Dtsteam said:
Thats excellent Greg., thanks very much. I use old lap top power supplies, so can I omit C1 as well as the Bridge rectifier ?
Could you extend the discharge cycle of C2 by, say, breaking the connection between the 4.7K resistor and the pot once C2 has charged ?
Yes, leave out diodes and C1.
Not quite sure what you mean by extend the discharge cycle. The train 'start up' time is when the capacitor is charging up through the 4.7k. The 'slow down' time is when the C discharges through thr 4.7k. If the train is running and you break the connection between 4.7k and C2, it will take a much longer time to slow down. C2 is then discharging through the 2 transistors, so the time depends on their gain and the load current. As the transistor gains (beta) vary widely, it's hard to predict how long, but will probably be minutes.
At the
min quoted spreadsheet gains of 100 and 20 for beta, and 1 amp train, the current from C2 will be 1/2 mA, so will take about 30 sec, but transistor gains are always well above quoted values.
Or are you trying to get different startup and slowdown times? If that is waht you want, put a diode in series with 4.7k with current flowinginto C2. Then parallel those two with a diode facing the other way and larger resistor.
Also note that your max track voltage be 1.5V less than your input supply due to the drop in the two transistor base-emitter junctions.