- Jul 5 11:05 AM--- In firstname.lastname@example.org, "Kari Sarmanne" <kaikari@l...> wrote:
> I have built some battery power packs using a 0.4 W potentiometerwrote:
> and a transistor ( BD244C ). They work just fine.
> Without a regulator or transistor you must have at
> least a 4 W potentiometer, but I do not recommend it.
> --- In email@example.com, "Paul Hertel" <paulhertel@c...>
...> > Last night I had the pleasure of burning up a potentiometer.
> batterySorry for the delayed response, have just been browsing recent
> > made the insides glow nicely.
messages and came upon this one.
A resistor (potentiometer) voltage control is fine for a layout with
no changes in load - ie flat landscape. If there are grades on your
layout, the regulated voltage option (transistor) will maintain a
more constant train speed.
When using a 4 watt (or more) variable resistance (potentiometer) go
for at least 50 ohms, better 75-100 ohms. Use only 2 terminals in
series with the track. Some uses of a potentiometer use all 3
terminals where the ends of the resistance terminals are placed
across the power source and the variable voltage is taken from the
central wiper contact. This is not required for locomotive speed
control and will waste battery life un-neccessarily if all 3
terminals are used since battery current is drawn through the
potentiometer and does no useful work.
If you have grades on your cigar box layout the regulated voltage
circuit helps to keep the speed more constant. The transistor
regulator maintains a constant voltage to the locomotive by
constantly self-adjusting it's electrical resistance to compensate
for variations in current demand by the loco (it measures it's own
output voltage and adjusts itself to keep this voltage at a constant
level). A fixed resistance (potentiometer) instead without this self-
monitoring feature will drop more voltage as the loco current demand
increases to climb a grade, this results in even less current
available and the loco slows even more. If drastic speed changes are
the desired effect, then go for the resistor approach. If you wish
to maintain a more constant speed, use the transistor regulator.
BTW, the simplest form of regulated voltage adjustment is a series
string of silicon rectifier diodes, about 10 cents each if bought in
bulk, like a pack of 25. Unlike a resistance, each diode in it's
forward conduction polarity drops a constant 0.7 volts regardless of
the current through it. A battery is already voltage regulated other
then a small internal resistance and gradually decreasing voltage as
it is used. Any common silicon diode rated for 1 amp or more is
suitable - 1N4001, 1N4002, 1N4003, 1N4004 etc. Ten in series will
drop 7 volts for minimum speed (2 volts left for the loco from the 9
volt battery), 5 in series drop 3.5 volts (5.5 volts left over for
the loco) etc. An alligator clip wire can be used to select how many
diodes are electrically in the string. If you want to get fancy you
can connect each diode wire connection to a screw, a series of screws
in the back of the cigar box let you select speed with the alligator
clip. As the battery ages move the clip to the next screw. Not
elegant, but for a small layout with grades this is the simplest form
of constant voltage supply.
Another use for diodes proposed and tested by Cliff Travis - series
diodes used on a helix in the downhill polarity help maintain the
speed so the same throttle setting results in the same speed both up
and down. Neat idea.
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