DCC power through a relay

[Mark_Hassall]

Happy new year - and now time for what may be a stupid question!

Is there any reason why the DCC track power to a block can't be used to drive a relay so when there is a loco in that block current is drawn and the relay is energised?  Granted the relay may need to be wired in parallel to the track feed so there is still a "clean" dcc signal to the track.

Bizarrely I can't seem to find anything online that says it can't be done, and if it works it would make my life so much easier - but something so simple just feels too good to be true!

[Chrissaf]

A relay coil is an inductor with a relatively large mH value. DCC is a high frequency about 7-16Khz digital square wave (pseudo AC). Inductors interact with AC waveforms. Putting inductors directly in the DCC signal path is probably not a good idea at all.

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This alternative circuit will do exactly what you want. It detects a current being drawn in a DCC block to switch a transistor to trigger a 555 IC. The output of the 555 can then be used to operate what ever you want including a relay. As there are less than 2 turns on the detector primary, the inductance created by the detection coil of wire is negligible.

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https://btcloud.bt.com/web/app/share/invite/5YpJikHA04

When the page above opens click the document file name in the top left corner to download.

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I should stress however that the PDF document that you can download on the link above was written by me as a theoretical exercise. I produced it as a possible future project that I might want to build later, it includes the RS part number for a critical key component that would have been difficult to source. I have not built this circuit. It is not even my design, it was designed by Rob Paisley for whom credit should be given. All I have done is put a text document description wrap around his circuit to explain how it works.

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I have included a web link to his web site below as there are literally 100's of railway related electronic circuits there.

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http://www.circuitous.ca/CircuitIndex.html

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PS - Some additional comments regarding your suggestion.

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Granted the relay may need to be wired in parallel to the track feed so there is still a "clean" dcc signal to the track.

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Even ignoring the inductor issue, putting the relay in parallel won't work. The DCC power will just operate the relay permanently and you won't be able to switch it off without removal of the DCC feed.

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so when there is a loco in that block current is drawn and the relay is energised?

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Again ignoring the inductor issue for a moment. So you put the relay in series to detect the current drawn into the block. A typical low voltage relay has an operating current of say 20mA. Let's be generous and assume a relay operating current of 50mA. That 50mA is ALL the current that the relay will pass through it even if the loco is providing a short circuit path across the track. The loco needs far more current than that. The loco will have a DCC voltage drop across it, leaving less voltage across the relay, this means that the relay passes even less current through it and may not even operate. Chances are that with the relay in the series DCC path, the loco won't have enough current to move at all.

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Bizarrely I can't seem to find anything online that says it can't be done,

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That's probably because nobody before has even considered it as a possibility. In other words others, like me, also think it is a non starter and therefore not worth publishing a statement that it can't be done.

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PS - Should have added that you can buy ready made DCC block current detectors.

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For example the BOD1-BR from Block Signalling

[Fishmanoz]

But Mark, the question wasn’t stupid, it’s just that the answer says it can’t work.

 

just adding a little to what Chris says - while the fundamental frequency of the square wave may be 16kHz, because it’s a square wave harmonics are also present.  So any circuit will also have to pass 32, 48, 64kHz etc.  As an inductor is an impedance that increases with frequency, putting a relay coil in series with the DCC circuit will effectively kill the DCC power and the DCC signal. 

[Mark_Hassall]

Many thanks for your replies.  I knew it was going to be too good to be true!