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[quote]QUOTE: Originally posted by timmatthews It's not that I have anything against buying MRC, but that I have enough electronic junk laying around and can design/construct something for less cost. Half of the fun for me is the electrical aspect of Model RR. I have already built and tested a straight DC throttle which consists of an Operational Amplifier circuit set up as a voltage regulator driving an NPN Darlington power device. [/quote] Pulsed gives more torque for better low speed performance, but can make the motor run hotter. MRC uses unfiltered full wave rectified sine shaped pulses, some models produce 60 Hz at low speed settings with lower voltage alternate pulses again at 60 Hz for a total of 120 Hz. As the speed is increased both sets of atlernating pulses increase in amplitude until they are at equal level at 120 Hz at full throttle. Works very well. If operating off batteries you don't have the sine shaped option or 60 Hz frequency reference. 30 to 100 Hz works, the best frequency is the lowest at which your locomotive crawls without vibrating, but if the headlight is a LED you want at least 50 Hz to reduce annoying headlight flicker. The motor growls more realistically at low frequency (diesels only; growling steam locos are very unrealistic!). Square shaped pulses are easiest to generate but result in the highest motor heating and loudest noise. Triangular shaped (ramped) pulses are also fairly easy to generate with opamps and are a reasonable approximation to a sine shape to reduce heating and motor noise, but the drive transistor will then require a heat sink to prevent it from overheating with anything other than square pulses. A good system is one which superimposes filtered DC at low voltage with short duration full voltage sine or triangular spikes, the DC voltage rises and the spikes can increase in duration as the speed is advanced. This combines torque with cooler motor operation. There's a lot of room for experimentation in this area, have fun. An important point - make sure you have a current limiting safety device in series with the power pack and track. This can be a PTC (positive temperature coefficient) device that limits current as it heats. They are available specially made for this purpose in a variety of trip currents; the automotive light bulb in common use is an example of a device made to work as a PTC through the ingenuity of RR modellers. Or use a current sense resistor in series with the track and a circuit to shut down the power if it detects an overcurrent of say 2 amps so you can MU locos.
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