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23948Re: Amps

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  • Glen Chenier
    Jul 5 11:05 AM
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      --- In z_scale@yahoogroups.com, "Kari Sarmanne" <kaikari@l...> wrote:
      > I have built some battery power packs using a 0.4 W potentiometer
      > 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 z_scale@yahoogroups.com, "Paul Hertel" <paulhertel@c...>
      wrote:
      ...> > Last night I had the pleasure of burning up a potentiometer.
      A 9V
      > battery
      > > made the insides glow nicely.


      Sorry for the delayed response, have just been browsing recent
      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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