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Update: 2009-09-29
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To run a briefcase layout at exhibitions and shows, a power pack with a
battery can be a possible solution. Here, we're not speaking about the
primitive battery box with double pole switch included in several
starter gift sets that offer the choice of 200 mph (in forward or
reverse) or stop. The operating voltage must be sensitively adjustable
to simulate prototypical running performance. Power packs operating
from house current (mains), with a pulse rectifier for low speed work,
are excellent for regular iron-core model motors while battery-operated
power packs make best slow-speed operation possible for locos with
coreless motors. The battery power pack explained in these instructions
is suitable for both (regular and coreless motors) but works best for
coreless motors.
The nominal voltage for Z-scale locos is 8 volts, but 10 volts over a
short period does not damage them. At voltages near the upper limit,
the locos have a speed that far exceeds prototypical values [:-(]. For
adjustment of the operating voltage, we install an integrated circuit
having a minimal voltage drop of 1.5 volts. With a 9 volt battery as a
power supply, we have at our disposal 7.5 volts maximum. The "adjust"
lead of the LM317T regulator is connected to an adjustable voltage
divider. It makes it possible to adjust the regulator's output voltage
between 1.5 and 7.5 volts. Two resistors in series with the
potentiometer are calculated to have its full rotation available to
adjust the speed. At an output voltage of 1.5 volts, a loco with a
coreless motor can crawl very slowly. Turning the potentiometer further
down will activate the potentiometers internal switch and disconnect the
whole power supply to stop the loco. Depending on what motor type you
would like to operate with this power pack, the series resistors can be
varied a bit to adapt the start and slow speed quality. The 0.1 µF
capacitor prevents damage to the integrated circuit caused by voltage
peaks at the motor. The double pole switch makes it possible to change
the running direction of the loco. For operation of locos with coreless
motors, the LM317T integrated circuit need not be cooled; but, to power
regular model motors with iron cores, a heat sink is recommended for the
LM317T.
The wiring can be arranged on a piece of universal printed circuit
board. Ready-to-use 9 volt battery snap connectors are available at
your local Radio Shack, as well as the other components (or you can
sacrifice an old battery to get the terminals and connect them to
terminal wires). The potentiometer, with the printed board and the
double pole switch, can be installed in your control panel.
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