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Miniature light bulbs have come a long way since I started modeling 40 years ago. No longer do you have to suffer
with pea sized light bulbs glaring at you in your close up photos. Of late, the 1.5 v bulbs have become universally
available and their miniature size and affordable cost makes them a regular choice of mine when lighting structures.
The biggest challenge I have is finding an acceptable 1.5 v power source which has been resolved through use
of a 12 V DC source, a couple of diodes and a 12 volt bulb.
The secret to using 12 volts to power a 1.5 volt bulb is through the use of a couple of diodes. When a diode conducts
in the forward direction, it has a voltage drop that is relatively constant at about 0.7 volts. It is this characteristic that
makes diodes work as a constant intensity lighting regulator. If we use a lamp that requires 1.5 volts to operate it,
two diodes in series conducting current will develop about 1.4 volts. This is sufficient to light the bulb. Figure 1 is
the schematic of this circuit. The +12v lead is connected to the anode (positive) side of the first diode that in turn is
connected to the anode side of the second diode. We then connect the 12 v bulb and back to the negative terminal
of the power pack. The 1.4 v drop across the pair of diodes means the 12 v bulb will see about 10.6 v which will
produce a warmer light colour and greatly extend the bulb's life. By connecting our 1.5 v bulb in parallel with the two
diodes, it will experience a 1.4 v drop thus it will also light. Additional 1.5 v bulbs can be added in parallel with the
diodes.
Figure 2 is the same circuit drawn with actual components. The diodes are cylindrical
shaped devices with a coloured band at one end indicating the cathode end. Make sure
you install them correctly otherwise the light will not light. I normally use the 1N4XXX
series of diodes, which are available at Radio Shack and Active Electronics. Make sure
you use ones with a current rating of 0.25 amps are better. The 1N4001 units are rated
at 1 amp - more than ample for our use.
Though you can locate extremely tiny 12 v bulbs, I normally stick to the Grain Of Wheat
size to keep costs down and install these inside the structures where they can't be
seen for interior lighting. I install the 1.5 v bulbs in a Campbell's lamp shade and
install these Grain Of Rice sized bulbs outside to light doorways, stair wells etc.
Over time I have come to discover that you should test each bulb before installation
to ensure it does work properly (all too often low priced bulbs have a corresponding
quality level) as it is next to impossible to change a bulb after it has been installed
inside the building. My favourite bulbs are made by Miniatronics and average about
$1.50 CDN each.
So through the judicious use of a couple of low cost diodes, I am able to use a 12v
power source to light my 1.5 v bulbs.
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