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DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
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INTRODUCTION
Lamps suitable for model railroad applications vary
according to voltage, current, bulb shape and size, type
of base, rated life and price. This sheet is devoted to
domestically produced lamps of reasonable service life
ratings, popular base (socket) types and fairly general
availability. Only incandescent lamps are covered.
OPERATING VOLTAGE
COMPARED TO RATING
10%
OVER
EFFECT ON LAMP LIFE
75% LESS
5%
OVER
Sheet #: D7i
Title: LAMPS
Updated by: Richard E. Napper, MMR
Updated: October 1999
First Issued: October 1956 (D7c.51)
Originally
Compiled by: Frank Saville
Revised by: John A. Dorsam
Revised: October 1962
Page: 1 of 6
References: Scalite company
5%
UNDER
G. E. Lamp Division
Westinghouse Lamp Division
10%
UNDER
45% LESS 100% MORE 300% MORE
EFFECT ON LAMP LIFE 40% MORE 18% MORE
12% LESS
22% LESS
These may be mounted in any position, unless manufacturer specification, such as "burn base
down" indicates otherwise, and operated on AC, DC, high frequency current or pulse power.
Lamp life is critically affected by applied voltage as shown in this tabulation.
When used as indicators, most lamps operate quite well on only 50% of rated voltage. In this
case service life expectancy is so greatly increased that leads may be soldered directly to the
lamp contacts, dispensing with the sockets entirely.
LAMP SELECTION
Unless size is critical or replacement extremely difficult, voltage is generally the most important
factor in lamp selection. High voltage lamps are generally more efficient due to reduced cooling
of the filament. Heavier current lamps are generally less affected by vibration because of their
coarser filaments. Bayonet bases are preferred because they are less likely to loosen in their
sockets. Because cost is often important, 1962 list prices are included in the tabulations, enabling
a comparison of lamp costs. Many retailers offer quantity prices when identical lamps are ordered
in lots of 10. Difficult-to-get-lamps may be procured without too much difficulty when ordered in
lots of 10 identical lamps. When a lamp is operated in a small, enclosed or poorly ventilated
space, a low wattage lamp operated on reduced voltage is indicated.
LAMP OPERATION
As indicated by the above table, it is best to operate lamps at or below rated voltage. Wherever
possible, lamps should be operated on AC to avoid loading down DC supplies. If it is imperative
that lamps be operated at higher-than-rated voltage, it is logical to use several identical lamps in
series. Two 6v or five 2.5v lamps might be operated in series from a 12v source, although the
burnout of one lamp will affect the entire series. When planning series operations, check by
multiplying the number of lamps in the series by the rated voltage of one lamp. The product
should not be less than rated source voltage. (Example: Three 6.3v lamps on 16v supply. 3 x 6.3
= 18.9; satisfactory series operation.)
Sheet #: D7i
Title: LAMPS
Page: 2 of 6
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
DROPPING RESISTORS
At times dropping resistors are essential to the application. To calculate the characteristics of the
required resistor for a No. 55 lamp (rated 7.0v at 0.4a) on a 16v source:
1. Subtract lamp voltage from source voltage;
16-7.0v = 9v across resistor.
2. Divide resistor volts by rated lamp current;
9 / 0.4 = 22.2 ohms.
3. Multiply resistor volts by lamp current;
9 x 0.4 = 3.6 watts.
Closest next higher resistor is 25 ohms at 5 watts.
BULB SHAPES
Bulb shape is given by a letter followed by maximum diameter in eighth inches, for example:
T1¾ designates tubular shape lamp 7/32" in diameter.
BASE TYPES (Common in model railroad usage)
MdS
MdF
Midget
Midget
Screwed
Flanged
0.3" long x 0.21" dia.
MnS
MnB
Miniature
Miniature
Screwed
Bayonet
0.5" long x 0.37" dia.
SCB
DCB
Single Contact Double Contact
Bayonet
Bayonet
0.8" long x 0.55" dia.
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7i
Title: LAMPS
Page: 3 of 6
LAMPS AS CURRENT LIMITERS
Because lamps have very low resistance when their filaments are cold, they are quite suitable for
use as current limiters, particularly for use with propulsion power supplies in the smaller scales.
Using 12v lamps in parallel, their total current rating should be such that none of the filaments
glow with normal train current. This will be about 6 times train current. Two identical 6 volt lamps
in series may prove to be less expensive than one 12 volt type. Number 1133 should be
especially suitable. The lamps are incorporated in the circuit in place of the usual circuit breaker.
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Notes: f - flasher bulb
& - available in clear
* - most widely used
Use: A - automotive
B - bicycle
C - coin machine
H - hand lantern
I - indicator
M - marine
N - novelty
P - telephone
R - radio/TV
T - trains (model)
V - aviation
Sheet #: D7i
Title: LAMPS
Page: 4 of 6
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7i
Title: LAMPS
Page: 5 of 6
LAMPS
For the modeler, many specialty lamps have been manufactured. They include micro-lamps,
grain-of-wheat, grain-of-rice, and small fluorettes. These lamps come in 16 volt, five volt, and 1.5volt ratings. They are available in clear, amber, green, and red colors. These small lamps have
allowed us to add realistic lighting to our modeling efforts. Such lamps, however, have one draw
back, they still produce more heat than light, and this prevents their use in any kind of plastic
model.
Modern electronics has provided use with an alternative, which greatly decreases the heat
problem of normal lamps. This marvel is called an LED, Light Emitting Diode. LED’s can produce
red, orange, yellow, green, or blue light; some of them produce very high light output, with very
little heat generated. LED’s come in different physical sizes, T-1-3/4, T-1, and T-1/2. Since LED’s
are electrical diodes, they will only produce light when the diode is forward biased. That is to say,
you must apply a Direct Current voltage to the diode so that the anode is positive, and the cathode
is negative. The LED will be marked with a flat cut into the plastic dome, which identifies the
cathode (negative) LED. Secondly, most LED’s have two leads with the anode lead being longer
than the cathode lead. LED’s are not current limiting. You must use a series current limiting
resistor with an LED. The calculation is very simple: Supply voltage minus LED Forward voltage
divided by the LED current rating. Most LED’s have a forward voltage drop of about 2.2 volts, and
LED’s pass about 20ma of current. Lets assume that your supply voltage is 12 volts DC.
12-2.2=9.8 volts/.020 amp=490 ohms. 490 Ohms is not a standard value, so you could use either
a 470-ohm or a 510-ohm resistor, ¼ watt.
LED’s can be driven with TTL logic circuits for use in trackside signal circuits. The LED can be
used as a position control indicator for switch machines on control panels, as marker lamps in
locomotives, as flashing crossing gates, and many other modeling uses.
There are special LED’s with either three or two leads, which are called Bi-Color LED’s. Such
LED’s will provide two different colors from only one LED depending upon which way the DC
voltage is applied to the LED. Such LED’s come in the following combinations: Red/Green,
Red/Yellow, and Green/Yellow. These combinations are very useful for trackside signals. Since
LED’s can be pulsed, you can use a yellow or amber LED as an operating rotary beacon on model
locomotives.
Gilway Technical Lamps, 800 West Cummings Park, Woburn, Mass. 01801-6355, Tel. 617-9354442 produces the smallest LED’s available. There Type E3, E4, and E5 LED’s are only 2mm in
diameter which means they are usable as HO scale Diesel Classification lights, and as trackside
signals. Gilway also produces the tiny T-1 LED’s as BI-Color LED’s. The part numbers are: E207
(red/green); E208 (red/yellow); E209 (green/yellow). LED’s are also manufactured as very bright
LED’s with very high light output, very low current LED’s, and very large LED’s, T-3-¼ size.
The modeler can produce many kinds of animated displays using LED’s. Infrared LED’s
can be used for track occupancy detection circuits. There is now available white light LED’s,
although they are of the T-1-3/4 size at this time. LED’s add much variety to the modern model
railroader.
DATA SHEET
© NATIONAL MODEL RAILROAD ASSOCIATION
Sheet #: D7i
Title: LAMPS
Page: 6 of 6
CALCULATING SHUNT RESISTOR VALUE
RESISTOR
+
R
SUPPLY
VOLTAGE
LED
R(ohms)=
Resistance in ohms =
SV - (LED) V
(LED) I
Supply voltage - voltage drop across the LED
Current through the LED in amps
R = resistance in ohms
SV = supply voltage in volts
LED(V) = LED forward voltage drop in volts
LED(I) = LED forward current in amps (usually about 20 mA)