Model train speed control

  
Virtually any NPN small signal transistor may be used in place of the BC 108 shown. Likewise any suitable NPN power transistor can be used in place of the 2N3055. The output transistor must be mounted on a suitable heat-sink. Short circuit protection may be provided by wiring a 12 volt 12 watt bulb in series with the output
Model train speed control - schematic

This will glow in event of a short circuit and thus effectively current-limit the output, it also acts as a visual short-circuit alarm.




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