Meters, Power Supplies and Generators

Transcription

1 1. Meters Meters, Power Supplies and Generators Generally analogue meters respond to the average of the signal being measured. This is due to the mechanical mass of the pointer and the RC response time of the electronics. Since the average of most ac signals is zero, ac meters require more circuitry to measure values of time varying voltages. Many meters can function as both current and voltage meters. Usually there are separate inputs for the voltage and current functions. Trying to use the instrument as a voltmeter, when the leads are connected to the current input will either destroy the meter or blow a fuse. Voltage Measurements Voltage is analogous to pressure in a water pipe. And like pressure, it has to be measured relative to a reference. A voltage is measured across a circuit component, to indicate the Figure 1. The Voltmeter voltage difference or voltage drop. The dashed lines in figure 1 indicate that all parts are contained within one physical unit. In this case, the resistance is a part of the meter and there is no way to separate it from the meter. An ideal meter has a resistance of 06 or 6 depending upon the type of meter (current or voltage) and the meter model (series or parallel). To measure voltage electronically, a voltmeter uses a known, stable, resistor. All voltage meters draw current, because of that resistor. A typical digital volt meter (DVM) has a resistance of 10 M6 while some older analog meters may be as low as 3.2 k6. Current Measurements Current is the rate change of charge and is, therefore, a measure of the number of electrons per second moving through a circuit. Since the current into and the current out of a lumped component are equal, you cannot measure a current drop across a component. To measure current you have to measure the actual charge flow through the wire. -1-

2 Figure 2. The Ammeter This measurement is done by inserting relatively small resistor in series with the circuit and measuring the voltage drop across the resistor. This is normally shown transformed into a series resistor and an ideal current meter. Both the series and parallel models are shown in figure 2. In the typical digital multimeter (DMM), on a 2A range, the resistance is 100 m6. Because a current meter has resistance it will cause a voltage drop and may affect the operation of the circuit. The actual amount of resistance depends upon the sensitivity of the meter. For example, the DMM on a 200 ua range has a resistance of 1 k6, while a nanoampere meter will be more than 100 k6. Power Measurements Measurement of power (product of voltage and current) may be made using a wattmeter. Measurement Errors This is a complex issue and we will only cover the basics. An error causes the measured value to be different from the actual value. Human, or gross, errors are usually due to mistakes in using or reading an instrument. Two examples are viewing an analog meter needle at an angle rather than straight on (parallax error), and reading the wrong meter scale. Other errors may be caused by the instruments themselves. Plots of data (figure 3) typically include error bars, which indicate the accuracy of the measurement, the effect of the meter loading upon the circuit, etc. Plots of theoretical calculations can also include a tolerance to account for the non-perfect circuit components (ie ±1%, ±5%, etc), temperature variations, etc. Figure 3. A Plot Illustrating Error Bars Accuracy The accuracy of a measurement is how close the value came to the actual value that exists, not the theoretical one. Instrument accuracy is usually broken down into systematic errors and random errors. The systematic errors are either instrument related (ie friction in a mechanical meter pointer bearing) or environmental. The method by which the meter works (mechanical, electrical,..), and the quality of design and construction, greatly affect how large these errors are. Quite often both of these are further broken down into static and dynamic errors. Dynamic errors would be caused by the meter not being able to respond fast enough. Precision Precision pertains to the number of significant figures and the repeatability of a measurement. High precision instruments provide more significant figures and have greater repeatability. -2-

3 Resolution This is the smallest change in the quantity being measured that an instrument will indicate. For a digital meter, this is simply one of the smallest digits. With an analog meter, this is usually ½ of the smallest divisions on the meter face. Repeatability The repeatability is a measure of the fluctuations which are due to noise and errors that are not repeatable. When a measurement is repeated several times, and then statistically analysed, the repeatability can be expressed as the standard deviation. Uncertainty This is the combined effect of accuracy, precision, repeatability, noise, and errors on the measurement. It represents how uncertain the measurement is. 1.1 Calculating Meter Error Analog Meters The error for an analog meter is usually not explained in detail. However, most of the error is due to the mechanical meter movement. This error is constant, across a given meter range, regardless of the magnitude of the variable being measured. An analog meter stamped as being accurate to 2%, is accurate to 2% of the full scale deflection (FSD). For example, suppose a reading of 35.0 V (dc) was obtained on the 50 V range of a dc voltmeter, that is accurate to 2%. The error is: 2% * 50 V FSD = ±1 V Therefore V = 35.0 ± 1 V (dc) or expressed as a percent V = 35.0 V (dc) ± 2.9% Since the absolute error of ±1 V is constant for the range, the smaller the measured voltage is in relation to full scale, the larger the percent error becomes. If this meter range was used to measure a 9V battery the error would be ± 11%. For analog meters, it is more accurate to measure values with the needle close to full scale. Digital Multimeters The DMM is the most common instrument you will encounter. The precision of a DMM is usually expressed in digits. The most common model has 3 ½ digits. That means that you have three full digits that are capable of indicating from 0 to 9 and one ½ digit. The ½ means that a +1, blank ie. 0, or -1 can be displayed. There are also 4 ½ and 6 ½ digit meters as well as 3/4 digit meters. A 3/4 digit meter can display +2, +1, blank (0), -1 or -2 as the leading digit. A DMM has two sources of systematic error. One part of the error is due to tolerances in resistors. It is expressed as a percentage of the input and therefore varies with the magnitude of the voltage being measured. The second part of the error is due to the digital conversion process. It is fixed for a given meter range and is expressed in the number of digits to which the reading is accurate. Each digit, has the value of a single count of the right-most digit on the display. For example, consider a reading of 1.345V on a 2 V dc range. The meter displays a maximum of V on this range. The smallest change on the meter display is V. Then the errors are, -3-

4 percent error: 0.1% * V = mv digit error: + 1 digit = 1 * V = 1 mv. Therefore, V = V ±2.345 mv or V = V ±0.17%. Now consider the same measurement on the 20 V range. The reading would have been 1.34 V and, percent error: 0.1% * 1.34 V = 1.34 mv digit error: + 1 digit = 1 * 0.01 V = 10 mv. Therefore, V = 1.34 V ±11.34 mv or V = 1.34 V ± 0.85%. This shows that the most accurate measurements are made on the lowest meter range that will accommodate the measured value. If the meter were to read V (rms) while measuring an 8 khz triangle wave on a V RMS scale; how large is the error? The meter measures True RMS and we can trust a reading done on a triangular wave. On the AC range the error is 0.5% + 2 digits. Then the errors are, percent error: 0.5% * V = 80.6 mv (rms) digit error: + 2 digits = 2 * 10 mv = 20 mv (rms) Therefore V = V RMS ± 101 mv RMS or V = V RMS ± 0.62%. Below is a table of the accuracy for various measurements made with the Fluke 8010A model. Measurement Accuracy: ± (% of reading + digits) DC DC current (upto 2A) AC, 45 Hz to 10 khz AC current (upto 2A range, 45 Hz to 2 khz) Resistance (below 200 k6) Meter Loading Every meter affects the circuit it is connected to by drawing current (voltage meter) or having a voltage drop (current meter). In later years you will learn more about other ways in which a meter affects a circuit (capacitance, inductance, adding noise,...). Analog Meters By the end of E&CE-100 you should able to calculate the resistance between the terminals of a voltmeter, and determine if it will affect your circuit significantly. An analog voltmeter s resistance is usually specified as a sensitivity in ohms/volt. The resistance for a particular voltage range is determined by multiplying the sensitivity by the full scale voltage on that range. Some meters may give the total resistance while others give the current required for full scale deflection. As long as the meter draws much less current than the component across which it is connected, you can say that its effect will be small. A better technique is to calculate the error introduced, by the current that flows through the meter, and see if it is less than the accuracy to which you want to make a measurement. Ammeters are more complex. The standard current shunt (precision resistor in the meter) has a 50 mv drop at the maximum current rating. Since this voltage is usually small, an ammeter should not affect the circuit too much. However, an ammeter does affect a circuit by adding inductance. This will be explored more later. Remember that every time you change the range, the shunt resistance changes. Since the shunt resistance, in a normal meter, can vary from -4-

5 milliohms to kilo-ohms, using the wrong current range can easily affect your circuit. In comparison, a voltmeter s resistance will typically only vary from 100 k6 to 10 M6. Digital Meters For all ac and dc voltage ranges, the DMM has a fixed input resistance which is typically 10M6,. On the current range, the meter will usually have a 200 mv drop at the maximum current, but the resistance is different for each range. Digital meters are usually more accurate, since they remove parallax and other mechanical errors. However, if you do not know how to select the range properly, it is very easy to obtain errors as large as any analog meter. This happens quite often on the current ranges. 1.3 Meter Waveform Shape and Frequency Dependance The more common meters measure the rectified average voltage, while better ones measure the root mean square (RMS). Unless you know what the shape of the waveform is, and understand how the meter measures a voltage, the number that you read will be useless. Each of the different methods gives the same voltage for a sinewave in a certain frequency range. However, they will all be different if given another waveshape. Ac meters are calibrated at one sinusoidal frequency to read the RMS voltage or current. However, most measure the rectified average of the ac component of a signal. The rectifier circuit acts like an absolute mathematical function. The ac component is what you have left when you remove the average or dc voltage from a signal. In real life, when you set a meter to read an ac signal you not only remove the dc signal but you also remove some of the low frequencies of a waveform (ie. the meter acts like a high pass filter). Meter bandwidth is the frequency range over which measurements are accurate. It more generally refers to the frequency range over which a circuit or instrument functions properly. For example, a Fluke 8010A digital meter should not be used below 45 Hz or above 50 khz. The error goes from 0.5% to 1.0% error when the frequency is over 10 khz, and 5.0% when the frequency is between 20 khz and 50 khz. Oscilloscopes and all other instruments also suffer from similar effects. Before you can use a meter, you must consider the Crest Factor of the waveform being measured. It is the ratio of the peak voltage to the RMS voltage. Mechanical meters are slow to respond because the needle, as a physical system with mass, takes time to accelerate. Electronic meters also have similar problems and this is responsible for the use of the Crest Factor. Some examples are given in the following table. Waveform Square wave Sine wave Triangular wave White noise Crest Factor to 4.0 The Fluke 8010A meters only measure the correct rms value if the Crest Factor is under 3.0. So if, for a sinewave, you measure the rms voltage and measure the peak voltage on an -5-

6 oscilloscope (that can handle the waveform without introducing more errors), you can see if the meter is reading the rms value properly. The Crest Factor is more difficult to calculate for rectangular waveforms. A good instrumentation book or instrument manual will explain this. An rms reading meter measures the root of the mean of the square of the signal (proportional to the power) or: V RMS T 1 v 2 (t)dt TP 0 ( V PK V PK for a sinewave) The purpose of an rms equivalent value of a waveform, is to simplify power calculations by providing an equivalent dc voltage, that would develop the same average power in a resistive load. If a voltage waveform is not described, but you know that it is equivalent to 10 V (rms), then you will have 100W of average power dissipated in a 16 load (ie.p AVERAGE = V RMS *V RMS /R). If you did not know the rms voltage, the only way to calculate the power would be to do an integration, from an approximation of the waveshape as seen on an oscilloscope. This only works for resistive loads because rms value is a magnitude and does not consider phase information. A meter that reads an average rectified value for a waveform of period T, indicates a value T V AVE 1 v(t)dt (0.637 V TP PK 0 for a sinewave) Therefore meter scales are calibrated to read 1.11 * V average, and since 1.11 * = 0.707, the reading is the rms value for a sinewave and a sinewave only. Notes: 1) As V PK is the average value of fullwave rectified sinusoids only, a rectifying averaging meter is only accurate for sinusoidal waveforms. The multiplier 1.11, known as the form factor, is specific to a pure sinusoidal waveform. Averaging ac meters are therefore, only accurate for sinusoidal waveforms. 2) The above discussion assumes that the meter uses a fullwave rectifier. A second type, called a half-wave rectifier, exists and would read a different voltage because it only responds to the positive parts of a waveform. Because of this, a half-wave rectifier can give very strange measurements if the waveform is not symmetrical about 0V. Here is a table of how some meters would respond to common signals of 1 V (peak) or 2 V (peak-to-peak). The dc average value will usually be 0V for a sinewave since sinewave generators usually have no dc voltage while square or triangular wave usually start from 0V and go to some positive or negative voltage (single ended signal generator). Waveform DC average 1 V PK voltage AC rectified average voltage * 1.11 (A) RMS volts (B) % difference (A), (B) Square Sine Triangular ±1 or 0 0 ±0.5 or % +0% -3.8% -6-

7 2. DC Power Supplies There are two types of dc power supplies. There are those with fixed voltages, which are usually accurate to ± 5%, and those which are adjustable. Adjustable supplies have a control which allows one to change the dc voltage. Both types have two output leads where one is indicated as being positive (with respect to the other lead). Many supplies also have current limiting. This feature, decreases the voltage when the current goes beyond a certain limit. Because of this, the power supply will act like a voltage source until the current limit is exceeded. Then it acts like a current source which is fixed at the current limit. In the case of the Anatek instrument, and other power supplies, the current limit can be set by the user using a control on the front panel. This control should be set before you perform the lab experiment. The best way to prevent circuit parts from exploding due to overheating is to limit the current to 100 ma. Most components will not explode, or be destroyed, with 100 ma the way that they would with 1.5A. If you short circuit the power supply you will see the current meter indicate the present current limit. Adjust the current limiting knob to the maximum current you require. At any time during the lab, a quick glance at the current meter, that is built into the power supply, will tell you if you have created a short circuit. Warning: Only perform this short-circuit test on power supplies which have current limiting. If the power supply is not limited you may start a fire or trigger an explosion. Power Supply Connections Most power supplies are floating. This means that neither the positive, nor negative outputs are connected to ground or any common point. This means that you can connect the power supply in any way, even several hundred volts above ground. A non-floating power supply has one lead grounded or connected to a common point. This means that there is only one lead which can be connected anywhere in a circuit since the common or grounded one cannot be moved. These power supplies may be referred to as single ended. The word single means that you can only have a positive or negative voltage from the supply. If the supply is double ended, you can obtain either a positive or negative voltage from it. A typical variable DC power supply is shown in figure 4 and its front panel controls are described below. -7-

8 ±. POWER: Push on. ². LED power on indicator. ³. Panel METER ZERO ADJUST.. METER SELECTION SWITCH V for voltage. A for current. CURRENT LIMIT switch: when pushed in the panel meter indicates the output current-limiting value set. µ. PANEL METER. Use as an indicator. No precision given for this meter.. VOLTAGE CONTROL: Adjust VOLTAGE of the output at the + and - terminals.. CURRENT CONTROL: Adjust CURRENT control for maximum output current allowable. Set the CURRENT CONTROL fully clockwise for maximum output current.. OUTPUT TERMINALS: Connect the load across the + and - Figure 4. Power Supply Front Panel terminals. The symbol designates the common terminal which is connected to earth via the power cord. NOTE: The + or the - can be connected to the common. Hence you can have a positive common (or ground) or you can have a negative common (or ground). -8-

9 3. Signal Generators A signal generator provides a time-varying signal to test circuits. This can be anything from a simple sine, square or triangular wave up to a TV station signal with different types of noise added. This symbol and basic circuit model for a single-ended (notice the grounded output) generator are shown in figure 5. Figure 5. Signal Generator R OUT is the output resistance and may sometimes be called R INT or internal resistance. Every electronic circuit has a certain amount of resistance. Signal generators are usually 6006, 756, 506 or some other value due to varying industry standards. You have to keep the output resistance in mind. Whenever you draw current from a signal generator, the voltage at the output terminals will change due to the voltage drop across the internal resistance. Signal generators, like dc power supplies, can have different properties. They may have one of the outputs connected to ground or have a circuit common point. This is a single-ended generator as opposed to a floating one, where neither of the outputs is connected to a common or ground. The generator may be unipolar, where the voltage is always of one polarity, or it may be bipolar, where the voltage is both positive and negative at different times. Generally, sinewave generators are bipolar and squarewave generators are unipolar. Unipolar generators are further broken down into positive and negative. A positive, unipolar, squarewave generator produces squarewaves that start at 0V, go up to some positive peak, and then return to 0V in each cycle. Signal generators have imperfections. The waveforms are not pure. There can be noise or distortion in the output. Noise is a random signal and distortion is a change in or distortion of a 'pure' signal. An ideal sinewave generator should produce v(t) = sin(t), while a distorted signal could be v(t) = sin(t) * sin(3t). The frequency or amplitude of a generator output voltage will not be stable. The user is expected to keep this in mind. Each generator has an amplitude and frequency control. The controls generally work in ranges so you will usually see a range control that limits the minimum and maximum frequency or voltage and a fine control to adjust within the range. With time, and as the instrument heats up, the frequency and amplitude of the signal generated will change. -9-

10 Function Generators A function generator is a signal generator that is capable of producing more than one waveform shape. You will be using the B&K Precision model 3010 function generator, shown in figure 6, for the IL and LS laboratory exercises. Figure 6. B & K Precision Function Generator ±. Power Switch ². Frequency Range Selection Buttons ³. Waveform Function Selection Buttons. Amplitude Dial µ. DC Offset Dial. Frequency Dial marked from 0.1 to 10 (multiplies by range). TTL Level Output Connector. Common Connector ¹. Function Generator Output Connector -10-