Modern Light Emitting Diodes (LEDs) have revolutionised the way hobbyists
can light model railway vehicles and buildings. LEDs are available in
almost every colour imaginable, from brilliant white to stunning purple,
and in a range of sizes that enables the modeller to represent close
to the correct dimensions for many applications.
It is essentail that LEDs are always used in conjunction with a ballast
resistor. If connected directly to a power supply, the likelihood is that
the LED will light brilliantly for a very short (and final) time.
This handy unit (shown above) comes in a little battery powered box. It can
be used to determine a number of things. The LED to be tested is connected
to the box by sliding its leads into the miniature rivet like terminals. The
tool firstly acts as a go/no-go tester – did that LED survive being
tried out directly across the battery terminals? It is also very useful
when the leads have been cut off and one doesn't know which is positive
and which is negative – a quick connection to the box both ways soon shows
which lead is which. The tester has a number of test points which enable
the LED to be tested at currents from 0.5 milliamp (mA) to 20mA.
(Normally, LEDs should not be run at greater than 20mA for maximum life.)
This facility allows the modeller to see how bright a LED looks at
various currents, and hence to decide at what current one wishes to
The Tool indicates the current, and the associated value of ballast resistor
at 12 volts. Two sets of terminals are provided at 10mA, to allow comparison
of different LEDs at the same current. To calculate the resistor required
if one is using a different supply voltage, one can apply Ohm's Law, or use
the handy resistor chart (RK-RESCHART) available separately, which shows
the ballast resistor values needed for supplies of 5,6,9,12, and 15 volts
at various currents.
This chart also shows the colour code for these resistors in both 3 band
and 4 band variations. The reverse of this chart shows the real world
size of various common LED sizes in different modelling scales, and
also provides a scale ruler for O/48, HO/87, N/160, and Z/220 modellers.
The diagrams above show the unit, and the correct way to connect a
LED through a ballast resistor.
All kits available from :
Aztronics Pty Ltd       Phone 8212 6212
170 Sturt St,                     Fax 8212 4212
Adelaide, SA, 5000.