ELECTRONIC SYSTEMS (ECT226)
Introduction
This exercise was used to prove the different electrical properties using Multisim, a simulation software. Different electronic circuits such as half-wave and full-wave rectifiers are used to prove that Multisim can be used to find values and characteristics of the wave produced by the half wave rectifier and the full-rectifier rectifier using different capacitor and resistor values. Multisim will also be used to prove the properties of a Wheatstone bridge and a bridged T network (pad).
A half wave rectifier is a circuit that is responsible for converting alternating (AC) input power to direct (DC) output power. The peak voltage refers to the maximum voltage reached. Ripple voltage refers to the unwanted periodic variation of the of the DC output power which is being derived from AC input power. Ripple is as a result of incomplete suppression. This exercise has enable the student to interact and use the Multisim software.
Worksheet 1 (2013)
Using Multisim® construct a half wave rectifier with input frequency of 50 Hz at 10 Vpk. Choose virtual components to start with. During the workshop you will be varying the smoothing capacitor (C) value (consider a logarithmic scale) and show that the following equation holds true.
V=qC so dVdt=1Cdqdt=iC giving Vripple=dVdt×t
Q = CV
I = C(dv/dt)
I(t) = C(dV(t)/dt)
Ci = dQi/dVi
How does the ripple voltage vary with capacitance?
Ripple is usually a result of incomplete smoothening of the alternating current (AC) waveform. The capacitance should be varied so as to get rid of unwanted ripple. Increase in capacitance leads to reduced ripple voltage.
What value is t?
18.0006 ms
Increase the value of C1 in a logarithmic manner. At what approximate value of C1 is there an insignificant decrease in ripple voltage for a doubling of value of C1?
0.01F
What is the effect of doubling the load resistance to 100 Ω?
Doubling the resistance reduces the rate at which the capacitor discharges. This is because a high resistance causes the less current to flow across the resistance. When the capacitor is not charged, it acts like a short circuit. When the capacitor is charged, it develops a resistance. The capacitor and the load are in parallel. Therefore, the current will pass through the component with low resistance. If the load has a low resistance, then the capacitor will not be charged. However, when the load has a high resistance, the capacitor will be charged.
What happens if we increase the load to 1 k?
If the load is increased to 1 k, the current will charge the capacitor. This is because due to the high resistance of the load, the current will opt to flow through the capacitor thus charging it.
Draw a graph of the affect of capacitance on ripple for a given load (give some thought to the scales used).
PTO
The capacitance reduces the ripple and smoothens the curve.
Now modify your circuit substituting the RESISTOR by a RESISTOR_RATED one.
What will happen if you simulate the circuit without changing any component settings?
The dissipated power (I2R) will increase above the rated power (0.25 watts). The resistor will burn out and it will act as an open circuit. This is because V = IR therefore, R = V/I; since the voltage remains constant, an increase in resistance will lead to a reduction in current. Therefore, since the power loss is given by I2R, the power loss will reduce with high values load resistance (Paul, 2010).
What setting of the RESISTOR_RATED needs to be changed and to what value?
Resistance of the RESISTOR_RATED should be changed. The resistance should be changed to 82
Now create a full wave rectifier using RESISTOR_RATED and CAPACITOR_RATED values.
How does the ripple voltage vary with capacitance value?
Increased resistance leads to reduced ripple.
At what point is there an insignificant decrease in ripple voltage for a doubling in C?
1.6 mF
What is the effect of doubling the load resistance to 100 Ω?
At 50 Ω
At 100
Increasing the resistance to 100 will reduce the ripple
What happens if we increase the load to 1 k?
An increase in the load to 1 k will produce a more smother curve thus reducing the ripple.
Draw a graph of the affect of capacitance on ripple for a given load.
The capacitance has the effect of reducing the ripple.
DIRECTED LEARNING
Could you use a 1N4001 diode instead of the DIODE_VIRTUAL?
The 1N4001 is contains general purpose silicon rectifier diodes that can be used in AC
Adapters. The 1N4001 diode may be used instead of the DIODE_VIRTUAL.
Electronic Systems (ECT226)
Workshop 2 (2013)
Simulations of Flash Analogue-to-Digital Converter (ADC)
Objectives
Procedures
For a 2-bit Flash A/D you will need ___3______ comparators (a zero output is the default). Hint: Use the formula 2N-1.
N = 2
Calculate the voltages (V1, V2 and V3).
V3 = 3/4Vref = ¾ x 5 v = 3.75v
V2 = ½ Vref = ½ X 5 v = 2.5 v
V1 = ¼ Vref = ¼ x 5v = 1.25v
Input the whole FLASH 2-bit ADC into a Multisim schematic.
If you use a real or virtual op-amps or comparators what will happen if you a bipolar supply (positive and negative)?
A comparator is used to compare two voltages and show which is the larger voltage. A comparator may also be used to compare two currents and determine the larger current.
Set Vref = + 5V,
Try the following values for Vin (you can either change the source voltage or replace it by a potentiometer:
0.5 V
2.0 V
3.5 V
(iv) 4.5 V
For each case, the comparators compares the V1, V2, and V3 with the 2 V to find which is the greater (Brimicombe, 1987).
