Track power failure / short circuit? [DCC layout]

[Chrissaf]

Hornby Standard DCC controller......

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Please be specific........what Hornby DCC controller model?......there is no DCC controller that is considered as being a 'standard' one.

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....used a multimeter on the tracks and they're live....

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Please specify the voltage that you measured? and confirm that the meter was on the AC Voltage scale?

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[Chrissaf]

Hi folks,

I'm fairly new to model railways but i've built a DCC layout which was working perfectly until this morning.

I have two ovals of track linked by points and this morning I had a derailment with a loco falling over both tracks and now nothing works.

I understand this might be a short circuit but nothing I do has any effect. I've disconnected the power (Hornby Standard DCC controller and re-started, used a multimeter on the tracks and they're live, tried different locos and everthing is dead, even the little lights on the buffer stops.

I've checked the track for debris and cleaned it all and I'm now at a loss to see the problem, I'ld be grateful for any suggestions.

Regards

Stu

[stunev]

Hi Chris, thanks for your prompt reply, I'm using the Select DCC controller and I've checked the track using the AC voltage function of the multimeter - I'm not familiar with  multimeters but each piece of track shows a voltage of an average of 5 (volts?)

[jane1707819582]

5 volts is very low . is the track still connected to the select?. Does your select go through it's  start up sequence OK ?.

[2e0dtoeric]

A DCC track should be showing about 16v AC.

As you only have 5v, I'd suggest the controller is 'testing' for a short-circuit, and not fully starting up because there is one somewhere.

Have a really close look around where the engine fell off, in good light. You might find a stray track pin, or a bit of metallic debris there.

[Rog RJ]

Disconnect the power wires from the track and then measure what voltage you are now getting on the end of the power wires.

[96RAF]

If you are reading 5vAC at the rails and you are using the 1A Select power supply unit (wall wart) then there is still a short somewhere, which is causing the psu to self protect and drop its output voltage. In this condition the Select will continuously try to reboot.

 

Have you possibly dislodged a power clip and wrongly reconnected it.

[Chrissaf]

I was intending to reply, but Rob got in first with the reply I too would have given. You still have a 'short condition' somewhere. It may however be a resistive short of a few ohms and not totally zero. Or a power supply fault....see below:

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On the AC voltage range you should get a voltage of about 13 to 15 volts on the track. Connect your Select to a short piece of totally isolated track and place a stationary loco on it. Accurate voltage readings need to be taken with a load. If you still get about 5 volts, then consider that the Select power supply has gone faulty due to the original 'short circuit' you had when the loco derailed.

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Just purely a logical deduction on my part, based on all the descriptive text you have provided. But the Select power supply would be my number one suspect on this occasion.

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If the Select 1 amp power supply is faulty, replace it with the official Hornby P9300 4 amp power supply. This uprated supply is officially recommended by Hornby for the Select and will be a very beneficial upgrade.

[stunev]

Hi folks, many thanks for the all the advice and helpful suggestions  - since my original post I've cleaned all the track and couldn't see any debris, checked the dropper wire connections and this morning I tested a loco on a small piece of spare track by touching the power wires from the Select to the track and it worked. I'm going to order the P9300 power supply suggested by Chris and see if that works.

Thanks again, best wishes.

Stu

[Chrissaf]

That test you have described just gives credence to a fault still existing on the main layout track. If that is the case, the P9300 won't fix it.

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You really do need a multi-meter and measure the resistance across the two rails with everything disconnected from the track. If all is well, you should get an infinite (open circuit) reading. Any resistance measured would need investigation.

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A multi-meter is an essential bit of test kit for this hobby. Since your opening post stated voltage readings, then you obviously have one.

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Disconnect the Select controller, remove all rolling stock from the track and take the resistance measurement across the two rails. If you get infinite resistance, replace the rolling stock one at a time and test again between each new addition to the track. A fault would be where you get a reading say less than 20 to 40 ohms.

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If you get any reading across the rails with nothing placed on it, then that needs to be investigated with the meter to find the source of it. One of the Buffer Stop lights (LEDs??) may have gone faulty, so disconnect them too if you get a resistance reading.

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Short circuits on DCC track generate transient ringing voltages that can exceed 3 times the track voltage. The meter on AC voltage will read an average value. The peak voltage value will be approx 1.412 times the average. So a measured track voltage of 15 volts AC will be 21 volts.....peak to peak will be a voltage swing of 42 volts and if ringing is added to the equation then the peak to peak swing voltage due to a DCC short circuit ringing condition can easily reach 120 volts or more. This is enough to damage an LED. You can only see these ringing voltages on an Oscilloscope with a capture function, they are too transient to be measured with a standard multi-meter.

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[stunev]

Hi folks, since my original post I've tried various things to sort my problem without success.

• Brushed and hoovered all the track - no sign of any debris.
• taken out the buffer stops in case the LEDs were faulty as suggested by Chrissaf
• I disconnected the Select from the bus and used some scrap wires going from the Select and touching them onto the tracks and finally got a loco to run, great I thought, it's a wiring problem. I've just stripped out all the wiring and installed a new bus and dropper wires and after all that - nothing,
• the only other thing I can think of is replacing the Select but I'm reluctant to spend money in case that doesn't work either.

As you can imagine I'm getting very frustrated, I'm grateful for all the good advice so far and if anyone has any other ideas I would appreciate it.

Stu

 

 

[Flashbang]

Hi

If the loco(s) run correctly on a totally isolated piece of track which is feed directly from the Select's "Track" terminals then the fault has to be somewhere on the main layout. 

 

If this isolated section of track check shows a problem and all the main layout wiring is removed from the Select when testing on the isolated section of track, then the fault is either with the Selects power supply or the Select itself.

 

Assuming the locos all run correctly as per para 1 above, then look at the layout and its wiring.  Question.... Is a DCC Bus pair used with droppers from the Bus pair to the rails?  If so, double check you haven't cross connected one or more sets of droppers to the rails  i.e. red dropper wire is on a black dropper wire fed rail somewhere.

 

If you're using more than one set of power rail sections or plug in power clips ensure the wires in the terminals of the connector are the correct way around.  That is a red feed wire always feeds the same rail regardless of which way around the connector or section is installed. 

Note: I'm referencing red and black feed wire insulation colours, but the actual insulation colour will be your choice.

 

Edit to correct typos!

 

[stunev]

Hi folks - I'm back in business! I'm still not totally sure what went wrong but the re-wiring seems to have worked after some extra strong crimping of the suitcase connectors from the bus to the droppers. Many thanks for all your advice.

Best wishes

Stu

[Chrissaf]

I've used those crimp connectors and have never had a problem, but for those type of crimps to be problem free you must use appropriately sized wires for both the Bus and the droppers. If either the Bus or the dropper is too thin, then the crimp jaws may not make proper contact through the wire insulation.