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 (only 1-3 and 2-4 are short circuit 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 inside the meter 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) (Ground is already built inside the decade boxex)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 (no breadboard needed) • 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 resistive 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 – No DC connect. • 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 (Remember GND, File – Save, and V and I) • Connect the following circuit in PSPICE only17 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 P-Spice Only • Obtain the voltages 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 … …