Fulgurex point motor control

This week's project is to add control circuitry for a Fulgurex slow motion point motor. The photo below shows the parts I intend to use:


Those familiar with driving Fulgurex will probably think this is a bit over the top, but there is method in my madness and each component has its place in my scheme.

First, it is my intention in the longer term to drive the point motor via DCC, but there may be times when I don't want to have to go to the bother of addressing the motor via the control unit and then selecting the direction I want. To avoid that, I need a local switch. Some DCC point motor controls provide a facility for local buttons, but I'm not sure which those are and whether or not I will use them. In the meantime, I need to control the motor now because unlike Peco solenoids, you can't manually "flick" a Fulgurex. My solution to this need is to use a normally open, momentary action DPDT switch which can be seen in the left of the photo.

The momentary nature of the switch means that once the limit has been reached and the current is switched off, I can release the switch and there will be no current at all. This should avoid conflicts with any DCC control unit I install later. The drawback with this method is that I lose the switch acting as an indicator for the current setting of the point blades. The solution to this problem is to use one set of auxiliary contacts on the Fulgurex to drive the pair of LEDs which are also on the left along with the resistors and matching mounting bezels. This will provide a visual indication by the switch of the current setting. When DCC control is added, any changes caused by the DCC unit will be reflected by the LEDs. This circuit may also double up as input to a pair feedback sensors to provide PC control some time in the distant future.

The ceramic disc capacitor at the bottom of the photo is to provide a degree of noise suppression. According to LDT, the Fulgurex is rather "noisy" electrically and this is a good precaution. Once the motor is installed under the baseboard, I don't fancy lying on my back trying to retrofit these things later, so they're going in now.

The other two devices beside the capacitor are a pair of Zener diodes. This again is an LDT recommendation for reducing the voltage supply to the motor to reduce the change over speed. I'm doing this because the motor will be supplied at about 14v DC which is a bit on the high side.

I will be using the other pair of microswitch contacts to change the frog polarity on the point. The switch blades themselves are hard wired to the outer rails. This eliminates the potential for shorts should the microswitch change over before the moving point blade has ceased making contact with the outer rail. A short on an analogue DC layout is no problem, but it will shut down DCC which is not a good thing.

The black stringy stuff in the top right is heat shrink tubing which I will use to make sure there are no bare wires straggling about.

Now I just need to find something to mount the switch and LEDs on. Panel thickness is a bit of an issue for these LED bezels because the LEDs are quite small.

David