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Postby garrett » Mon Jul 11, 2016 8:19 pm

Hi all

I stumbled upon T gauge by accident over on reddit where someone posted a 3d printed SD70ACe (I think he's a member here too.) So I then went on ebay and bought the starter track kit for a whopping $30 now I need to get everything else!

Quick question - I see some mention of the different control boxes, what are the differences? I think the tan one is the old style, correct?

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Re: Newbie

Postby WildMarker » Mon Jul 11, 2016 10:02 pm

Hi Garrett,

Glad my ACe pulled you into the world of T! :)

I dont know the controll box differences myself, but im just using the Railpower 1370 from MRC. Its max output is 16v, which would fry the 4.5v motors in T. But so long as your ever concious to keep it below 20%, it works great.

I am interested in if/ how the PWM controller would improve performance over straight DC though, so hopefully someone will help explain that.
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Re: Newbie

Postby jerrybigcat » Mon Jul 11, 2016 10:27 pm

Tan box is the old one. The blue box, the new one, is PWM.
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Re: Newbie

Postby dkightley » Tue Jul 12, 2016 12:43 am

I am interested in if/ how the PWM controller would improve performance over straight DC though, so hopefully someone will help explain that.

I'll attempt to do that....although please accept my apologies if I tell you things you already know. I'm assuming zero knowledge as a starting point for my description!

PWM......or it's full title Pulse Width far superior for controlling the speed of DC motors compared with the basic analogue method of controlling speed.

The analogue method is simply varying the voltage from zero (or near zero) up to line voltage using a rheostat or variable resistance. Analogue is fine for controlling the brightness of filament bulbs, as the brightness of the glow is proportional to the voltage. Increase the voltage a bit....and you increase the brightness a bit.

Controlling DC motors is different with an analogue method. Start at zero and raise the voltage to approx. 1/3 full voltage...and the motor will probably not create enough torque to overcome the standing resistance of the motor, the gear system and the train. Increase the voltage a little bit...and the motor suddenly overcomes the standing friction, and with the motor now running, it's efficiency is improved slightly...and it will accelerate. The result is the train is reluctant to move...and then takes off like a rocket!!! Not good for scale movement!

PWM....on the other hand...puts the full voltage straight onto the motor so it has plenty of power available....but keeps switching this full voltage on and off so the speed of the motor can be finely controlled. This pulsed switching on and off is done at something like 30 to 100 times a second depending on the size of the motor...which is why PWM systems tend to hum. The clever bit is that to control the motor speed the amount of time the power is "on" compared to the time the power is "off" is varied from around 3% of the time to almost all of the time. Short pulses...and the motor will run slowly, and long pulses...and the motor will run faster. Hence the name PULSE WIDTH MODULATION.

Using PWM, the motors have full power capability virtually all of the time...which means running at a very slow speed is possible, giving a smooth control of train speed.
Doug Kightley
Webmaster here and volunteer at the National Tramway Museum
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