RealTrax Switch Wiring - Good or Bad Idea

I'd just beef up the wire that cooked, it's obviously undersized.

The wire didn't cook, it was the trace on the board the wire is soldered to. It's a problem we've heard of before but never seen one until now. 

I think it resulted from his running long lighted passenger cars and the next nearest lock-on connections are about 3 feet down the track from the switch. 

We like the idea of that trace having it own dedicated power feed, but noticed that MTH didn't put anything in the manual about wiring the switch to allow for this connection.

Back when I was a club member, we had 50 such switches, 7 of which were inoperative when I joined.  So in fixing this I learned a few things.  Only one had a burned trace as you described.  I became curious, as undersized Chinese traces are not a stranger to me.  I calculated the theoretical ampacity (how many amps continuously) of this trace after measuring its thickness and width.  The answer is 5 amps (although I cannot now recall if that was at room temperature @68F, or the next step up @86F.  In our case some summer days we had 100+F on the layout. or the 104F step where our traces were down to a 4 amp rating.  We were using two Z4000's with power set at 18v.  Due to the long feeds, the short circuit current was sufficiently reduced that the Z breakers did not operate rapidly.  (Generally you could cover the typical 30 feet back to the Z's quicker than the breakers.) 

The intent was to use a maximum running current of 10 amps on each of the 4 outputs.  As each output served several power blocks (4 or 5?  I forget.) the 3d rail gaps were set to make these blocks as nearly equal length as convenient.  Thus the switches might wind up in the middle of an end-fed power block.  The trace is on a very small circuit board (half by one inch?) so it was not the reliable heat sink some boards are.  The traces take the wiring under the throw bar as there is not enough clearance for the insulated wires.  The fix was to add an insulated wire jumper for the 3d rail under the layout from one end of the switch to the other, attaching to the adjacent track.  This was planned for all 50 switches.

The more common problem was a burned-out coil in the switch.  There was no way to repair these but to check for this condition a bench test on the coil assembly alone was most convenient, using a spare bench switch lacking a coil set, but having factory controller and power connections.  This dealt with the problem of removing the switch itself out of the screwed-down track.  [These coil assemblies will not stand excessively repeated mounting and dismounting, however.]  The several spare switches (due to layout changes during construction) were sufficient for needed spare coil assemblies.  One or two may have had trouble with their plug-in contacts to the switches.  There may have been a problem with one coil-clearing microswitch, which was a hard problem vs all the other problems (I may have just changed the switch out).

Not all inoperative switches had burned coils.  I discovered that to get reliable complete throws that would operate the coil-clearing microswitches, the applied voltage had to be right up to the intended 18v.  There were special small timing boards which would limit the application of the throw connection to one-second as a precaution against burn out, serving 5 or 6 switches each.  These were in local areas of switch concentrations, to keep the smaller output wires short (comm wire as found, whether 22, 24, or 26 I cannot recall).  But there were a couple of switches at a rather long output distance, and the small comm wire caused trouble.  The controls were local (layout edge) to the switches but in these two cases a one-second board was not available nearby.  All the switch circuits initially  operated off the reduced amp output of the Z's at one time; I cannot now recall how I got to 18v on these outputs-- perhaps I moved them to one of the variable Z outputs.  (The layout had a triple track main loop, plus a connected smaller upper-level loop.)

One switch had a misgaging causing derailment in one direction and had to be changed out.  Altogether, we had 7 spare switches (some requiring repair), and these were adequate to provide the spare parts to make all the 50 in the layout fully operational.  By dim memory, I just changed out the switch with the burned trace, salvaging its coil-pack.

I don't intend the layout wiring description as a recommendation one way or the other, only a a background to the switch problems.  Hopefully this account will help with a range of problems.  Good luck,

--Frank

bad idea - the amperage draw from the coil when the switch fires - even momentarily- is not something that you want on track power. But, the AUX power is designed to use the same return (common) black terminal of the track power.

I found that using an alternating pattern with the lock-ons or 1 or 2 of the 10" Ground Tracks to make both outside rails common helps.

The switches are what they are, as well described by Frank. We have even had the magnets from old switches stay in a switch because the glue failed when swapping the motors. So, have bad ones marked as such for parts donors and a spare or two handy.

Ahh, the beauty of the Fastrack switches, no huge power surge when the switch changes position, just a very small DC motor running.

Moonman posted:

bad idea - the amperage draw from the coil when the switch fires - even momentarily- is not something that you want on track power. But, the AUX power is designed to use the same return (common) black terminal of the track power.

This was a concern so we did some testing to check the current draw from the switch to see how much it would affect the common side of the track power. 

A simple light bulb test was done with two 18v bulbs wired to the switch & track power using the common ground. The switch and track are run from separate (phased) transformers.  We didn't see any noticeable change in the bulb on the track power when the switch was thrown, but the Switch power bulb did flicker when triggered. 

We also tested the peak amp draw the switch needed when triggered and averaged about 2.5 amps in five tests.

Our conclusion was that the immediate draw on the transformer powering the switch was the dropping voltage vs overloading the common wire to the switch. 

The problem with the switch power isn't the actual max current, it's the sudden current spike that will upset things and create high voltage spikes on the track power.  That's why all the talk about TVS protection diodes.

gunrunnerjohn posted:

The problem with the switch power isn't the actual max current, it's the sudden current spike that will upset things and create high voltage spikes on the track power.  That's why all the talk about TVS protection diodes.

Thanks for your input John!

Both transformers are TVS protected, I do suppose that a voltage spike from the switch transformer can carry to the track beings that the common is shared. This wasn't something we considered but I think we have this covered.

Should we be putting more TVS protection diodes (perhaps on the switch itself) to additionally protect voltage spikes from one transformer to another?

We are currently using these TVS units available from Digikey: 1.5KE36CA If there is something better please correct us!

If you're going to keep powering the switches from track power, it might be a good idea to put one of the TVS diodes directly across the switch power right as the switch.  The 1.5KE36CA is the correct part to use.

gunrunnerjohn posted:

If you're going to keep powering the switches from track power, it might be a good idea to put one of the TVS diodes directly across the switch power right as the switch.

Would this still be an issue if the Track and the switch are powered by separate (phased) transformers? In bulk, TVS is cheap so I don't mind added them.

Having the TVS is never a bad thing, but separate power will go a long ways to mitigating any issue of spikes on the track power.