1 Basic Circuitry and Measurements Lab 32 Equipment List • Multimeter • DC power supply • Breadboard • Decade resistance box (2) • R-C combination box • Resistance: 1kΩ, 2.2kΩ3 Circuit Testing • To see if a circuit works, voltages and currents have to be measured • Multimeters are used to measure DC voltages and currents4 Using the Multimeter • Switch the multimeter on. We will first use it to measure resistance • Push the third soft key in from the left on the top row. The display should say that it is measuring ohms • There are five terminals on the right in two columns. Never use the terminals on the left column • Place one wire in the ‘HI’ and and the other in the ‘LO’ terminal.5 Using a Decade Box • Find one of the decade resistance boxes. The four terminals on it form six possible pairs e.g., top left and bottom right. List these six combinations • Set the box to 1 kilo-ohm and then use the multimeter to measure the resistance for each of the six possible pairs • Record the values on your data sheet6 Valid Terminal Pairs • Internal circuitry of a decade box: • Any valid terminal pair consists of one red and one black terminal 1 2 3 47 “Measuring” Resistance • The multimeter generates a small test current which it runs through the resistance being measured • The multimeter then measures the voltage drop across the resistance • Ohm’s Law is used to compute the resistance value which is then displayed8 Measurement Limitations • Can the multimeter be used to measure the resistor’s value without first removing the resistor from the network? • Record your answer clearly and legibly on your data sheet9 Connecting a Circuit • Can we connect the following circuit using one decade box? (Assume R1 and R2 have the same values)10 Can These Connections Be Used? • Why or why not? Record your answers on the data sheet 1 2 3 411 Connecting a Voltage Divider • Connect the following circuit using two individual decade boxes and a DC power supply • Values for R2 are given on your grading sheet12 Taking Measurements • Use the multimeter to measure the voltages across the resistances and the current flowing in through the circuit – Voltage Measurement: Connect the multimeter in parallel. Use the Rightmost HI and the rightmost LO terminals. Push the DC V button – Current Measurement: Use Ohm’s Law – It is usually easier to measure voltages rather than current13 Limitations of Measuring Devices • For the voltage divider circuit, set both decade resistance boxes up to their maximum value (999,999 ohms) • Experimentally check KVL by measuring the voltage drop across each resistance individually – Do the voltage drops add up to the source voltage? Explain your findings14 Reason For The Limitation • Connect two decade resistors in parallel. Set one decade box to 100 kilo-ohms, There is NO source in this circuit • Use the multimeter to measure the resistance of the parallel combination for different resistance settings of the second decade resistor given in the grading sheet15 Reason For The Limitation • When two resistances are connected in parallel, the resistance of the overall combination decreases – Less voltage is dropped by the same current across a lower valued resistance • Therefore, if the resistance across which the voltage is being measured is comparable to the internal resistance of the multimeter, errors in measurements are introduced16 Effect of Lower Internal Resistance • Connect the following circuit in PSPICE17 Circuit Significance • This circuit is to simulate the case when a multimeter with low internal resistance 100k (depicted by R3) is used to make measurements – This circuit is similar to the case when an actual multimeter is connected across R2 in the original voltage divider18 Measurements • Measure the voltage across R2 for each of the 3 values of R2 • Record the data on the grading sheet19 Understanding the Difference • When we use the multimeter to make DC voltage measurements, it has an input resistance of 10 MW • In the second circuit, R3 depicts a multimeter with relatively low input resistance. Use the measurements you made to explain why low input resistance is not ideal20 Building Circuits Differently • Circuits can be built with components used in front end circuitry of equipment such as oscilloscopes, wave form generators • Components commonly used are – Resistances, inductances and capacitances – Chips, integrated circuits21 Component Specifications • Information supplied about each component – Nominal value – Range of variability about the nominal value – Power ratings – Temperature ratings – Noise issues • We will need only nominal values and the variability for our purposes22 Reading Resistance Values • Resistances come as 3 band or 4 band resistances. We deal only with 3 band resistors • The colour of the bands indicate the value of the resistance 0 - Black 1 - Brown 2 - Red 3 - Orange 4 - Yellow 5 - Green 6 - Blue 7 - Violet 8 - Grey 9 - White 1st band = 1st digit of resistance value 2nd band = 2nd digit of resistance value 3rd band = Multiplier Example: Brown = 1, Black = 0, Red = 2 Resistance = 10 (Brown, Black) X 10^2(Red) = 100023 Variation About Nominal Value • There is usually an extra band that denotes the variability of the value of the resistance about the stated value – Gold - 5% variability – Silver - 10% variability – No band - 20% variability24 Reading and Measuring Resistance • Using the colour codes, figure out the value of the two resistances • Use the multimeter to measure the actual resistance of each of the two resistors (Clip a wire onto each end of the resistance) • Record your values on your data sheet25 Breadboard • The breadboard has two halves separated by an indentation. The holes in the breadboard are electrically connected on the bottom of the breadboard26 Breadboard • The holes between the indentations are electrically connected column wise • The holes at along the red and blue lines are electrically connected along the rows • The indentations denote electrical separation • The top and bottom halves are symmetrical … … Indentations Electrical connections27 Placing and Removing Components • Make sure that the pins on chips or wires are straight and that they line up well with the holes you intend to use • Extract things slowly and carefully.
View Full Document