IP digital point motors on dcc layout

[terrysoham]

Last night, I was fitting a Cobalt IP Digital motor to an N gauge point.

When fitted it wasn't switching the frog polarity. I removed it and checked it out on the bench. The S2 switches had stopped working.

Fitted a second motor having previously checked that the S2 polarity switches were working. Tested it. Second motor wouldn't switch frog polarity!

Clearly there was a power problem. Started to remove adjacent point (fortunately I glue my track down with Copydex) and suddenly there was the answer. I hadn't fitted one rail off the frog with an insulated rail joiner which of course meant that when the point was switched in one direction there was a short on the frog.

 

I opened up the motor's casing and started to check the pcb' s switch contacts - there was no connection from the common slider pad. Looked carefully at the track on the pcb and found that a track had shorted and burnt through.

Carefully I soldered some fine wire across the damaged section. Checked it, put the casing back together and it worked on the bench.

Did the same on the other damaged motor.

 

Lesson. Always check that you have fitted the insulated track joiner to both frog rails.

[Nigelcliffe]

Terry, 

 

There is something fundamentally wrong with your wiring for the burn-out to have happened.    Yes, you should have fitted the insulated joiners. 

 

The short via the contacts in the Cobalt was to the DCC bus.   If your layout wiring was "up to snuff", then the DCC system (or a circuit breaker for a power district) should have shut-down due to the short circuit.   

That it didn't shut down implies there are areas of resistance somewhere in your layout wiring, which means the short cannot be detected at the power delivered by your DCC system, and instead enough power was flowing to burn out contacts on a switch.   

 

 

- Nigel

[terrysoham]

Nigel,

You have me worried.

I wonder whether I could take this off line and send you a private message

Thanks

[Grovenor]

Actually may just be that the PCB track is underrated. Doing the usual coin test nearby (BUT NOT AT THE ACTUAL FROG) will check if your DCC system cutout is working.

Regards

[Nigelcliffe]

Keith's (Grovenor) comment about PCB tracks may be correct, but its a very high resistance in those tracks if they don't allow a command station to trip.  I think it more likely that they are eroding due to the high make/break currents they are subjected to, see below.   Just fitting an insulated jointer on the output of the frog isn't a total solution given the problems experienced. 

 

 

I'll take "an N gauge point" to be a Peco live-frog turnout.  Within those, both of the blades are connected electrically to the frog.  In normal use, as Peco intended, no extra wiring is required, the blade touches the fixed stock rail and connects that stock rail to the frog for one direction, and the open blade is far enough over that the back of a wheel shouldn't touch it and cause a short.

( Some other N gauge points are different, such as the "Finetrax" kit built turnouts.  The Finetrax turnouts are a "classic" live frog where the blades are not connected to the frog )

 

With the introduction of a second switch path to the frog, via the Cobalt motor, there is a BIG problem.  

As the Cobalt motor operates, at some position in its travel, the contact switch will break, go through a brief "no contact" period and then make with the opposite side.   Unless that "no contact" exactly coincides with the turnout blades moving through their "no contact" position (middle of movement from one position to the other), then a short circuit is created when one item makes contact before the other has broken its old contact.  That exact "no contact" has to work in both directions of travel, and is difficult to setup reliably (probably impossible without some modifications to the turnout motor, regardless of maker of motor).  The short created is maximum track current for the power district in question (or maximum current for the command station).  

If the short is created by the movement of the Cobalt motor switch, the short will probably erode the contacts on the Cobalt motor as they won't be rated for multiple amp break/make currents (or even 1amp make/break), more likely the contacts are fine once the connection is established to carry a current to a loco. 

 

There are several fixes for this problem. 

a - use Peco N gauge live frog turnouts as Peco designed them, and feed the frogs via the blades.  Do not use extra frog switching.   This is usually reliable if the track is as per factory.  But, ballasting (glue) and painting rails, can make the contacts less reliable. 

b - modify the Peco turnout to make it a "classical" live frog turnout.  For this, the rails between frog and blade need to be cut (usually near the frog), and the resulting cut-blade can optionally (and recommended) be electrically connected to its corresponding fixed rail.   Then, the frog is now isolated from the blades and requires a switch (on the turnout motor) to change its polarity. 

c - modify the switch contacts on the turnout motor to have a much larger "no contact" area, and so they only make contact at the far ends of the motor's movement.  With this, provided the turnout moves its blades after the motor is in the "no contact" area, then things will be fine, but the movement does need to be reliably set this way.

d - introduce some electronics which disconnect the turnout motor switch path before the turnout motor starts to move, and only re-establishes contact after the motor has finished movement. Not that complicated to design.

e - use an automatic frog polarity switching device, such as the Frog Juicer, or Gaugemaster equivalent product in place of the turnout motor switches.  These still have the short circuit, but the device is designed to withstand the short and then cut-over to the correct polarity. 

 

 

 

 

- Nigel

[terrysoham]

Nigel,

 

The layout that I'm working on is a friend's. This is the first time that I've used Cobalts and an NCE PowerPro. The other two layouts have used a mixture of solenoids and Tortoise motors and the dcc components are by MERG. This is the third layout that I've wired using exactly the same wiring regime. All my layouts have used a bus wire having a minimum diameter of 1.5mm and all the droppers are 16/.2 equipment wire soldered to the bus wire. In terms of resistance I wouldn't imagine such a specification would be problematic but I am willing to be corrected.

 

The points are modified Peco Electro-frog types code 55. I have sawn through the stock rails before the frog so that the frog is not fed from the switch rails. The stock rail and the cut rails have also been electrically bonded. So it is the Cobalt's switches S2L, S2R and S2C that do the frog switching taking the power from the dcc bus rails via S2L and S2R and into the frog via S2C.

 

The common track on the Cobalt's pcb is quite narrow and I suspect that it could be susceptible to this sort of problem. I think that an email to the manufacturer might be a good idea.

 

By the way at present none of the layouts are using DCO's but I have built a couple of MERG ones and I wonder whether I should try them on the present layout to see if it avoids any further problems - apart from remembering to fit the IRJs

 

If you can see any obvious source of error in my wiring, I'd be happy to hear from you.

 

Kind regards

[terrysoham]

Actually may just be that the PCB track is underrated. Doing the usual coin test nearby (BUT NOT AT THE ACTUAL FROG) will check if your DCC system cutout is working.Regards

Keith,

Thanks for your reply. The coin tests works fine and the NCE PowerPro's status light indicates that there is a short circuit and shuts down the booster for 2-3seconds and then tries again as per the manual.

[Nigelcliffe]

Terry,

 

it appears that you've already adopted ( b ) of my list of solutions to the Peco turnout issues.  So, the frog switching should not be an issue for the layout.

 

What therefore remains is the burn-out of the switches when there was a short due to the lack of insulated jointers on the exit of the frog onto the main running lines.    That's either the make/break arcing current destroying the contact places (5A due to the lack of insulated joiner mentioned), or a narrow track on the cobalt being unable to handle 5A and burning out before the breaker in the PowerPro trips.  If the latter, then a Cobalt is likely to burn out the contacts in use if there are 5A shorts at the frog (a DCO could reduce this current). 

 

 

You reported that the coin test passes around the layout, so your wiring is fine for the 5A system - that's a pragmatic test of wiring.

 

If it is N gauge, then yes, I'd fit DCO breakers to a 5A system. An N gauge loco draws perhaps 100mA to 200mA at maximum load, and tends to emit smoke from malfunction at 0.5A - the most recent N motors are even lower currents.  

The MERG DCO a decent design.  Run it at its lower setting (1.5A), or even put two turns of track wire through the coil to make it 0.75A on the lower setting.    Ideally break the layout up into different zones, each with their own DCO.  

 

 

If you run a DCO at 1.5A, that's under 1/3rd of the current of a 5A PowerPro system, so the wiring which passed the coin test on the PowerPro 5A is now at least 3-times less resistance (ie. 3 times better) than required to trip the DCO.   At 0.75A it becomes more than six times. 

 

 

If using a turnout / accessory bus, that's the least urgent to get a DCO, and arguable doesn't need one.  Though, with an accessory bus, its important to do as you indicate you have done, and wire to the three contacts on the auxillary switch on a Cobalt (not the single frog output which some digital Cobalt products offer, otherwise you're bridging the accessory bus with the track bus, and who knows which DCO will operate! ).

 

 

 

- Nigel

[terrysoham]

Nigel

I will definately fit the spare MERG DCOs to the busses.

 

Thanks for your response

 

Regards