Basic Circuitry and Measurements Lab 3 1 Equipment List • • • • • • Multimeter DC power supply Breadboard Decade resistance box (2) R-C combination box Resistance: 1kΩ, 2.2kΩ 2 Circuit Testing • To see if a circuit works, voltages and currents have to be measured • Multimeters are used to measure DC voltages and currents 3 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. 4 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 sheet 5 Valid Terminal Pairs • Internal circuitry of a decade box: 1 3 2 4 • Any valid terminal pair consists of one red and one black terminal 6 “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 displayed 7 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 sheet 8 Connecting a Circuit • Can we connect the following circuit using one decade box? (Assume R1 and R2 have the same values) 9 Can These Connections Be Used? 1 3 2 4 • Why or why not? Record your answers on the data sheet 10 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 sheet 11 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 current 12 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 findings 13 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 sheet 14 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 introduced 15 Effect of Lower Internal Resistance • Connect the following circuit in PSPICE 16 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 divider 17 Measurements • Measure the voltage across R2 for each of the 3 values of R2 • Record the data on the grading sheet 18 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 ideal 19 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 circuits 20 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 purposes 21 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) = 1000 22 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% variability 23 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 sheet 24 Breadboard • The breadboard has two halves separated by an indentation. The holes in the breadboard are electrically connected on the bottom of the breadboard 25 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 connections 26 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. Take your time. For chips, slide something (a wire or jeweler’s screw driver blade that you have handy) under the item you are extracting and gently raise it 27 Build a Voltage Divider + Series Connection Power Supply R2 (2.2k) R1 (1k) 28 Measurements • Use the multimeter to measure voltage across each resistance individually • The multimeter just clips on to either side of the resistance across which you are measuring voltage 29