Tapiola Parish Model Railway Club


Why not use ordinary variable transformers?

Variable transformer [2K]

Most of European model railway suppliers bundle variable transformers with their starter kits. It's heart is a double insulated transformer with secondary windings so arranged that it may be tapped with a slider attached to the speed knob. The latest versions with special slow speed controls (like Fleischmann MSF) have two sliders attached to the knob to enable half wave rectification at slow speed.

If these are compared with rheostat-controllers/throttles, the variable transformer has much lower internal resistance, thus the output voltage is not so current dependent. With rheostats the less current the motor takes, the more voltage is applied. With conventional motors, having permanent magnet field and iron core armature, the current needed to to start the motor is higher than the current needed to keep it in motion. With rheostat controller, the speed control must be advanced quite far to get the loco into motion. Once the loco starts moving, one should adjust the speed setting back, in order to prevent the loco from speeding. If you have such a controller and have ammeter attached to it, you can easily prove this to be true! With rheostat we don’t actually control the voltage the motor gets or sees, but the current passing through it. As the rheostat is in series with the motor, the current passing through the both components is roughly the resistance of the motor and rheostat divided by the power source voltage (yes -- this slightly simplified). As the resistance of the dc-motor reduces as the motor starts to rotate, the resistance of the circuits becomes lower. Now due to fallen resistance in the circuit the current rises and the voltage accross the motor becomes higher and thus speeds the motor even more.

And now, why not use the variable transformer? Firstly, we wish to be able to follow the train, and that is quite impossible with a huge transformer. The second reason was money! We can make the hand held electronic controllers from scrap box parts, and only buy a cheap car battery charger at less that one third of the price of a toy train transformer! The third reason is "bells and whitsles dept." — the desire to have all kinds of nice things like inertia/momentum.

The basics of a dissipative controller

The main parts are illustrated below.

Voltage follower [1.2K]

Potentiometer P1 is a linear 10 kilo-ohm potentiometer. It is connected as a voltage divider. As the graphical representation shows it has a resistor which is tapped with a slider. The pot can only deliver 1/8 W of power, so we need an amplifier.

Enter: power transistor T1.

The transistor is a kind of electronic valve. The tiny control current flows from base (attached to the control -- or base current limiting resistor R1) to emitter (with the little arrow). Now if we have a motor attached across the output terminals (on the right after the direction switch) a current may flow from the top end of the pot, through the resistor R1 through the transistor, and via direction switch back to the bottom left corner terminal. This flowing of current causes the transistor valve to open. The valve lets heavy current to flow from collector (the one leg of the transistor we haven't yet mentioned) to emitter. This current depends on the control current (base-emitter current), and current magnification factor of the transistor, which may be 10 to 15,000! Now, as the valve opens, more current flows through the motor, and the voltage accross the motor rises. If it rises so much, that the emitter of the transistor becomes higer than the voltage we choose from the pot, no base-emitter current flows, and the transistor valve starts to close. Apparently a steady state will be reached, where the base-emitter current still flows, and the valve is somewhat open to let collector current pass to do the hard work! The resistor R1 limits the current that will travel through the base-emitter. Ah I almost forgot to tell that as the current flows from base to emitter, the transistor will tax 0.7 volts of it! Now this means that the emitter will at its best be 0.7 volts less the voltage we get from the pot. Now as the transistor valve is only slightly open, the transistor operates like a resistor, letting some of the current to pass through the collector. This is hard work, and develops heat! Hence the name dissipative controller. How much power the transistor will tolerate depends on design. The transistors designed to handle heavy loads (power transistors) tend to have poor amplification, around 10 or so. So we could not quite handle our locos with this transistor alone.

And now the Darlington connecton!

By adding another transistor to control the control current of the power transistor we get sufficient amplification. The way the transistors are connected in below is called Darlington. To make it properly, we should add a few resistor here and there, but we have not yet burned enough of these to learn the hard way.

The Darlington pair [1.5K]


Yes, now we are entering the field of bells and whistles! To drive the train realistically, we desire to have slow and smooth starts and stops. Would it not be nice, if we could just swing the speed control to desired setting and the electronics would take care of this slow acceleration or braking.

Inertia simulation [1K]

By adding a capacitor and an extra resitor (or adjustable resistor, like the speed control) we can achieve this. The voltage we just a few lines ago got from the speed pot could be taken from an electronic reservoir, capacitor. We can feed or drain it with our speed pot, and the rate of drain or feed is adjusted with P1, the inertia/momentum control. If we got sufficient amplification from our darlington amplifier, the capacitor could be left without a connection to the speed pot, and our train would coast at only slightly dropping speed. (yes, this is the so called the “memory function”). We could replace the speed pot with push buttons, the other would charge the capacitor trough a resistor and the other would drain it. With a connection like the one below we could get two acceleration rates and two braking rates quite easily.

Control with buttons [1.5K]

It would be a good idea to add yet another button as an emergency brake, that would short the capacitor almost instantly.

The problem assocciated with this kind of brake operation is that as the speed becomes lower the rate of deceleration becomes less too. This is often referred as the capacitor discharge curve. If we compare this with the full scale world it should be quite the opposite. Why does this rate of deceleration become less as speeds goes down? It is the fact that as the voltage in capacitor becomes less, the voltage across the brake resistor becomes less, hence the current through it - -- the capacitor discharge current -- will become less too. As a remedy one could connect the other end of the brake resistance to a point that is constantly certain volts lower than the voltage in the capacitor. There are numerous methods for overcoming this effect in model railway electronics publications.

What about the overload trip!

Yes, as we are feeding these units from a car battery charger, a current limit of some kind is needed!

If we place a resistor in the current path, and measure the voltage across it, we get the current that flows in the circuit. By placing a resistor R4 in the ground path like in the picture below, we may monitor the voltage across it with a transistor.

Overload trip [1K]

As we learned from above, the transistor will only work if it is allowed to tax 0.7 volts across base-emitter. If the current in the motor circuit rises over one amp, the voltage across the resistor R4 will be over 0.7 volts, and the transistor valve opens and starts to drain the voltage going to the darlington pair. For a one ampere overload trip a 0.33 ohm resistor is used. A diode D2 is added as we also want to get a LED illuminated at overload. The diode prevents the sneak path trough the LED to amplifier base.

The works!

Add a bit here and a bit there to get the result, as seen below.

The works! [3K]

What is added? Firstly the capacitor accross power transistor T2. That will help emphasing the valleys of the output voltage and aid slow speed running. The capacitor should be of 1000 mmf or more. We also added a diode across the output. This is to kill the back-EMF of the motor. Normal DC-motors act as generators when the power source is disconnected. This may be due to dirt in rail or dead crossing of a turnout. The green LED L1 is a power indicator. I’d suggest using a center off switch as direction switch as that may act as a fail safe device if the electronics get out of control.

For further reading I'd suggest visiting Mainline Modeler pages, especially the CoolerCrawler [TM]. The design rose quite a debate at News:rec.models.railroad (thanks to "Cathy" for finding link to discussion!) and as a result Mr Ken Willmott has made a technical representation of the matters at his "Throttle Design Notes" (used to be at "http://wits.on.ca/rail/electronics/throttle/throttle.htm" but no more. What has happened to Mr Willmott or to his data?).
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