DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
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INTRODUCTION
Data Sheet D7d.1 described the workings of meters and
set forth criteria for choosing those which will offer
maximum service value. From that base, this sheet will
explore the applications of meters and the circuitry
associated with them.
APPLICATIONS OF METERS
Sheet #: D7d.2
Title: METERS: APPLICATIONS
Updated by:
Updated:
First Issued:
Originally
Compiled by:
References:
AND CIRCUITRY
John A. Dorsam
October 1999
July 1969 (D7d.15)
John Dorsam and
Bob Johnson
Electrical Measuring Instruments, Part 1
Drysdale & Jolley, revised by G.F. Tagg
Wiley, NY
How to Use Meters, Rider & Prensky
Manual of Electrical Instruments
General Electric Company
Power Magazine, May, June, July,
Sept and Nov 1966
Various Electronic Supply Catalogs
Page: 1 of 5
A meter should be connected only to the type of current
indicated on its face. If a DC meter is connected to AC
(or vice-versa) an incorrect value (or no reading) may be indicated. Such errors do not usually
damage the instrument.
The Ammeter measures the current in a conductor. Our most common application is in the track
feeder where it is recognized as the most important meter in the hobby. If only one meter is applied
to the circuit, it should be this one. It gives the information listed below under "Meter Reactions"
and the value of train current. When the propulsion motor is the only current-consuming device in
the train ( no lighting or car resistors) the ammeter will indicate motor current, which should be
limited by the operator to the known safe value for the engine. Lamps and resistors will
proportionately increase the meter readings.
The Voltmeter measures the electrical pressure (voltage) between a pair of conductors, and is
most frequently found in the propulsion circuit. If economics suggest the substitution of a pilot
lamp1 for the more costly meter, the panel should be so designed that a voltmeter may be installed
later.
The Multimeter (Fig. 1) is a selfcontained electronic tool of broad
application. It combines in one case an
AC/DC voltmeter, an ohmmeter and a
DC ammeter, usually with several
overlapping ranges of each function.
Range and function are selected singly
by switch and connections. Commonly
referred to as a "V-O-M" (Volt-OhmMilliameter) this instrument is most
useful in servicing and circuit-tracing
work. A comprehensive (but not
necessarily model-railroad oriented)
instruction book is usually included with
each V-O-M. A sample V-O-M scale is
displayed as Fig. 2.
Fig. 1: Multimeter
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7d.2
Title: METERS: APPLICATIONS
AND CIRCUITRY
Page: 2 of 5
APPLICATIONS OF METERS - continued
Fig. 2: Multimeter Scale
The V-O-M may be used to check for open circuits, short circuits and high-resistance connections.
It will assist in identifying unmarked connectors in a cable or raceway. Permanently connected
meters being rare in auxiliary circuits, the V-O-M will aid periodic checking of their pressure and
current. It can be used to determine resistance values of coils, rheostats and resistors, and to
some extent for checking capacitors. Its greatest value will probably be had as a self-contained
continuity checker. Useful on almost any model railroad, it may become essential to the electrical
group of a large club. Its value is limited only by the imagination and experience of the user.
METER CIRCUITRY
Ammeters must always be connected in series with the load. Note from Data Sheet D7n that it is
correct to install the ammeter in the protected conductor, between protective device and load.
Voltmeters must always be connected in parallel with the circuit to be measured. In Figs. 3 through
9, meters are conventional (zero left) unless marked (*), which indicates a zero-center instrument
(note 1). Other abbreviations and symbols are these: A-Ammeter, V-Voltmeter, DC-Any Direct
Current source, PP-Propulsion power pack, PS-Any power source.
Note 1: A pilot lamp may furnish almost as much information as a voltmeter; at low voltages, the
filament barely glows and lamp brilliance increases with applied voltage. A 28 V, lamp (No. 1819 or
1829) would burn less brightly than one of lower voltage, and a blue lamp cap might increase
operator comfort. Connect pilot lamp same as voltmeter.
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7d.2
Title: METERS: APPLICATIONS
AND CIRCUITRY
Page: 3 of 5
METER CIRCUITRY - continued
Fig. 3: Meters connected to any power source,
as for auxiliary circuits.
Fig. 4: Optimum location of meters in custombuilt or commercially-available power packs.
*
*
Fig. 5: Meters located between DC power
source and direction controller as might be
found when using a handheld or similar
trackside control unit. Ammeter will read
correctly but voltmeter will read source
voltage only.
Fig. 6: Meter located between direction controller
and track, using hand-held or trackside control
unit. Ammeter will read train current. Voltmeter
will read track voltage. Both meters must be
zero-center type because they are subject to
polarity reversal.
Fig. 7: Zero-center meter installed in
conjunction with ready-to-use propulsion power
pack, involving no other circuit changes.
Fig. 8: Conventional meters employed in
conjunction with ready-to-use propulsion power
packs, with an additional direction controller
applied. Original direction controller is no longer
used. Preferable to Fig. 7.
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7d.2
Title: METERS: APPLICATIONS
AND CIRCUITRY
Page: 4 of 5
METER SWITCHING
At times it is desired to switch ammeters between two propulsion power packs. This may be done
as shown in Fig. 9 but is generally less satisfactory than two separate meters and saves less
money than might be expected. A zero-center type ammeter must be employed. When it is
desirable to determine the current in a non-metered circuit, the system shown in Fig. 10 will be
valuable; it embodies an ammeter (may need to be zero-center type in propulsion circuits),
protective device, flexible leads and spring clips. When the spring clips are connected across an
open circuit breaker or removed fuse, the current in that circuit will be indicated on the ammeter.
Voltmeters may be switched among the various circuits. The system suggested by Fig. 11 is
employed among accessory power circuits located in close proximity to each other. The diagram
suggests plugs, but a selector switch is as good or better. No circuit Breaker is required in a
voltmeter circuit.
PACK "A"
TRACK "A"
PACK "B"
TRACK "B"
Figure 9
Figure 10
Figure 11
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7d.2
Title: METERS: APPLICATIONS
AND CIRCUITRY
Page: 5 of 5
METER REACTIONS
Ammeter Reading
High
Zero
Voltmeter Reading
Low
Problem
Ammeter Reading
Flickers
low to
normal
Voltmeter Reading
Voltmeter Reading
Flickers
normal
to high
High
SHORT
CIRCUIT
Ammeter Reading
OPEN
CIRCUIT
DIRTY
WHEELS
OR
TRACK