Point Motor Control Problem

[RobertW]

Hi All,

 

I'm currently designing the electrics for my new layout.

 

I'm aiming to switch the points using a custom point controller. The aim is that the front panel will consist of a single push to make switch per point, with 2 LEDs to indicate the direction of the point. The circuit will also be able to work with live frog points.

 

I am using a Peco PL13 switch to switch a pair of DPCO relays in parallel (in that the relays switch together to form a PCB mounted 4 pole changeover relay). The first contact is the push button switch, which can be switched through the relay to the base of one of 2 power transistors depending on the lie of the points. The second changeover contact switches between the pair of LEDs, so that the indication is of the actual position of the points. On the second relay, one set of contacts switches the polarity of the point frog, and the second is spare. 

 

I've built this on a breadboard, and when I push the button, the point tie bar on my test bed rattles across between the 2 positions. I thought that perhaps the control voltage was dipping when the points were being switched, so I've tried using a high value capacitor to hold the 5V up while it switches, however it still does it. I'm feeding the CDU from 24VDC from my bench power supply with a 5VDC feed from the bench power supply to the control side. 

 

Can anyone suggest a solution to allow the point to change from one direction to the other when the button is pressed. I'm trying not to use ICs, and I think that the circuit isn't far off, but its not quite working correctly.

 

Thanks and apologies for the wordy post.

 

Thanks,

 

Rob

Point_Controller.pdf

[John ks]

 

I suspect that when you push the point control PB

One coil is energised & the point changes 

The PL13 switch changes

The relay changes which connects the other coil & the point changes back

This repeats until the CDU is discharged or you release the PB Sw

If this is the case then  give the PB Sw a quick tap. If this works it would tend to  confirm my suspicion

I am not sure how to fix it if this is the problem

I have added an image of your PDF so the next person doesn't have to download it

 

John

[RobertW]

Hi John,

 

Thanks for this, its been really useful. I've updated the design as below.

 

I think that the way you could mitigate this would be to fit 2 electrolytic capacitors as per the diagram to sustain the voltage for a small time after the switch moves, and arrange the time constants for the circuits such that the relays won't change until twice the time that the solenoid control line is energised for. This means that hopefully the point won't oscillate.

 

The CDU has 4000µF of capacitance so it should have a discharge time constant of 16ms, and a discharge time of 80ms. The solenoid should be energised through the capacitor C2 for 558ms, meaning that the CDU should be completely discharged through the motor by the time that the capacitor C2 is discharged. To prevent the point oscillation, the direction control is delayed by 1.65s, meaning that the relay shouldn't switch to the other transistor base before the CDU is fully discharged, hopefully fixing the oscillation.

 

Let me know what you think.

 

Thanks 

 

Rob

 

 

 

 

 

 

[John ks]

Hi Rob

I am retired electrician who has dabbled in electronics from a hobby standpoint.

Looking at the modification to the circuit it looks like it may/could/should work but to be honest its getting a little above my pay grade

Build it on your bread board & test it to see if it works as designed ( tests could include short press, long press, double press of PB Sw)

Hope this helps 

John

[RobertW]

Hi John,

 

That's fine. It occurred to me tonight that there was a better way of doing it. If I use latching relays instead and fire one coil with one way on the PL-13 and the other coil with the other, then only when the PL-13 has moved over will it try to fire the other coil. I need to order the relays as I've only got non-latch ing ones in stock, and I'll update when I've got some news.

 

Thanks for your help.

 

Rob

 

[John ks]

Rob 

Should you have a resistor between the PB Sw & the bases of the solenoid transistors

John

[Nigelcliffe]

I think you only need one latching....  Because it doesn't need a set of relay contacts for the LEDs. 

 

The LEDs could be lit from the 5v going via the point motor switch.  

That frees up a set of contacts for the frog switching, thus making the second relay unnecessary.   

 

 

- Nigel

 

 

 

[Junctionmad]

What do you mean by ” rattling across “. What’s actually happening 

Edited March 15, 2021 by Junctionmad

[RobertW]

9 minutes ago, Junctionmad said:

What do you mean by ” rattling across “. What’s actually happening 

 

Hi Junctionmad,

 

When I push the push button, the point tiebar moves across. The test rig I have has an ordinary microswitch recovered from a mouse with the end of the tiebar actuating it at the end of the travel. As the tiebar releases the microswitch, the relay controlling the solenoid drops out before the tiebar has overcome the spring in the point, which causes the tie bar to move back across into the microswitch, starting the cycle again.

 

Looking at the diagram, I think I might end up needing a microswitch at each end of the tiebar, and just using the N/O contacts on each microswitch to actuate the relays.

 

Thanks

 

Rob

[AndyID]

Hi Rob,

 

You can use a non-latching relay if you "slug" the coil. Put an electrolytic capacitor in parallel with the coil and feed the current to the coil through a resistor. That will delay the relay state change and give the solenoids enough time to throw the points properly. You can't use a very large resistor unless you increase the voltage a bit but if they are five volt relays you can probably afford to drop half a volt across the resistor if you use the five volt supply.

 

Cheers,

Andy

[RobertW]

Hi All,

 

Just for completeness, I redesigned the module to Andy's suggestion, and it worked perfectly on breadboard. I've attached the updated schematic, and I'm moving on to produce the Veroboard this weekend.

 

 

 

The way that I've implemented the slugging is to maintain the supply to the transistor as suggested through a 470μF capacitor. I did try to slug the relay coils, however I found it more effective this way. The result is that the move is completed properly, as the motor remains energised for longer. I'm now at the Veroboard design & manufacturing stage, as there will be 4 of these for my new layout.

 

I'm hoping to start building the baseboards next weekend, and I'll start a layout thread in due course.

 

Let me know if you have any comments.

 

Thanks

 

Rob

Edited March 21, 2021 by RobertW
Edited to update drawing

[Junctionmad]

My only comment would be that the direction leds will be wrong if the operator ever forgets  to push the “ activate “ button , the usual way these circuits are arranged is to use a latching  relay driven off the coil pulses , hence the led follows the point motor , not the switches 

 

you could also replace the led relay with a simple two transistor set/reset toggle  from the pm drive which would do the same thing 

Edited March 22, 2021 by Junctionmad

[RobertW]

Hi Junctionmad,

 

My take on it was that by arranging it so that the LEDs are controlled by the direction switch SW2, they will reflect the actual position of the tiebar, rather than the intended position as it's a PL-13 SPCO switch mounted on the motor.

 

I've had a similar application on a previous project where the LEDs were controlled by a 555 bistable with a DPCO non maintained switch ganged to the point motor switch. If the point didn't move for some reason, the LEDs were out of sequence with the point position, hence why I'm basing it on the mechanical movement of the point, rather than the electrical control pulses to the point.

 

Hope this explains things a little.

 

Thanks

 

Rob 

[Junctionmad]

how can the circuit illustrated actually follow the tie bar, as I understand it when you switch the " direction" switch the LEDs change immediately , even though the button to fire the point motors hasn't actually been pressed ?

[AndyID]

The direction switch is attached to the tiebar. Basically it's a divide by two circuit. Every time you push the button it changes state.

[Junctionmad]

On 26/03/2021 at 14:55, AndyID said:

The direction switch is attached to the tiebar. Basically it's a divide by two circuit. Every time you push the button it changes state.

Oh that wasn’t clear from any discussion to date 

[AndyID]

4 hours ago, Junctionmad said:

Oh that wasn’t clear from any discussion to date 

 

Yes, it took me some time to figure that out