Model railway electronics help

[ABB125]

Good morning,
I have recently bought an electronic controller for my model railway.

The potentiometer, in its current position, will be relatively awkward to install on my control board (partly because it's hinged). I noticed the white connector next to the fuse, and another potentiometer I have will fit perfectly into it.

The problem is, I don't know what effect doing so will have. I assume the two potentiometers would be connected in parallel (but I'm not sure), but surely this would create anomalies if both are adjusted (perhaps accidentally). Also, the potentiometer on the board is a B100k, whereas my spare one is B10k; would this cause any issues?
Here is a picture of the bottom of the board. The built-in potentiometer is the three pins between the screws on the right, the white socket is the three pins to the left of the screw hole top right.

And hopefully you will just be able to make out what looks like a connection from the potentiometer to a resistor on the top.

Many thanks for any advice, if you need any more detail please ask.

 

[edwin_m]

I can't work out what the connector does without a better view of the top of the PCB. You can find out using a continuity tester if the three pins of the connector connect to the three terminals of the fitted potentiometer. If so it is a parallel connection - connecting in series would need a "dummy" plug to be inserted in the connector to make the on-board potentiometer work. Can you search online for any instructions (quoting the make and model) that may explain what the connector is for, and if for an external control how it should be wired?

In principle if you leave the original potentiometer on its maximum resistance setting then putting another one in parallel would be OK. However replacing a 100K with a 10K would at best give very poor speed control (maybe only 0-10% of the speed range!) and may burn out other components. There is also the question of whether the pinout on your connector matches the one on the controller board.

If it was me and I couldn't find any instructions or work out the connector, I think I'd desolder the original potentiometer and connect it to three wires soldered in to the three PCB holes it came out of.

 

[ABB125]

I can't work out what the connector does without a better view of the top of the PCB. You can find out using a continuity tester if the three pins of the connector connect to the three terminals of the fitted potentiometer. If so it is a parallel connection - connecting in series would need a "dummy" plug to be inserted in the connector to make the on-board potentiometer work. Can you search online for any instructions (quoting the make and model) that may explain what the connector is for, and if for an external control how it should be wired?

In principle if you leave the original potentiometer on its maximum resistance setting then putting another one in parallel would be OK. However replacing a 100K with a 10K would at best give very poor speed control (maybe only 0-10% of the speed range!) and may burn out other components. There is also the question of whether the pinout on your connector matches the one on the controller board.

If it was me and I couldn't find any instructions or work out the connector, I think I'd desolder the original potentiometer and connect it to three wires soldered in to the three PCB holes it came out of.

Thanks very much, here are some more pictures of the top. Hopefully they're useful.

I suspect it'll be easiest for me to try to mount the controller in a different position to where I was planning (perhaps the side of the control panel rather than the "proper" face), but thanks for your input.

 

[John Webb]

I'd be inclined to unsolder the potentiometer (pot) and fit leads to it connected to the appropriate holes in the circuit board to enable the pot to be mounted remotely. But due to the dense packing on the board, we can't see everything there - I suspect there is some board circuitry on the top surface as the pot otherwise seems to have only the centre (wiper) contact connected to anything, looking at the underside view! If that's the case then unsoldering it would not be so easy.

 

[ABB125]

Thanks for your replies. I've had a closer look at the top and bottom of the board with better light, and I think that they are wired in parallel.

• The two centre contacts are obviously connected on the bottom
• The right-hand contact on under the white socket has a "wire" (what's the correct term for an individual link printed on the board?) that disappears near the other contacts; this reappears on the top and connects to one of the contacts
• The other two contacts also appear to be linked, by a "wire" on the top of the board

I've had a go at drawing a circuit diagram (beware! I've never tried this before properly, and I've probably mistaken some of the components!); all the ground points are connected to the -ve input contact. I haven't managed to get beyond the 555 chip, it gets rather crowded!

I hope you find this interesting, and thanks once again for your assistance.

 

[D365]

I've had a go at drawing a circuit diagram (beware! I've never tried this before properly, and I've probably mistaken some of the components!); all the ground points are connected to the -ve input contact. I haven't managed to get beyond the 555 chip, it gets rather crowded!

I hope you find this interesting, and thanks once again for your assistance.

Looks good to me, most of the symbols are correct. For future diagrams, my suggestion would be that you use a triangle shape to represent the supply voltage, as seen below:



I'd also suggest drawing dots to represent where an electrical connection is made between three or more different wires. This makes it clear that the wires are connected, rather than just overlapping.

 

[John Webb]

Looks like a 'Pulse Width Modulation' circuit. Trouble is that wiring an external potentiometer in parallel with the one on the board will alter how the circuit works, possibly not for the best! I suspect the makers use a standard board and the socket is provided for having a remote pot when that is desired.

One possibility is to take the pot off the board and use the wires and socket from the 10k pot you already have and fit them to the 100k pot off the board, if you can do soldering!

 

[ABB125]

Thanks very much all of you - I'll put a picture here showing what I've done with the controller once I've installed it.

 

[ABB125]

As promised, here are some pictures of the installed controller.

However, I now have another problem: the controller is a PWM device, and the output is rated at 10-100% (ie: doesn't go down to zero). I had hoped this wouldn't be an issue, but sadly trains will move at a relatively good speed even when the controller is on minimum power (which is, I suppose, the whole point of PWM!).
I'm currently running the controller from the 12V accessory output on my previous "proper" model railway controller (Gaugemaster Model D), so I tried plugging it into the controller output on the Model D with that at half power, but it hasn't really worked. Also, my new controller is rated 12-40V, so this would potentially cause damage.
So, as to a solution, I was thinking simply putting a voltage control device in between the controller and track, but would that work? What would perhaps be better?
Many thanks

 

[edwin_m]

Why not just put a switch on the power input to the controller?

 

[ABB125]

Why not just put a switch on the power input to the controller?

I have got one, but the trains go way too quickly (for minimum power) as soon as the power goes on, probably about 60% of a "normal" maximum speed. There is a noticeable drop in speed if I put a second train on the track, but this has little effect overall.
Probably the easiest thing to do is buy another controller that does a 0-100% duty cycle. After all, they aren't that expensive and it's not like they can't be used for anything else.