electronic devices and circuit prototyping

  1. Voltage regulator design

This exercise assignment requires you to analyse the design of a type of voltage regulator, simulate its performance in Multisim, calculate some component values and answer a number of questions about the circuit.

 

The use of Multisim was covered in the module practical assignments and all information about how to use the software, with examples of simulation results, are still available on the module Study Direct site (see the section ‘Week 4-6 practical sessions’.

 

You should submit your answers either hand or type-written on paper, along with printouts of the Multisim circuit and screen snippets of the meter and oscilloscope readings obtained during simulation.

 

  1. The circuit

Figure 1 below shows a type of voltage regulator circuit, in the dashed box. It takes an 18 V average DC input voltage and regulates it to provide 10 V DC to a 20 W load. All component types and values are labelled and you should use these to construct the circuit in Multisim.

 

 

 

 

 

BC108BP

 
BZX79-C5V6
Load

20 W
DC power source

18 V
RZ

549 W
RQ

715 W
R1

634 W
R2

374 W

 

 

Figure 1

 

 

  1. Questions

Answer the following questions.

(i) What is the name of the circuit of Figure 1?

(ii) What components in the circuit determine the value of the output voltage applied to the load?

(iii) Given that the zener diode voltage is nominally 5.6 V, calculate the ideal values for R1 and R2. Explain all steps in your calculations and justify the choice of any unspecified parameters.

(iv) Given the results of these calculations, explain the justification for the resistor values given in Figure 1.

(v) Tables 1 and 2 show extracts from the datasheets for the two transistors specified in Figure 1. Identify the two most important ratings to be considered in the design of this circuit and explain the justification for choosing each of these transistors for this design.

 

 

Table 1: TIP31AG – Absolute maximum ratings

 

 

Table 2: BC108BP – Absolute maximum ratings

  1. Simulation exercise

(i) Construct the circuit of Figure 1 in Multisim and include a printout of the circuit with your submission.

(ii) Add three multimeters connected to measure the following; the load voltage, the load current, and the current through the chain of resistors R1 and R2.

(iii) Run the simulation and record all of the voltage and current values obtained.

(iv) Comment on the values recorded by explaining whether they are as expected and, if not, what the possible reasons could be.

 

The following tasks are designed to simulate the effect of applying an average 18 V DC input with a finite amount of ripple.

(v) Replace the DC source in Figure 1 with an AC voltage source. The Multisim symbol for this is shown in Figure 2. Edit the AC source parameters to give 1 V pk, 18 V voltage offset and 100 Hz.

Include a printout of the revised circuit with your submission.

Figure 2

 

(vi) Connect an oscilloscope to measure, on channel A, the output from this AC source and run the simulation. Provide a snippet image of the oscilloscope screen and comment on what the result shows.

(vii) Re-connect the oscilloscope to measure, on channel A, the signal across the load. Provide a snippet image of the oscilloscope screen and comment on what the result shows.

(viii) Increase the level of peak voltage on the AC source until you can observe ripple on the load voltage. Record the peak AC voltage and provide a snippet image of the oscilloscope screen showing the resulting signal across the load.

(ix) What do you conclude about the performance of this circuit as a voltage regulator?