Page 269 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 13 Meters 231

typical VTVM. These meters generally have the same func-
VOLTS
tions as a typical multimeter, but with boarder ranges. They
1 2 3 4 5 6 7 8 also have a high input impedance and are accurate over a
Voltmeter 9 10 Multifunction broad frequency range. To accomplish the broader parame-
10 9 8 7 6 5 4 3 2 1 0 Scale
Needle OHMS Batt ters, the high input impedance and broad frequency range,
these meters are equipped with an internal amplifier similar to
ZERO ADJ BATT TEST
Zero Adjustment the circuit shown in Figure 13-15. Another feature that is
Battery Test
1 OFF 1 common to VTVM’s is a 0- to 10-volt output. Regardless of
AC VOLTS 10 10 DC VOLTS
the input range selected, the output mirrors a 0- to 10-volt
100 100
Range/Function 1000 1000 High-Impact signal. This allows easy interfacing to other instruments.
Selector Switch .1 .1 Plastic Case
AC AMPS AC AMPS
1 1
10 100K
100 1K 10K
OHMS
10-amp Terminal volts/ohms Digital Multimeters
10A A COM V/ Terminal
Most modern multimeters are equipped with a digital readout,
amps Terminal Common Terminal as shown in Figure 13-27. A digital multimeter combines the
Figure 13-25 Portable Analog Multimeter compactness of an analog unit with the sensitivity of a
VTVM. These instruments have become very affordable and
are an excellent addition to any toolbox or workbench.
housing. They are easily placed in a toolbox or the techni-
cian’s pocket.
Most multimeters will read AC volts, DC volts, AC amps,
DC amps, and ohms. Many units also provide a beeper for
Four Digit Display Multifunction
continuity testing and a battery check function. As previously Read-Out
discussed, analog units will have a fairly low impedance, so
sensitivity to very low voltages will be poor.
Plus/Minus Low Battery
Indicator SB Stand By
Vacuum Tube Voltmeters
1 OFF 1
AC VOLTS 10 10 DC VOLTS
Vacuum tube voltmeters, or VTVM’s as they are sometimes 100 100
referred to, are a type of multimeter that is designed to pro- Range/Function 1000 1000 High Impact
vide high sensitivity. Figure 13-26 shows the front panel of a Selector Switch .1 .1 Plastic Case
AC AMPS AC AMPS
1 1
10 100K
100 10K
1K
OHMS
10-amp volts/ohms
Terminal Terminal
10A A COM V/
Leather
Strap 3 4 5 6 7 Multirange amps Terminal Common Terminal
2 8
0 1 9 10 Scale Figure 13-27 Portable Digital Multimeter
6
4
5
Strap 10 20 30 40 50 60 70 80 90 100
3
7
2
8
1
Buttons 10 20 30 40 50 60 70 80 90100
9 10
OHMS -1 0 520 OHMS
-2 DB
Voltmeter
Bench Built Multimeter
Zero On Power
Adjustment Switch Building a simple multimeter is an excellent way to gain some
Zero Adjust Power Off Power experience with electromechnical devices. This is a very inex-
Function OFF .01
Amps OHMS 10M .1 Indicator pensive project that can be built in just a few evenings. In
Selector
1M 1
uA AC uV Range addition, when you are finished you will have a useful instru-
100K 10
Selector ment that will provide support for future efforts.
DC uV AC VOLTS
10K 100
Case DC VOLTS 1K Figure 13-28 shows a schematic and list of components for
Function Range
Instrument Front Panel a basic analog multimeter. The meter is a 50 A, panel mount
GROUND
Ground INPUT OUTPUT 0- to 10-volt movement with an internal resistance of 1800 ohms. The
Output
selector switch is simply a banana jumper set. The volt and
Dual Banana ohm resistors are common 2% carbon units. The current
Rubber Foot High-Impedance Input Terminal resistors are high wattage 5% units. The battery is an ordinary
Figure 13-26 Vacuum Tube Voltmeter 1.5-volt AA cell.