Download Digital Multimeter Advantages

SOE JRE GROUP OF INSTITUTION
CT-I EIM EEC 403
Answer Key
Section-A
SECTION – A
2*3=6
Q.1 Explain different types of forces operating inside the PMMC instrument.
Solution Deflecting force causes the pointer to move from its zero position when a current
flows is magnetic force; the current sets up a magnetic field that interacts with the field of the
permanent magnet
Controlling force is provided by spiral springs retain the coil and pointer at their zero position
when no current flow Damping force is required to minimize (or damp out) the oscillations
must be present only when the coil is in motion, thus it must be generated by the rotation of
the coil is flowing
Q.2 What are the essential parts of a ramp type digital voltmeter?
Solution:
Q.3 What are the different types of amplifiers used for CRO’s?
Solution: VERTICAL AMPLIFIER SECTION
Position: Controls vertical positioning of oscilloscope display.
Sensitivity: Selects the sensitivity of the vertical amplifier in calibrated steps.
Variable Sensitivity: Provides a continuous range of sensitivities between the calibrated
steps. Normally the sensitivity is calibrated only when the variable knob is in the fully
clockwise position.
HORIZONTAL-SWEEP SECTION
Sweep time/cm: Selects desired sweep rate from calibrated steps or admits external signal to
horizontal amplifier.
Sweep time/cm Variable: Provides continuously variable sweep rates. Calibrated position is
fully clockwise.
Position: Controls horizontal position of trace on screen.
Horizontal Variable: Controls the attenuation (reduction) of signal applied to horizontal
aplifier through Ext. Horiz. connector.
SECTION – B
3*3 =9
Q.1 Explain frequency ratio measurement with block diagram.
Q.2 Draw a complete diagram of a emitter follower voltmeter using transistor input stage.
Explain the circuit operation and the effect of transistor base voltage.
Solution VTVMs are not used due to their disadvantages, but the same circuit principle is
employed, replacing the vacuum triodes with BJTs. Such a meter is called a Transistor
Voltmeter (TVM). The basic bridge circuit configuration used is as shown in Fig. Q1 and Q2
form the lower arms of the bridge circuit. RC1 and RC2 form the upper two arms of the bridge.
R2 is the zero adjust resistor. Base B2 of Q2 is grounded through RB2. Input is applied to RB1.
Zero adjustment can be done using R2. The differential output (VC1 – VC2) is proportional to
the input. A potential divider circuit modifies the input to make it suitable to be applied at RB.
Current measurement can also be done.
.
Q.3. Explain integrated type digital voltmeter with waveform and block diagram
Solution DUAL SLOPE INTEGRATING-TYPE DVM
In this method, the accuracy of conversion will not depend on the precision of the resistor and
capacitor of the ramp generator current, or the op-amp. In this technique, an integrator is used
to integrate an accurate reference voltage for a fixed period of time. The same integrator is
then used to integrate with the reverse slope, the input voltage. The time required to return to
the starting voltage is measured. The block schematic of this type or DVM is shown in Fig
The input voltage to be measured Vx is applied to the integrator by means of an electronic
switch, usually a JFET device. The integration is done for a fixed amount of time, as
determined by the counter. As soon as integration starts, the counter is also initiated by the
control logic. When the counter reading reaches a predetermined value, integration is
stopped. The output of the integrator will have opposite polarity to that of the input voltage
Vx. The counter is reset. Reference voltage Vref, which is of opposite polarity to the input
voltage, is now connected to the integrator. The integrator output voltage will now go in the
opposite direction. At the same time, the counter is also initiated. Integration is done till the
output voltage of the integrator becomes 0 V. The counts accumulated during this period are
a measure of the input voltage. The counts are accumulated and the digital display or readout
is given. The waveforms are as shown in Fig.
SECTION – C
5*3=15
Q.1 Sketch the circuit of a transistor difference amplifier type of voltmeter. Explain the
Circuit operation; give its advantage over emitter follower voltmeter.
Solution The temperature drift terms are major limits to amplifier sensitivity. The method of
compensating these is to balance out these effects with an equal but opposite drift signal. This
is the principle of operation of the differential amplifier shown in Fig. If a signal is applied to
the input terminals of Q1 and Q2 it appears as an amplified signal at the collector of Q1 and
Q2. Kirchhoff's Voltage Law loop equations are written for the two input signals Vi1 and Vi2
as follows.
Q.2 Explain digital multimeter with their classification and ranges. Differentiate between
analog and digital multimeter
Solution A multimeter or a multitester, also known as a VOM (Volt-Ohm meter), is an
electronic measuring instrument that combines several measurement functions in one unit. A
typical multimeter would include basic features such as the ability to measure voltage,
current, and resistance. Multimeters may use analog or digital circuits—analog multimeters
(AMM) and digital multimeters (often abbreviated DMM or DVOM.) Analog instruments are
usually based on a microammeter whose pointer moves over a scale calibrated for all the
different measurements that can be made. Digital instruments usually display digits, but may
display a bar of a length proportional to the quantity being measured. Digital multimeters
have all but replaced analog moving coil multimeters in most situations. Analog multimeters
are still manufactured but by few manufacturers.
A multimeter can be a hand-held device useful for basic fault finding and field service work,
or a bench instrument which can measure to a very high degree of accuracy. They can be
used to troubleshoot electrical problems in a wide array of industrial and household devices
such as electronic equipment, motor controls, domestic appliances, power supplies, and
wiring systems
Analog Multimeter Advantages
Analog meters are older and still preferred by many engineers. One reason for this is that
analog meters are more sensitive to what is happening in the circuit that is being measured. A
digital multimeter samples the quantity being measured at a particular time and displays it.
Analog multimeters sample a quantity as it is happening. If there are slight changes in DC
voltage, the needle of an analog multimeter will track them -- the needle moves -- while
digital multimeters often miss them. This continuous tracking feature becomes important
when testing capacitors or coils. A properly functioning capacitor should allow current to
flow when voltage is applied, and the current slowly decreases to zero -- this "signature" is
easy to see on an analog multimeter but not on a digital multimeter. It is similar when testing
a coil, except the current starts small and increases.
Digital Multimeter Advantages
Digital multimeters are simpler to use and read, and more accurate than analog multimeters.
For example, calibrating a digital multimeter is simply a matter of pressing a button. More
expensive digital multimeters have "automatic ranging" -- to measure voltage you just select
"V" and the meter determines the range. On an analog multimeter you must choose if the
voltage is less than a volt, less than 10 volts, less than 100 volts, etc. If you make a mistake in
selecting the range on an analog multimeter you can damage the meter. Digital multimeters
are more robust in general -- if you drop an analog multimeter, it is likely ruined; a digital
multimeter has no moving parts so it is more likely to survive a drop.
Q.3 A 500 Hz triangular wave with peak voltage of 40 V is applied to the vertical plate of
CRT. A 250 Hz sawtooth wave with a peak voltage of 50 V is applied to horizontal plate. The
CRT has 0.1 cm/V deflection sensitivity Determine the waveform display on CRT screen.