Speed limiter for DC?

[hartleymartin]

Hi everyone. I'm building a small shunting layout and I am using a Gaugemaster model W handheld controller. I was wondering if there is any way to limit the voltage sent to the track so as to limit the top speed of the locomotives. I want to prevent speeding locomotives derailing things.

 

The reason for this is that it is basically an Inglenook where I intend to invite members of the public to try shunting. I want to prevent some idiot turning the dial up to 100%, as undoubtedly someone will sooner or later. Perhaps limit the maximum voltage to the track to 6 volts?

[JohnnyDMonic]

There are a couple of options I can think of without opening up the controller and modifying it:

 

1. You could try powering it from a lower voltage transformer. Their website suggests 16vAC, so you could try 9v or even 7.5v. You would have to build yourself an earthed metal enclosure with appropriate fuses, etc.. but could be done fairly cheaply (£10 - £20).

For example: https://www.rapidonline.com/vigortronix-vtx-126-020-2075-chassis-transformer-230v-20va-7-5v-7-5v-88-3920 (you would have to wire the secondary outputs in parallel.)

 

2. You could use a variac to lower the mains voltage supplying your existing transformer. This would be more expensive (£60 upwards).

For example: https://uk.rs-online.com/web/p/variacs/8902765/

 

Or, if you aren't fussed about modifying the controller and know a bit about electronics,  you could open up the controller and add a resistor in the correct place to form a voltage divider with the existing speed pot.

 

Of course, there's also the good old fashioned method of attaching something to the knob / housing to physically prevent the knob from turning beyond a certain point.

[cliff park]

There are plenty of wall wart type power supplies at very cheap prices supplying 5v or 6v or 7.5v, look around the house at power supplies for anything rechargeable, check max current and give it a try. 

[AndyID]

Looks like Johnny and Cliff have covered most of the likely options. There is another more complicated option - increase the reduction gear ratio in the locomotives, a lot. This will limit the top speed and also make them much more controllable for shunting maneuvers.

[wasdavetheroad]

That good old fashioned method looks the easiest. Modify the knob or buy a replacement with a protruding 'pip' that can hit a stop on the casing, maybe superglue a plastic or rubber pad at the appropriate location

[hartleymartin]

Seeing as I am using the Model W controller, it makes sense that perhaps using a 9-volt AC adaptor might be the first thing to try. I doubt that any harm can come to the controller by feeding it less than the specified voltage. It now seems to obvious that this should be the first thing I should try.

[kevinlms]

6 hours ago, hartleymartin said:

Seeing as I am using the Model W controller, it makes sense that perhaps using a 9-volt AC adaptor might be the first thing to try. I doubt that any harm can come to the controller by feeding it less than the specified voltage. It now seems to obvious that this should be the first thing I should try.

Agreed, that is the easiest thing to do, it will not cause any damage to feed in a lower voltage.

 

The idea behind limiting the amount of travel by mechanical means, is also good. An adjustable stop, might be required for different locos used, since almost certainly the top speeds will vary.

[hartleymartin]

I'll try using a lower-voltage power input. I would rather not modify the controller itself (and void the warranty). The controller will also be used on other layouts, so this seems the best solution.

[JohnnyDMonic]

Just make sure you get an AC -> AC adapter. They can be got in the UK for not much:

 

https://uk.farnell.com/ideal-power/77db-10-09/power-supply-ac-ac-15w-9v-1-1a/dp/2368019

 

 

 

Edited April 26, 2019 by JohnnyDMonic

[kevinlms]

12 hours ago, hartleymartin said:

I'll try using a lower-voltage power input. I would rather not modify the controller itself (and void the warranty). The controller will also be used on other layouts, so this seems the best solution.

This one is closer to home.

 

https://www.jaycar.com.au/9v-ac-1-amp-plugpack/p/MP3027

[sharris]

There was a thread here some years ago

 

https://www.rmweb.co.uk/community/index.php?/topic/57463-gaugemaster-model-w/

 

Where someone had reverse engineered a Gaugemaster W. The circuit appears to be little more than a bridge rectifier followed by a Darlington emitter follower. 

 

I couldn't see any reason why it shouldn't work with a lower input voltage. Since there's no pulse width modulation or feedback involved that might need a 50 or 100Hz timebase any ready made power supply that can supply enough current to run your engine should work whether it's ac or dc out.

[Clive Mortimore]

Of course there is the eye, hand and not twist too far method. Really cheap.

 

Practice driving your locos at slow speed, you will soon get to know how far to turn the knob to achieve the required speed with each loco.

[sharris]

I think the idea was to let the unpracticed have a go on it.

[hartleymartin]

22 hours ago, sharris said:

I think the idea was to let the unpracticed have a go on it.

 

Indeed. The idea behind this layout is that it is an Inglenook shunting puzzle that members of the public are invited to try out during exhibitions. I wouldn't necessarily give it to little children, who undoubtedly want to turn everything up to 100% and ram my nice locos through the buffer-stops!

[hartleymartin]

I was thinking ideally I would have some sort of circuit which could be set to a maximum voltage no matter what was being fed into it, but would allow variable lower voltages. For example, it would basically do nothing to an input voltage of say 0-9 volts, but any voltages up to 12 volts would be limited back to 9 volts.

[AndyID]

9 hours ago, hartleymartin said:

I was thinking ideally I would have some sort of circuit which could be set to a maximum voltage no matter what was being fed into it, but would allow variable lower voltages. For example, it would basically do nothing to an input voltage of say 0-9 volts, but any voltages up to 12 volts would be limited back to 9 volts.

 

It's possible to do that although it is fairly complicated. The bigger problem is that it would have to be connected inside the controller before the reversing switch.

 

It might be a lot easier to build a simple regulator/controller that has a second pot to control the output voltage range.

[hartleymartin]

By the sounds of that I might just get a 9 or 12V AC adaptor for my controller instead of the specified 16V AC.

[DCB]

Get a handfull of diodes and wire them in series between the controller and the track, give the controller some welly and see if your loco is too fast. It will only run one way.  Too fast add diodes, too slow take one or two out of circuit and try again.  When the max speed is right wire a similar number of diodes together and connect the string in parallel to the others but the opposite way round so it runs both ways.

Diodes drop voltage pretty much irrespective of load.  some folk say 0.7 volts per diode.  Jouef used diodes in their battery controllers. I built an emergency controller with 11 notches built from 11 diodes and a 12 way rotary switch with a separate reversing switch. 

Actually the 11 diode solution should be near ideal for a shunting layout though you might well only need three positions, slow, slower and stop.

[sharris]

11 hours ago, DavidCBroad said:

Get a handfull of diodes and wire them in series between the controller and the track, give the controller some welly and see if your loco is too fast. It will only run one way.  Too fast add diodes, too slow take one or two out of circuit and try again.  When the max speed is right wire a similar number of diodes together and connect the string in parallel to the others but the opposite way round so it runs both ways...

 

What this will do is put a large dead-zone at the bottom end of the control knob, until you get into the top half (or so, depending on how many diodes you put in series) where the speed will start to increase from zero to some slower value than normal. 

 

Since, assuming the thread I linked to earlier is correct, the controller doesn't actually care whether it gets ac or dc as an input, you could, as an alternative use a dc power supply and the string of diodes between the power supply and controller to give the same (approximately) speed range but give you the whole knob range for better control (effectively you're dropping the dc supply voltage), or use an ac power supply with 4 of the diodes as a bridge rectifier followed by a string of 2 less diodes to give a similar effect. In the latter case I'd add a large smoothing capacitor immediately after the bridge rectifier too or your output will look more like a pulse train of variable height, with the pulse width set by the number of diodes in the chain, and the pulse height set by the control knob and diode count than a dc voltage.

[kevinlms]

34 minutes ago, sharris said:

 

What this will do is put a large dead-zone at the bottom end of the control knob, until you get into the top half (or so, depending on how many diodes you put in series) where the speed will start to increase from zero to some slower value than normal. 

 

Since, assuming the thread I linked to earlier is correct, the controller doesn't actually care whether it gets ac or dc as an input, you could, as an alternative use a dc power supply and the string of diodes between the power supply and controller to give the same (approximately) speed range but give you the whole knob range for better control (effectively you're dropping the dc supply voltage), or use an ac power supply with 4 of the diodes as a bridge rectifier followed by a string of 2 less diodes to give a similar effect. In the latter case I'd add a large smoothing capacitor immediately after the bridge rectifier too or your output will look more like a pulse train of variable height, with the pulse width set by the number of diodes in the chain, and the pulse height set by the control knob and diode count than a dc voltage.

 

Some modern controllers don't like a DC input. I can't think of specific brands off hand, but I do remember that topic being discussed.

[dhjgreen]

6 minutes ago, kevinlms said:

 

Some modern controllers don't like a DC input. I can't think of specific brands off hand, but I do remember that topic being discussed.

Then use the strings of back to back diodes on the AC side.

[sharris]

9 minutes ago, kevinlms said:

 

Some modern controllers don't like a DC input. I can't think of specific brands off hand, but I do remember that topic being discussed.

 

Generally those will be the pulse-width modulation and feedback type controllers that use the 50Hz (or 100Hz full-wave-rectified) signal as a time-base for synchronising pulses and back-EMF sampling points. From what I've seen, the Gaugemaster W doesn't have any of that clever gubbins inside. 

Edited April 30, 2019 by sharris

[kevinlms]

2 minutes ago, sharris said:

 

Generally those will be the pulse-width modulation and feedback type controllers that use the 50Hz (or 100Hz full-wave-rectified) signal as a time-base for synchronising pulses and back-EMF sampling points. 

Thank you, I knew I wasn't imagining it and makes perfect sense.

[DCB]

21 hours ago, sharris said:

 

What this will do is put a large dead-zone at the bottom end of the control knob, until you get into the top half (or so, depending on how many diodes you put in series) where the speed will start to increase from zero to some slower value than normal. 

 

It gives a zone below the minimum speed some controllers will allow and limits max voltage and speed.   Actually for shunting if you pick your diodes correctly you could probably get away with a bunch of diodes and a DPDT switch straight off a 12 volt supply, the  speed will be pretty much the same irrespective of load.  I often just drive on my direction switch on the diode controller if the loco has a flywheel and the diodes are a good match.  Some controllers are really horrible bits of junk, we don't all have access to Gaugemasters and the like.'

[long island jack]

The option I went for on Brewery sidings, is a simple DPDT c/off slide switch which plugs into the handset socket, this works of a Gaugemaster controller with a 12v controlled output or a 16vac output, is just a mater of swapping wires over, if there's a show with lots of eager children I plug the slide switch in and connect to the controlled 12v,setting the speed for what ever engine is in use, if it's a more grown up show, i plug a feedback controller in and connect to the 16vac output, it works really well

 

Edited May 1, 2019 by long island jack