DC throttle control

Discussion in 'HO Scale Model Trains' started by stanC, Mar 31, 2007.

  1. stanC

    stanC Member

    I was thinking about having power supply for a few different sections for a HO setup by buying one 15v DC transformer and then connecting this in parallel to the different sections with a variable resistors. This way it should be very cheap and easy to setup. I am interested to know if this is possible and if so what resistance should I need.
    I have not been very long with The Gauge but in that short time I have really enjoyed reading the very interesting and informative letters. Thanks to all of you
  2. Torpedo

    Torpedo Member

    You need rheostats for what you want to do. A 25 ohm rheostat capable of handling one amp of current should work and have ample power handling capacity. You can find such a beast here. The part number is (RWA) RP102SD250KK. I would buy just one at first to see if it had the appropriate range. You might need to go up or down a few ohms.

    Don't forget reversing toggle switches for each section. Also the power supply with have to be well regulated and capable of supplying sufficient current to power all sections at full throttle at the same time, or the section throttles will interact with each other.

    Somehow, I don't think you will save much money.
  3. pgandw

    pgandw Active Member


    Not a great idea. Used to be more popular when motors drew more current, rheostats were cheap in comparison to rectifiers, and smooth operation just wasn't common.

    Variable resistor control works very poorly today because most - but not all - locomotives have high efficiency can motors. The older open frame motors would typically draw 0.3-0.5 amps in operation on a layout, with Athearns going as high as 0.7 amps. A variable resistor of 40-50 ohms was needed to be able to control these. The rheostat needed to be able to dissipate as much as 10-15 watts running the train at slow speed. Wire wound potentiometers (variable resistors) were typical due to power requirements and smooth resistance changes.

    A newer can motor drawing 0.15 amps cannot be run at slow speed by a 50 ohm rheostat. In this case, the rheostat can only drop 7.5 volts at most, which leaves 4.5 volts as your starting voltage. Most modern locomotives will jump into action with 4.5 volts applied. If you use a 100 ohm variable resistor to control the can motor, then you have to turn the variable resistor to at least half scale to start the older locomotive moving.

    For the above reasons, direct transistor control of output voltage is preferred, and is available in better commercial power packs like the MRC Tech series.

    Next issue with your independent sections is when the train crosses the section boundary from one section to another. At that point, the 2 rheostats are in parallel, which reduces the total circuit resistance to half the total of the 2. The locomotive will suddenly speed up until it is wholly contained in one section again, where it will slow back down with another jerk. Metal wheels on cars which bridge the rail gaps on both rails simultaneously will have the same effect. If the direction switches happen to be opposite when the locomotive bridges the section - instant short circuit.

    Last problem with section control is that buying/finding appropriate variable resistors is niether trivial nor cheap anymore. They just aren't made. Before heavy acceptance of transistor throttles, the preferred rheostats were wire wound, non-linear tapered resistance, with at least 270 degrees of rotation - all in the interest of better control.

    These short-comings of section control led to the development of block control. In block control, each train has its own power pack that follows it wherever the train goes on the layout. Each block, or electrically isolated section of track, has an electrical switch which selects which power pack will control that particular train. This avoids any problems unless you accidentally get 2 trains inside the same block at the same time. When that happens, both trains go in the same direction and at whatever speed the track voltage gets them. You have simply lost independent control of the trains. Setting up block boundaries correctly is key to operational flexibility while avoiding 2 trains in the same block.

    In block control, the power packs don't have to be the same, and can be as feature-laden or not, as one desires. They only have to have enough power to run one train - 12 watts is sufficient unless you are double-heading Athearns or have a long trains of lighted passenger cars.

    yours in DC block wiring

    my thoughts, your
  4. JR&Son

    JR&Son Member


    My research shows that a 4 to 6 ohm pot. works best
    Tough to find but putting a wirewound resistor in with one of these would work
    Notice the price!
    Im using this
    I swapped the LM317 to an LM350 for a bit of head room (3 amps)
    VERY simple circuit that works very good
    The other one Im building is
    I have lots of Pancakes running around, low speed is not there forte so Im crutching them until..................
  5. stanC

    stanC Member

    Fred I very much appreciate your time for the very interesting reply. The point about halving the resistance was something I would never have thought about . It just makes me realise how much there is to learn.
  6. stanC

    stanC Member

    Thanks to everyone for the replies. It never fails to surprise how helpful the members are and what a wealth of knowledge they have. Incidentally anyone have any information about the volume control for a radio. Could this be used as a variable rheostat
    Best wishes
  7. pgandw

    pgandw Active Member


    On the radio, the volume control is a small potentiometer (pot) mounted at the front end of the audio amplifier. It has high resistance - usually several thousand ohms - and very low current. The pot sets the input voltage of the amplifier, which sets the amount of gain (amplification) and hence volume, of the amplifier. Stereo volume controls work much the same way. Preferred characteristics of the volume control are non-linearity (our ears are very non-linear devices), small, and cheap. Since the volume control is high resistance and very low current, power is very low (less than 1 watt), and simple, very cheap carbon compression pots will do. That is why on many portable radios, the volume control has dead spots (static and noisy) after a few years.

    Unless the rheostat came from a tube amplifier (from the days before transistors), it will not have the power handling capability needed to be used as a direct throttle for model trains. It would overheat and burn up quite quickly. Also, the resistance is way too high. Only a tiny fraction of the rotation would be the control range for your train.

    Now that I've been entirely negative: a volume control pot could be used as the speed control in a "transistor throttle". A transistor throttle uses the speed control pot to set the input voltage of an 1-3 transistor amplifier (note that opamps are a modern substitution for the early transistor stages). Rather than a lot of voltage gain, the amplifier in a throttle "amplifies" the current. The final output transistor usually needs a heat sink because it will get warm from the current being passed. Transistor throttle circuits are available here - http://home.cogeco.ca/~rpaisley4/Throttles.html.

    yours in throttling up
  8. JR&Son

    JR&Son Member

    I spotted the cogeco some time ago.
    I figured K.I.S.S keep it simple supid and went with the awrr
    Less wiring less headaches.

  9. stanC

    stanC Member

    Thanks for the reply. I was away hence the delay in replying. When I searched the site for
    RP102SD250KK I received the following message. Please can you help me locate it or let me have an indication of price. You are probably correct about the saving money but when it works with your own circuit that has to be worth a fwe dollars [FONT='Verdana','Arial','Helvetica']No document matches your query [/FONT]
    [FONT='Verdana','Arial','Helvetica']Regards [/FONT]
  10. stanC

    stanC Member

    Fred thanks for the reply Sorry that I have not thanked you before but I was away
    In connection with your reply how then does the power pack create the variable voltage. does it not have to have a built in rheostat. Regards
  11. pgandw

    pgandw Active Member

    Modern throttles use a transistor amplifier arrangement to directly control voltage to the track. The front end potentiometer on the input of the amp sets the drive level, and hence the final output voltage. Since voltage is set directly rather than as a result of a voltage drop that varies with the current drawn, transistor throttles can be used with any 12V motor as long as the final output transistor and input source can handle the current load.

    Other electronic circuits are easily added to a transistor throttle:
    • pulse generator for improved very slow speed running
    • momentum and braking effects
    • maximum and minimum voltage controls
    All Tech 2, 3, and 4 series power packs from MRC are transistor throttles, as is the Control Master 20 and 9500.

    You can also build your own transistor throttle; some circuits are here:

    yours in powering up

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