I constructed the circuit in post #14 and can confirm it does work.
However the slow speed control and torque of Hornby type trains seems to be only very slightly better with a PWM controller than when powered by a LM317T type of regulator which can supply a variable voltage from a low impedance source.
The drawback with the PWM controller is the mechanical noise generated through the motor due the the pulsing current. It initially was like a high frequency hum, (about 260Hz) and is there all the time even at stationary. It improved slightly and was less noticeable when I changed C5 from 100nF to 47nF.
But adding what is termed "inertia Control" (automatic slow start, coasting, and slow stopping) has proved difficult to do using the PWM circuit and is beyond my knowledge and understanding of the workings of the 555 timer chip..
Anyway I went back to experimenting with a linear regulator, LM317T chip, used in a "Slow turn on" circuit as below.
In theory this circuit should work with a larger capacitor to give a 30 second slow rise in the output volts, and together with suitable switching and selected resistors to shunt the capacitor the circuit can give the effect of breaking and finally stopping.
I found that a 100uF cap in place of C1 gave a slow rise in voltage of about 40 seconds to full output. (full output is set by a 4.7k variable resistor in place of R2) Various resistors were experimented with to give the breaking effect.
During testing the setup worked very well with a 300mA bulb, 40 seconds to full voltage, 30 seconds to about half voltage to simulate coasting, and 5 seconds to the final stop at the station.
However in practice things were totally different. When powering a train on the track the voltage was all over the place, rising then falling randomly, and even coming to almost a stop.
The odd thing though was, if the engine was held still allowing the wheels to spin on the track the controller worked perfectly, the voltage rose steadily to full volts. Release the train and it momentarily sped off but the voltage was soon all over the place again.
My conclusion, the slow start circuit in the Texas application notes DOES NOT work with a model train. I dont know why. Possibly because of back emf and connection to the track issues being amplified by the transistor.
Role back a day.
I had made this circuit on a little piece of Veroboard, and I had thought with out any wiring faults, it worked first time (allowing for the experimentation with capacitors and resistors) We were happily playing with it for most of the day.
It was not until later in the day that I discovered that the collector of the PNP transistor was not connected to ground, I had used the wrong copper strip for ground.
Oops, why was it working at all, oh well I will rectify that and try again.
The capacitor C1 value had to be drastically reduced from 4700uF to 100uF, the breaking resistors all had to be changed to cater for the gain of the transistor.
Final circuit.
I reverted back to the circuit with the collector disconnected. The engines need about 3 to 4.5 volts to start to move from stationary pulling a load with 4 carriages.
The final value for C1 was 4700uF.
Shorting C1 with a 2.7k resistor simulates slowing down for points or approaching a station, dropping the voltage to about 6 to 7 volts.
A further resistor 270 ohms switched in discharges the capacitor to 3v and simulates final breaking.
There is a short video here https://www.youtube.com/watch?v=Y1Ci2T-RVkc
What fun
Mike
However the slow speed control and torque of Hornby type trains seems to be only very slightly better with a PWM controller than when powered by a LM317T type of regulator which can supply a variable voltage from a low impedance source.
The drawback with the PWM controller is the mechanical noise generated through the motor due the the pulsing current. It initially was like a high frequency hum, (about 260Hz) and is there all the time even at stationary. It improved slightly and was less noticeable when I changed C5 from 100nF to 47nF.
But adding what is termed "inertia Control" (automatic slow start, coasting, and slow stopping) has proved difficult to do using the PWM circuit and is beyond my knowledge and understanding of the workings of the 555 timer chip..
Anyway I went back to experimenting with a linear regulator, LM317T chip, used in a "Slow turn on" circuit as below.
In theory this circuit should work with a larger capacitor to give a 30 second slow rise in the output volts, and together with suitable switching and selected resistors to shunt the capacitor the circuit can give the effect of breaking and finally stopping.
I found that a 100uF cap in place of C1 gave a slow rise in voltage of about 40 seconds to full output. (full output is set by a 4.7k variable resistor in place of R2) Various resistors were experimented with to give the breaking effect.
During testing the setup worked very well with a 300mA bulb, 40 seconds to full voltage, 30 seconds to about half voltage to simulate coasting, and 5 seconds to the final stop at the station.
However in practice things were totally different. When powering a train on the track the voltage was all over the place, rising then falling randomly, and even coming to almost a stop.
The odd thing though was, if the engine was held still allowing the wheels to spin on the track the controller worked perfectly, the voltage rose steadily to full volts. Release the train and it momentarily sped off but the voltage was soon all over the place again.
My conclusion, the slow start circuit in the Texas application notes DOES NOT work with a model train. I dont know why. Possibly because of back emf and connection to the track issues being amplified by the transistor.
Role back a day.
I had made this circuit on a little piece of Veroboard, and I had thought with out any wiring faults, it worked first time (allowing for the experimentation with capacitors and resistors) We were happily playing with it for most of the day.
It was not until later in the day that I discovered that the collector of the PNP transistor was not connected to ground, I had used the wrong copper strip for ground.
Oops, why was it working at all, oh well I will rectify that and try again.
The capacitor C1 value had to be drastically reduced from 4700uF to 100uF, the breaking resistors all had to be changed to cater for the gain of the transistor.
Final circuit.
I reverted back to the circuit with the collector disconnected. The engines need about 3 to 4.5 volts to start to move from stationary pulling a load with 4 carriages.
The final value for C1 was 4700uF.
Shorting C1 with a 2.7k resistor simulates slowing down for points or approaching a station, dropping the voltage to about 6 to 7 volts.
A further resistor 270 ohms switched in discharges the capacitor to 3v and simulates final breaking.
There is a short video here https://www.youtube.com/watch?v=Y1Ci2T-RVkc
What fun
Mike
