Decoder short circuit after derailment

[Britannia Builder]

My Hornby DMU with 8-TXS decoder derailed at slow speed over wrongly-set points yesterday, and since then it has had a permanent short circuit. The eLink trips whenever the DMU is on the track. Prior to this the decoder had worked perfectly for 3 months or more.

I removed the decoder and fitted a blanking plate, and the motor ran with a 9v battery against the wheels. I then reinstalled the old non-sound decoder and the DMU runs nicely with that.

I guess the decoder must have blown, although it seems strange that a simple track short could have caused that. I'll contact Hornby and see if they will replace it under warranty, unless anyone has any other ideas?

Regards, John

[96RAF]

Have you tried a CV8 value 8 reset a couple of times.

[P-Henny]

As previously discussed historically in the forum. Certain types of transient short circuits can cause 'ringing' of the DCC signal. These near instantaneous ringing events can create peak spike voltages that exceed 2 or 3 times the normal DCC track voltage. These voltage spikes can damage decoders. The ringing is brief (milliseconds) and occurs in the time window between the mechanical short occuring and the resultant current rising to a level that finally trips the controller short circuit protection.

To protect the decoders in the future. The adverse affect of short circuit induced ringing, can be alleviated by fitting a 'snubber' across the track. The snubber 'filter' dampens these spike voltages to levels below those that cause decoder damage.

Snubbers can be easily made DIY. They are a 100 Ohm 1 Watt resistor in series with a 0.1uF Ceramic capacitor with an AC voltage rating of 50 volts or higher. These two series connected components are then wired across the track, preferably at the farthest point away from where the controller is connected.

Want more information on snubbers. Then open the 'Useful Links' pinned sticky thread at the top of the 'General Discussion' forum and follow the link for "Mark Gurries - Snubber/RC Filter".

[96RAF]

I have seen snubbers with a diode in circuit, not sure what that achieves on DCC which is an alternating phase waveform. Any comment from our electronics gurus.

https://www.bclstore.co.uk/product-page/lf100-dcc-bus-bar-line-filter

[Brew Man]

To protect the decoders in the future. The adverse affect of short circuit induced ringing, can be alleviated by fitting a 'snubber' across the track. The snubber 'filter' dampens these spike voltages to levels below those that cause decoder damage.

Snubbers can be easily made DIY. They are a 100 Ohm 1 Watt resistor in series with a 0.1uF Ceramic capacitor with a voltage of 50 volts or higher. These two series connected components are then wired across the track, preferably at the farthest point away from where the controller is connected.

Want more information on snubbers. Then open the 'Useful Links' pinned sticky thread at the top of the 'General Discussion' forum and follow the link for "Mark Gurries - Snubber/RC Filter".

I would suggest much more than 50V capacitors. I had 50V ones on mine only to find some time later, (don't how much later) they had both shorted leaving my 100 Ohm resistors cooking nicely. Since I changed the 50V capacitors to 1000V ones, (yes I know, that's somewhat overkill but they were the only other ones I had) the problem has gone away.

 

 

[Britannia Builder]

Have you tried a CV8 value 8 reset a couple of times.

I can't change cvs via the app because the power won't stay on, but I suppose I could try it on the programming track if you think that might still work in the face of the apparent short circuit? Is it worth reinstalling the decoder and trying this, do you think?

Interesting to hear about the transient spikes and snubbers.

Regards, John

 

 

[LTSR_NSE]

Since this is the HM7000 section, would these RC Filters be appropriate for those powering their layouts using a DC PSU?

(If so I imagine they’d be similarly useful for decoder protection?)

[P-Henny]

No, they would have absolutely no effect at all.

The snubber is in essence an AC tuned filter for the frequencies generated by ringing on a DCC powered track. The DCC nominal frequency is 7Khz.

Plus DC doesn't pass through a capacitor, thus DC would not interact with the snubber.

[96RAF]

Snubbers used on DCC power bus originate from use on power line transmissions which are AC albeit those snubbers and the applied voltages are a tad more industrial.

The theory is explained here…(

). There are follow on videos if you haven’t fallen asleep before the end of this one.

In our case sticking more DCC locos on track can act as a controlling impedance and damp out ringing.

[Britannia Builder]

Have you tried a CV8 value 8 reset a couple of times.

I can't change cvs via the app because the power won't stay on, but I suppose I could try it on the programming track if you think that might still work in the face of the apparent short circuit? Is it worth reinstalling the decoder and trying this, do you think?

I decided to try this, drove the DMU onto the programming track using the old decoder, reinstalled the TXS decoder and found that I could read and write CVs, so the decoder wasn't entirely blown up - I then set CV8=8 (twice, as recommended) and lo and behold it all works again!

I was really surprised because a short circuit seemed like a hardware rather than software fault - I wonder where all the current was going?

Anyway, many thanks for your help Rob, you're a genius.

Regards, John