mechanism telemetry (especially current) with DCC?

[nickelplate759]

The standard way of diagnosing recalcitrant mechanisms is to remove any decoder and run it on DC while monitoring voltage and current.
A thread about problems with an N scale 2-6-0 got me thinking: is there any way to get data on current draw of a mechanism without removing a decoder?   

I suppose you could insert an ammeter in grey or orange motor leads, but in most of my engines that would be difficult at best, and it would be pretty much impossible to measure the current as the engine was running on track.   I know decoders can send some data back to a controller; does DCC allow for any telemetry of this sort, and if so has anyone implemented it?

[peteski]

The standard way of diagnosing recalcitrant mechanisms is to remove any decoder and run it on DC while monitoring voltage and current.
A thread about problems with an N scale 2-6-0 got me thinking: is there any way to get data on current draw of a mechanism without removing a decoder?   

I suppose you could insert an ammeter in grey or orange motor leads, but in most of my engines that would be difficult at best, and it would be pretty much impossible to measure the current as the engine was running on track.   I know decoders can send some data back to a controller; does DCC allow for any telemetry of this sort, and if so has anyone implemented it?

Only few select decoders (none of the American brands) send info back to the command station using RailCom protocol. Standard DCC decoders only receive packets from the command station.  The only feedback DCC decoder can send back is on the programming track and it is simply a current pulse sent as an acknowledgement that a programming instruction was received.  Nothing on main track.

I think a compromise would be to use an ammeter which can deal with the DCC signal, shut off all the sounds and functions, then run the model observing current draw.

[nickelplate759]

Isn't there some risk that the ammeter will degrade the DCC signal?

[peteski]

Isn't there some risk that the ammeter will degrade the DCC signal?

It shouldn't - ammeter just measures voltage drop across a shunt (basically low ohm resistor), but the reading will not be very precise (since the typical multimeter has a circuitry designed to measure sine-wave AC signal, not the funky DCC square-wave.  But for purpose of looking of current changes (not absolute values), it should do ok.

[Iain]

Oscilloscope

[peteski]

Oscilloscope

Ok, but what will it show?  George is looking to monitor current - oscilloscope shows voltage.  I suppose you could hook up the probe across a shunt in series with the DCC output and the track.  But then you have to pay attention to the ever-changing DCC signal (it is not a static waveform).

IMO, the easiest way to observe current changes would be on an old ammeter with analog movement, where the pointer clearly indicates the current variations.

[lyled1117]

Or use a true RMS multimeter. It's not expecting a sinusoidal signal, performs the math to determine the actual voltage/current. They're not expensive these days. A few decades ago mine cost $300+, you can get an equivalent these days for $30-$40

Lyle D

[freedj]

Could you throw a dc ammeter between the layout 12v power supply and your dcc controller assuming you only have one loco on the layout?  The controller would obviously add some base load but it might be a really easy place to measure.