Model Railway Forum

I thought I would add my half finished project as it may be of interest to someone.
It started for two reasons. The first is that I wanted to run a controller from 12v DC, and when I tried a commercially built panel that was really designed for a higher 16-18 volt AC current the trains ran a bit too slow. I know we modellers like slow trains but even the older Hornby 0-4-0's could do with a little more. The second reason is that I like making things myself. At least giving it a go to see if I can.
My plan is to keep things as simple as possible and I like using traditional methods.
Now I already had in my spares box the remains of the resistance part from an old Triang controller. The problem I had was that there was a big crack in the plastic remains from top to bottom. The remains give me variable speed via an electrical wiper much the same way old Scalextric controllers work, and the parts also enabled the current to be reversed via a centre off position. Turn one way and it is forward. Turn the other and it is reverse.
So all I needed was to make a new panel for it and reclaim the Triang parts, and provide a form of short circuit protection. As I am running the controller from a 12v battery I don't need to rectify the current from AC to DC. (If I did I would need four diodes). The answer came from an older gent who was using a car bulb in series. The idea is that the bulb is not lit and acts as a wire when the train is running but if a short circuit should occur, the bulb will light. A little experimenting and I found the theory works. I have a plan to use two different wattage bulbs as some of my locos hardly draw any current while others do.Therefore I plan to have a two way switch which I can turn to a higher wattage if I use certain locos. (5 watt to 15 watt or it could have been 5 watt and 10 watt bulbs I bought. I recall puzzling Halfords staff (Good job I know them ).
Also on testing the low current motors of today would move a little too fast with the controllers lowest setting, so I will be adding a little more switchable resistance via resistance wire reclaimed from the coils of an old electric fire. That way if I have a higher current loco I can throw the switch to accommodate it and switch the extra resistance for the low current locos.

Dont forget as a loco has load added - Carriages or wagons etc its motor current will increase!

While its not 100% accurate, using a rule of thumb that uses basic Ohms law gives 21watt/ 12 volts = 1.75Amp (21 watt car indicator lamp)
10watt/12volt = 0.83Amp (Side light or festoon lamp) while 6watt / 12 = 0.5Amp. (Festoon lamp or small bayonet)

Depending on the Ampere rating of the battery, which would normally exceed 1.0Amp/hr then I would use a 10Watt lamp. It only needs to come on when there is a short circuit.

BTW using lamps is not new, but do remember no power is turned off under fault conditions. Its just shunted by the short into the lamp. Therefore the lamp while lit is drawing battery power. I personally prefer self resetting thermal cut outs like this example http://www.gaugemaster.com/item_details ... aster+GM41 But not at those prices - Cheaper ones should be available elsewhere. Wired in series with one feed wire usually the positive side from the battery to the rail.

Thanks Brian. Only just seen your reply. The info is very useful, especially as it was a little guesswork for me. I started off by using the same watt bulb that someone else was using. Can't remember what wattage it was but I expect it was a 21 watt, as I found I needed to get a lower wattage to get the results I needed. I then found the odd older loco was drawing more current, hence the idea of using two bulbs and a two way switch. If I also use one of those circuit protectors, I assume it is wired in series on the output side of the controller rather then the input side? £4 may be expensive for the component but it isn't too bad as I only need to buy one of them.

Hi MG

You have a choice.... Protect the battery or protect the controllers output

I would be inclined to wire into one lead of the Output to protect both battery and Controller.