805 is a Positive Voltage Regulator IC in the family 78xx series. This generates an output regulated at 5V. For this IC to operate the minimum Drop-out voltage should be 2V, therefore the input range starts from 7V up to 35V of max. 

    A LM 7805 is recommended to use with Capacitors connected in parallel to the input and output terminals even though they are not mandatory. But to have a clean DC output for some applications, capacitors are highly recommended.  This is because, in case if the Power supply filter and the regulator are at the considerable distance, there could be noise added in the transmission path. To eliminate this, input decoupling capacitor is added and the output decoupling capacitor is added for even better resolution.  

    In addition, we could add higher rated capacitors in parallel to these depending on the transient requirements on the regulator application as shown in the diagram below.

This additional capacitor at the input is used to smoothen the input and the capacitor at the output to compensate the transient needs of the circuit in case of power shutdown.

Furthermore, the diode connected in the circuit is responsible to eliminate the reverse current that could originate from the charged input capacitor or by the regulator itself.

PART V

LM 317 is a Positive Linear Adjustable Voltage Regulator where you can get a desired regulated output between 1.2V to 37V. 

Simulation in the figure above, we can see that our output is regulated to 3.78V.  In here the voltage between the external resistors R₁ is used as the Reference voltage where it is maintained at a constant level of 1.25V. In here R₁ and R₂ are the responsible components to regulate the output voltage regardless of the input variations.

For the above two simulations with LM 317 regulator an AC source with 1V peak is connected at the input so that the input voltages are fluctuating in the range of 10 V – 11 V for an input of 10V DC and 6 V – 7 V for an input of 6 V DC. Comparing the output result in both scenarios, the output voltage seen at the Output put terminal were the same regardless of these input fluctuations and show a value of 3.78V in the simulation and the theoretical values also happened to be the same with 3.77V. When comparing the theoretical and simulation value, we could see a high precision in the obtained values as both the values are equal to the 2^nd decimal point.

When considering about the adjustment terminal, as it also contributes to the Output voltage, for more stability, the Quiescent current (I_adj) is maintained constant by the LM317 circuit itself to be equal to 100μA or less. Therefore, in our case, the Quiescent current is maintained at 49.8μA which leaves the Output Voltage manipulation solely in the External Resistors.

Apart from that, when we observe the output voltage when the Input is 6V, we can see smaller fluctuations. This is mainly because of the Drop-out voltage. Drop-out voltage seen for the LM371 regulator at the data sheet is 2.5V. Therefore as for the regulator to work at the minimum condition, the input voltage should be,

                                    V_in = Drop-out voltage + V_out

                                      V_in = 2.5V + 3.78V

                                      V_in = 6.28V  

Therefore, it is apparent that, for the regulator to work, the minimum input voltage should be 6.28V. As the voltage of 6V is given by the DC source and 1V peak Voltage is also given, the voltage oscillates between 6V and 7V, until it reaches 6.28V the disturbance could be seen in the output.