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How To Modify Model Train Controller?

Discussion in 'General Electronics Chat' started by muso52, Aug 18, 2017.

  1. muso52

    muso52 Member

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    Guys,
    After many years I have decided to resurect an idea iv'e had with respect to a model train controller and would apreciate your help and guidance. This entails modifying the circuit below which is a bit of a classic in the model train world. This circuit uses full wave rectified but NOT smoothed ac as helps the slow performance of the dc motors. We have R2 our speed controller pot and also (switchable) R7 which is a pot which acts as a initia/braking feature.
    What I would like to do is modify the circuit such that when the speed pot (R2) is at its minimum, approx 4 v is present on the output to the track.My simplistic view is that a suitable resistor in series with the bottom of R2 is all that is required though I do not believe it to be that simple.
    So why do it I expect you are wondering. By doing so I can configure the model loco's to only run when a voltage of greater than 4 volts is applied. Again why? This 4v rms can be smoothed to give me a 5v (approx) supply for onboard engine auxileries such as lights and sound. The very simple arrangement of an inverse parrellel diode array is connected between track supply and dc motor to inhibit this 4 volts thus rendering the loco stationary.
    But of course there is more! Traditional dc controllers work on a 0 -12v range. Now by reducing this range, my limited knowledge tells me that the resolution and hence controlability via R2 would be improved. So looking at a initial 4v ( no movement) + 10v for engine so nominally 14v.
    This circuit is I believe about 40 yrs old and I think it could be further modified by replacing the darlington pair with a ' single' darlington component, the individual transistors probably not available anymore here in uk. Codar.jpg

    I hope you will find this project of interest and I would truly welcome your comments and advice.
    Yours
    Willy
     
  2. ci139

    ci139 Active Member

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    it sets more thermal load to a single new -- right now it's somewhat shared . . .
    what you could do is find / buy a backup parts for this existing board and store it for future sales
    then design and build entire new one including line TF or some modern Flyback SMPS
     
  3. alec_t

    alec_t Well-Known Member Most Helpful Member

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    That could indeed provide the 4V you're looking for. However, the current available would be extremely limited because of the 10k resistance presented by R2.
    You could instead use a capacitor to smooth the bridge rectifier output, then use the smoothed voltage to drive a switched mode DC-DC converter to get the 5V.
     
  4. dave

    Dave New Member

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  5. granddad

    granddad Active Member

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    The transistors in your circuit are available here https://www.esr.co.uk/home.htm
     
  6. muso52

    muso52 Member

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    Thanks for that granddad.........2 d's? Will take a look.
    Alec_t
    R2 is in fact the speed control pot and as such is to me, a simple voltage divider so not sure of the relevence of what you are stating wrt current. Again I should point out that the output is variable to the model railway track to source the loco 12v dc motor. Forgive me but as stated in original post, smoothing the dc is a no no as it effects the performance of the dc motor. The nominal 4v is utilised onboard the loco via a regulator and capacitor to give 5v smoothed for auxilary electronics independant of the motor control voltage. i.e. irrespective of voltage above 4v the onboard electrics (independent of the motor) on the loco source sound and lights at 5v. So again the idea is to bias the darlington pair at a nominal 4v when the control pot is fully off and when fully on at a voltage determined by the (approx - allowing for drops) source voltage.
    Willy
     
  7. AnalogKid

    AnalogKid Well-Known Member

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    Yes, it really is that simple.
    Both transistors still are easily available here in the states. Plus, the 2N3055 will be available *forever*. Both parts are way oversized for the application. If it ain't broke ...

    ak
     
  8. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I misunderstood what you were intending to do with the 4V. I was thinking track-side power, not in-loco power.
     
  9. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    R8 helps to linearize the adjustment and i dont think you'll find that in a Darlington.

    A modern 12v supply would have a voltage reference diode to adjust the output with. It may even be PWM, although that gets more complicated.

    What is the switch for?
     
  10. alec_t

    alec_t Well-Known Member Most Helpful Member

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    If you remove R3 and put a 8k2 resistor in series with the bottom of R2 I think you'll get the result you want.
     
  11. muso52

    muso52 Member

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    Hi,Guys,
    Thanks for your input and guidance.

    Now theres an idea. Hadn't thought of that one!

    What is the switch for?
    S1 + R6,R7 and C3 are a simulator circuit for inertia/braking. With the later and components switched in, the rate of charge on C3 (and hence change on Q1, voltage = speed), is adjusted via R7. When voltage is reduced below that of C3 , the same components control the rate of discharge and hence speed of slowing down of dc motor.
    Its a play factor thing.
    Ref the resistor size Alec, yes will try. But as normal with my dabbling it will be very much'suck it and see'. But simple.

    and I think it could be further modified by replacing the darlington pair with a ' single' darlington component,
    What I should have said is " can anyone suggest a suitable replacment darlington pair in a single package?"


    Thanks,
    Willy
     
  12. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    It is as simple as a resistor in the ground side of r2 to get 4v min, try a 4k7, it depends on your trans voltage so you'll need to experiment, a 5v1 zener might be a better idea if your powering other motors from the same transformer to give a bit of stability, cathode to R2.

    A Tip122 is a 5a/100v readily available darlington, zrx450's are still available and 2n3055's are still common (although some electronic hobbyists will tell you they are old), a more modern version of it is a Tip3055 which is the same tranny in a more modern package.

    To power electronic accesories within the loco's you'll probably need a 1n4001 series diode and a 1000u capacitor, especially if these gizmo's contain a microcontroller, as with pulsed dc they'd get all upset. it would make sense that these devices would already be installed on such accessories so have alook first.

    Hows the weather in Plymouth?
     
    Last edited: Aug 19, 2017
  13. muso52

    muso52 Member

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    Hi dr pepper,
    many thanksfor your comments.
    Not sure about the zener. With the change in voltage drop and current through R2 (Rv2) would this not effect the Zener?
    A single diode is no good as polarity and hence direction of train is changed via S2 dp switch on the diagram.To power the accessories within the loco will require a bridge rectifier, cap and 5v regulator. Of course in my original post I did not take into accound Vd across former so may have to raise that 4v. No worries. To inhibit this 4v from reaching dc motor and hence moving the loco I cascade or series up 3 or 4 Bridge Rectifier packages between variable track supply and motor.This basically connects 6 / 6 or 8/8 diodes in inverse parrallel which provides a constant voltage drop irespective of the variable track voltage.Good eh? Well that is the theory and it do'es work.
    Hows the weather in Plymouth?................................................ You 'avin a laugh? Sunday afternoon so of course it's rainin just like the rest of the week!

    Willy
     
  14. crutschow

    crutschow Well-Known Member Most Helpful Member

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    If you subtract 4V to the motor with the diodes than you need to increase the supply voltage to 16V to still get 12V across the motor.

    A more elegant (and also more complex) technique would be to output 7VAC or 7V short pulses to the track which could be rectified to provide the 5V power for the on-board electronics, but would not cause the motor to run.
     
  15. muso52

    muso52 Member

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    Hi cruschow,
    Thanks for your interest.
    If you subtract 4V to the motor with the diodes than you need to increase the supply voltage to 16V to still get 12V across the motor.

    Yes you are right. However at a max 12v across a loco motor they tend to race around the track at unrealistic speeds! Leaving the supply voltage as was will give me improved resolution of and hence control of, loco speed via R (v) 2.

    Maybe so but what would happen when I wanted to apply voltage to move the loco? Connecting the diodes in the manner stated may not be elegant but has it's benefits. The drop across the diodes will, irrespective of changes in voltage (above Vd) or load (number of trucks behind loco) not alter. Also they are passive devices. Now you say what you are envisaging is more elegant? Maybe not. If I simply wanted to source 5v to a loco without it moving I would disconnect motor from track and connect a walmart 5v plugin adaptor to it! The idea has always been to source the dc motor while also providing aux 5v for the accessories. Again back to the diodes. I still need sound and lights when the loco is stationary hence why they are installed between track and motor.

    Hope this of interest to you all.

    Willy
     
  16. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    A zener would change a little with supply variances, but a lot less than a plain resistor would, hence my suggestion.

    Weathers not much better here, up north we think you get all the best weather.
     
  17. crutschow

    crutschow Well-Known Member Most Helpful Member

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    As you apply normal voltage to the motors, the auxiliary power rectifiers would then rectify the motor power instead of the pulse power as the motor voltage exceeded the pulse voltage.
    The motor power would supersede the pulse power.
     
  18. muso52

    muso52 Member

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    Hi Gents,
    Again my thanks for your interest and advice.
    Yes I am beginning to understand where you are coming from. Basically the zener is acting as a voltage reference? For a better understanding on my part I would appreciate if you you could pse explain its operation in the circuit as proposed.
    Arh...............a myth! Spent 2 1/2 yrs in the north west whilst in RN. Manchester ver Plymouth in rainfall?

    Hi crutschow,
    Firstly my appologies if on re-reading my last message if I appear a little arrogant,pompous, ungratefull, pig headed etc etc. Just trying to be friendly.
    I guess what you are suggesting is applying a seperate or independant low voltage source which would be swamped by the motor drive voltage when the latter is exceeded? I can understand that but It would mean the loss of resolution on R(v) 2 as the initial movement of the pot, until a voltage was reached that was greater than the secondary aux supply, would result in no motor volts. Basically the motor would not start to turn until further in the travel of the pot.Hence loss of resolution and play factor. Me thinks.

    Willy
     
  19. JimB

    JimB Super Moderator Most Helpful Member

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    Barrow in Furness by any chance?

    JimB
     
  20. crutschow

    crutschow Well-Known Member Most Helpful Member

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    No, you are still not understanding my scheme.
    The motor would see the motor control voltage as if there were no pulse voltage.
    The motor voltage is superimposed on the pulse voltage.
    It's response would be the same as if there were no pulses.
     
  21. muso52

    muso52 Member

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    Hi JimB
    Nah.......... Inskip (Kirkham) W/T station.

    Ok, But the motor voltage is full wave rectified but not smoothed dc. To superimpose or swamp a pulsed voltage surely would be problamatical?
    But again if you are suggesting as I think, that this pulseed voltage is applied additionaly to the motor drive voltage then again I have to suggest that resolution of the motor control would be reduced no? I would have to turn up the pot to a level which overcomes the onboard diode network before any movement occurs.
    Not arguing, maybe misunderstanding.
    Willy
     

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