Model Railway Point Switching Circuit

Discussion in 'The Projects Forum' started by vonsworld, Oct 14, 2014.

  1. vonsworld

    Thread Starter Member

    Apr 27, 2011
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    Hello

    I'm trying to design a simple circuit for switching point (turnout) motors on a model railway and found this circuit on a railway website, please see the attached jpeg.

    The point solenoid on the left needs a short burst of current to switch the point one way or the other, since constant current would quickly burn out the solenoid.

    The switch needs to be two position spdt switch. You could wire this circuit just using a couple of push button switches, but the advantage of the spdt switch is that you can see at a glance from the switch position how the point was last set.

    Do you think this circuit would work please and can you suggest any improvements?

    The circuit description says "When the switch is thrown, the current flows through the point motor until the capacitor is fully charged and then it stops. When you then throw the switch in the opposite direction the capacitor discharges through the other side of the point motor making it move in the opposite direction from before until the capacitor is fully discharged. Both times the switch is thrown the point motor only gets a very small burst of power."

    When it is fully charged would the capacitor block all DC current reaching the solenoid, or would a small amount still trickle through?

    The circuit relies on the capacitor discharging to switch the point back again. If you switched the point and then turned the power off, (ie. stopped using the model railway for a while), wouldn't the capacitor slowly discharge on it's own? So that when you returned there would be no power left to switch the point back again?

    Thanks for your advice :)
     
  2. MikeML

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    Oct 2, 2009
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    It is a horrible circuit, obviously created by a novice.

    The first time the switch is operated, the current pulse is half as subsequent pulses.

    The electrolytic capacitor is reverse charged half the time (reverse polarity should never be connected to an electrolytic capacitor.)
     
  3. djsfantasi

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    Apr 11, 2010
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    Have you considered putting a momentary switch in series with the SPDT switch, to activate the turnout once the position is chosen? This is the way prototype CTC panels are wired.
     
  4. ErnieM

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    Apr 24, 2011
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    Electrolytic caps will leak a fair amount of current, though I would expect not enough to really matter here. Of course that depends on the quality of the caps.

    Yeah, the caps will self discharge but I don't think that is really a problem as the "green" side normally is discharged, and if left on the red side when power is reapplied it will just tap the switch the way it should be anyway.

    There never is any reverse polarity applied to the cap so it should work as drawn, though I am a bit concerned (meaning it's too late for my brain to do the math) to size the cap or suggest a series resistor to limit the total ZAP of power.

    I trust you have noted you do need one cap per switch machine.
     
  5. vonsworld

    Thread Starter Member

    Apr 27, 2011
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    Thanks for your replies.

    This would be the simple answer, but I just wanted to explore if there was a neater solution where just a single SPDT switch would be required.

    I've looked at the circuit for a capacitor discharge unit, which are commonly used with a momentary switch to change the points on model railways. They protect the solenoid by ensuring it only receives a short pulse of power. They typically use a 2200uf capacitor directly supplying the solenoid with no limiting resistor, so I guess this must be ok.
     
  6. djsfantasi

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    Apr 11, 2010
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    Has this circuit been tested. It looks to me that when the switch is in the position shown, no power will be applied to the coils. The coil's power connections both will be connected to negative (assuming that the conventional color coding scheme was used, with red as + and black - or ground.) also, in this position, the capacitor will never charge.
     
  7. Alec_t

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    Sep 17, 2013
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    I think there are two coils, the centre connection being common.
     
  8. ErnieM

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    Apr 24, 2011
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    Exactly so.

    When the switch goes to the right the cap charges thru the top coil giving a brief pulse of current.

    When the switch goes to the left the cap discharges thru the bottom coil also giving a brief pulse of current.
     
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  9. djsfantasi

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    Thanks, Ernie
     
  10. MikeML

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    Oct 2, 2009
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    Well, this circuit is not as horrible as I thought. I was wrong about the voltage across the electrolytic reversing, too.

    Here are the results of a simulation, showing the various voltages and currents.

    I modeled the Points Motor as two uncoupled coils, each with a resistance of 24 Ohms, for lack of any real information...

    The form C toggle switch is modeled as two LTSpice switches, where one turns on as the other turns off. The V2 voltage source is used to control when the switches are thrown in the simulation.

    Look at the top plot pane:
    The voltage at the common terminal of the switch V(c) Blue trace alternates between 0V and 12V as the simulated switch is moved back and forth. The current through the two coils I(L1) Red trace and I(l2) Green trace alternates each time the switch is operated, causing the points to move back and forth. The duration of the current pulse comes from the resistance of the motor coil and the capacitor C1.

    Look at the bottom plot pane:
    The current through C1 I(c1) Yellow Trace is the sum of the two coil currents. It is the diodes D1 and D2 that steer the current to the two respective coils. The voltage between the negative end and the positive end of the capacitor V(C,N) Purple trace shows that the voltage across the electrolytic capacitor C1 is always net positive.

    68b.jpg
     
  11. vonsworld

    Thread Starter Member

    Apr 27, 2011
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    Thanks for your replies and MikeML for simulating this circuit.

    Looking at a description for a capacitor discharge unit, commonly used for changing points but with a push button switch, they use a 2200uf cap, the diodes are both 1N4002, and they say the impedance of these solenoids is quite low at around 3 ohms.

    If you had perhaps 6-10 of these circuits all running a different point solenoid, what type of power supply would you need to supply them all?

    If just one solenoid is being switched at a time (normal case) then I know a 1 amp 15 volt would be fine. However, if some of these capacitors are left charged when the model railway is no longer being used (switch left in the right position), they will slowly discharge on their own. When the power is reconnected they will all immediately recharge switching the solenoid again even though it doesn't need it. Would this affect the power supply required, would it overload the usual 1 amp supply?
     
  12. sirch2

    Well-Known Member

    Jan 21, 2013
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    We had a model railway as kids with points motors and I seem to recall that on the switch board the switches were momentary action but stayed in the "up" or "down" position. So you pushed the switch "up" and and it energized the solenoid but letting go disconnected the power but left the switch in the up position. I've Googled this but all I can find is momentary On-Off-On toggle switches where the switch returns to the centre when released. Am I misremembering or or they just not made anymore?
     
  13. djsfantasi

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    Apr 11, 2010
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    You probably aren't misremembering. Atlas made a switch for turnout control that operated as you described - the Atlas 56 Switch Control Box.

    They've improved it, so that it includes indicator lights and looks more like a CTC control. See Atlas 57 Deluxe Switch Control Box.
    http://www.atlasrr.com/Trackmisc/57switchcontrol.htm
     
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  14. MikeML

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    I resimed with 3 Ohms for the motor coil. With no inductance, just resistance, the peak coil current would be 4A. With some reasonable value of inductance, the peak current through the point motor coil is ~3A. The peak supply current is also 3A, but only when the switch makes V(c) transition from 0V to 12V; zero current as the switch makes the 12V to 0V transition. The steady-state supply current is zero.

    Since the likely hood of energizing two point motors at the same time is very small, the supply can be rated at what it takes to operate only one point motor coil, regardless of how many are connected.
     
  15. djsfantasi

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    Just as a side note, there are special cases where it is very likely that two or more turnout coils will be activated. For example, in a yard ladder, or in a crossover. The former case will see 'n' turnouts activated (where 'n' is the number of tracks in a railroad yard); the latter case will always have 2 turnout solenoids activated. This is the more common case, suggesting that the supply be able to handle twice the current draw of a coil.
     
  16. MikeML

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    Only if you can actuate the two switches that control the two point motors at exactly the same time. Most likely, your finger will actuate first one switch, and then the other...
     
  17. djsfantasi

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    In the cases I describe, they are often wired with only one switch. Particularly for crossovers.
     
  18. MikeML

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    Then double the current requirements, or wire a 20,000uF electrolytic across the output of the existing 1A or 2A power supply.
     
  19. eetech00

    Active Member

    Jun 8, 2013
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    Hi

    I know this is a model railroad turnout, but does it have contacts that indicate the position of the switch?
    A real electric switch machine has position indication contacts that are used to provide feedback to the control circuit logic that the switch has actually moved to the requested position. When power is applied to the switch motor, power is maintained to the motor until it completes movement to the requested position. The actual position is then checked against the requested position before a switch position indication is provided to the operator. This type of logic check is called a "switch correspondence" check.

    For what its worth...I used to design TCS/CTC systems back in the day...hope that helps.

    eT
     
  20. vonsworld

    Thread Starter Member

    Apr 27, 2011
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    It's true that more than one point/turnout coil might be operated at once, as in the case of a junction where two points have to work together. However I believe that a 1 amp/15v supply is enough to cope with this eventually.

    My only concern is what happens if you have say 10 of these circuits connected to one power supply. There is no problem if you only switch one or two points at once...

    But if you switch off the railway for a while, depending on the position of the switches, all the capacitors that are already charged will slowly lose their charge, and when the power is reconnected they will all immediately recharge and switch the points again even though they are already in the correct position.

    This won't damage the points, but won't it overload the power supply?

    If so how could you prevent all the capacitors recharging at once when the power is reconnected?

    Thanks :)
     
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