Electrical Engineering Technology EE306 Lecture 6 Measurement of AC current and Voltage Oscilloscope Digital Measurement instruments Northrop : 8.5 – pgs 528 – 539 8.6 – pgs 547 - 550 Hughes : chapter 46.1 – 46.6 – self reading 46.7 – 46.9 until pg 943 46.10 & 46.11 – self reading

AC Voltage Measurement AC voltage measurements can be divided into a number of frequency ranges and amplitude. The ac voltages are generally sinusoidal. Non-electronic, electromechanical ac voltmeters:- electromechanical ac voltmeters are designed to work at power line or low audio frequencies, and at voltages ranging from about one volt to tens of kilovolts. all ac voltmeters have their scales calibrated in root mean square volts of a sine wave

Electromechanical AC voltmeters Dynamometer AC Voltmeter – This type of ac voltmeter is generally a large, insensitive, bench top instrument used to measure sinusoidal voltages with frequencies from about 20–133 Hz. Accuracy is typically 0.2% of full scale voltage, and meter deflection.

Iron Vane Voltmeters These voltmeters are found as large, insensitive, bench top instruments which can be used to measure sinusoidal voltages with frequencies of about 15–133 Hz, with accuracies of about 0.5% of full scale voltage. Typical meter efficiency is about 29/V. This means that the meter draws about 34mA at full scale deflection. Typical full scale voltages for these meters range 8–750Vrms

Rectifier/D’Arsonval AC Voltmeters These voltmeters generally find application in bench top and pocket (multimeters). A bridge rectifier converts ac current in R1 to a full wave rectified current with a dc average value. The D’Arsonval microammeter movement deflects proportional to the average current through its coil, hence deflection of the rectifier/D’Arsonval meter is proportional to the full wave rectified, average value of the input voltage. The ac voltmeter calibration is, of course, in terms of the RMS of a sine wave input voltage.

The average value of a full wave rectified sine wave, If the input voltage source has zero resistance, Rs = 0, the full scale meter deflection, The value of R1 required to make an ac voltmeter with a full scale voltage reading of Vs(rms, FS), assuming ideal diodes: The ac, full wave rectifier voltmeter’s efficiency The above relations are generally valid for ac input voltages above 10 Vrms. A problem commonly encountered in using ac voltmeters of this sort is their response to non-sinusoidal voltages, such as square waves, triangle waves,

4. Vacuum Thermocouple/D’Arsonval AC Voltmeters These voltmeters are the only type of electromechanical ac voltmeter which will remain calibrated for input voltage frequencies of 15–20 Hz to about 50 MHz. The reading of a vacuum thermocouple ac voltmeter is proportional to the true root mean square of the voltage being measured. A schematic of a vacuum thermocouple ac voltmeter is shown in Figure 8.28. The input circuit consists of a series resistor, R1, and a heater filament with resistance, RH.

Analog Electronic AC Voltmeters 1. AC Amplifier-Rectifier AC Voltmeters The ac voltage under measurement is conditioned by a flat bandpass, R-C amplifier of accurately known gain. The amplified ac voltage is then converted to dc, which causes meter indication of the ac voltage. Most ac electronic analog voltmeters make use of the common D’Arsonval movement as the output indicator.

2. Peak Reading Electronic AC Voltmeters The second type of ac electronic voltmeter (peak reading) uses a low capacitance diode to half-wave rectify the ac voltage under measurement. The rectified ac is smoothed by a capacitor low-pass filter and then, is amplified by a drift free, dc amplifier with known gain, such as a chopper stabilized or commutating auto zero circuit. The dc output of the amplifier causes the deflection of the voltmeter’s D’Arsonval meter.

3. True rms AC Voltmeters of the feedback type The heart of the circuit are two, matched vacuum thermocouples The heater current of the input thermocouple is derived by ac amplification (or attenuation) of the ac input signal, VS. The heater current of the feedback thermocouple comes from the dc current through the D’Arsonval meter movement.

4. True rms AC Voltmeters using the direct conversion approach There are several dedicated ICs available that will perform true RMS conversion on ac signals. The dc output of these ICs can be conditioned and used to drive a D’Arsonval dc microammeter. The maximum frequency response for a specified % error in conversion is generally obtained for maximum ac input.

AC Current Measurement 1. Electromechanical (Analog) AC Ammeters Most of the electromechanical meter movements in ac voltmeters can be used for ac ammeters, with attention to their range and input current frequencies. To measure currents above the full scale sensitivity of the meter movement, a shunt must be used. For currents in excess of about 5 A, a 50mV external shunt is generally used. Shunts are typically used for high current measurements at audio or power line frequencies using thermocouple, electrodynamometer, or iron vane millivoltmeters. They are not used with electrostatic or D’Arsonval/rectifier type ac meters. The thermocouple ammeter can be used at radio frequencies up to 50MHz or so.

Another means used at power line frequencies to measure high currents on conductors at high potentials is the current transformer. One or more turns of the alternating current carrying conductor are passed through the center of a high permeability, toroidal iron core, inducing a sinusoidally changing magnetic flux. Also wound on the toroidal core are a number of turns of wire which are connected to an ac ammeter, generally having a dynamometer or iron vane movement. Current transformers should never be operated with their secondaries open circuited since dangerous high voltages can exist there. The clamp-on ammeter is a portable version of the current transformer

1. Electronic and Magneto-optical AC Ammeters The operational transresistor op-amp circuit offers a means of measuring very small ac currents flowing into ground, ranging from picoamperes to about 10 mA. Another means of measuring low level ac currents is to use a current transformer followed by an ac amplifier, followed by an ac analog indicating meter, or digital readout. Another means of measuring ac current in a conductor is to measure the magnetic field around the conductor. A sensitive Hall effect sensor probe can be put next to the conductor to intercept the solenoidal B field. The ac output voltage of the Hall sensor is amplified and then used to drive an appropriate meter.

Digital Voltmeter The analog to digital converter (A/D converter) is found in the digital voltmeter (sometimes given the abbreviation DVM). In this instrument, the analogue input is converted to a BCD-code representation which is then decoded and displayed on a digital display. In the electronic instrument, the analogue signal is one which represents an electrical or other signal. Thus if a voltage were sufficiently small, it could be applied directly to the converter – but if it were too large, we could apply it to a preset potentiometer, the output of which could be applied to the converter. A typical digital voltmeter has a four-digit display.

Cathode Ray Oscilloscope (CRO) The principal features of the modern cathode-ray tube :- C – directly heated cathode G - a control grid with a variable negative bias to control the electron emission of C, thereby varying the brilliancy of in order to control the brightness of the spot on the fluorescent screen S.

The combination of A1, A2 and F is regarded as an electron lens. Anode discs A1 and A2 are connected together and maintained at a high potential relative to the cathode, to accelerate the electrons passing through G. Electrode F – to focus the bright spot by varying its potential, hence vary the distribution of the electrostatic field in the space between discs A1 and A2. The combination of A1, A2 and F is regarded as an electron lens. the system of electrodes (C & G) producing the electron beam is termed an electron gun.

Deflecting System The electrons after emerging through the aperture in disc A2 pass between two pairs of parallel plates, termed the X- and Y-plates

The input signal is amplified by the Y-amplifier, so called because it causes the beam to be driven up and down the screen of the cathode-ray tube. The time base serves to move the beam across the screen of the tube in the X direction. The time base may be controlled by the output of the Y-amplifier. The power supply serves to energize the grid and anode systems of the cathode-ray tube, as well as to energize the brilliance, focus and astigmatism controls of the beam. The power supply also energizes the amplifiers for the control of the beam. Example 46.1 and 46.2 pg 949

Digital Oscilloscope – 46.11 Digital storage oscilloscope is now the preferred type for applications. Digital oscilloscopes are used in the same manner as the analogue oscilloscopes, but they have greater versatility. It is has digital memory, which can store data. They can process the signal by high-speed digital signal processing circuits, normally controlled by a set of menus.

The vertical input is digitized by an A/D converter to create a data set that is stored in data memory. The data are processed and then sent to the lcd display (which was cathode-ray tube). They can be networked and the data can be sent for processing or storage elsewhere. The screen image can be directly recorded on paper by means of an attached printer or plotter. The signal can be analyzed by the in built software to performs many useful processing functions, including measurement of rise-time, pulse-width, amplitude and frequency spectra.