Model Railway Forum

Hi All,
Just a general query as I'm currently still in the planning stages of my layout, but am thinking of control options. I intend to run primarily recent RTR locos on DCC, and will have the layout wired for DCC. I do have some older locos which I don't want to convert to DCC and intend to include an isolated loop of track which will be DC controlled for these locos to run on. I had initially intended to buy a separate DC controller for this, but wondered about the possibility of wiring in a loco decoder such that the input is fed direct from my DCC bus, and the motor outputs wired to this isolated loop of track - I think this should then allow the DC locos to run.

So would this work? And if so, are there any particular decoders which might be good for this? Size isn't an issue obviously, and functions such as sound/lights would not be needed, but I'm thinking one with a large current drive would be a good choice as my elderly loco collection includes die-cast Wren/Tri-ang models which I know are power hungry.

I'm aware of the possibility of running one DC loco as 'loco 0' on DCC track, but I'm not keen on this option, especially for the older models which have some sentimental value, and I'd hate to risk burning the motors out (having tried it on a friend's DCC layout, the buzz from some old locos is worrying to say the least, even standing still!).
Thanks for any advice.

You can indeed wire a loco decoders output - Orange and Grey wires to the rails and the Red and Black wires to the DCC systems 'Track' output (or the rails feed by the DCC system). Leave it at address 03 and power one DC loco on the loop. Note you can only power one DC loco and all others must be removed from the rails. I would use a Lenz Standard + or a Zimo decoder if possible but the final choice is yours. On no account must a DC feed be applied to these rails with the decoder connected. DC applied to the decoders Orange and Grey wires will kill the decoder immediately!

Note: not all DCC systems can operate a DC loco on address number 0 (zero). In fact many systems do not offer this option! However, even if its is available, this is not a recommended practice and is not suitable for long term use.

On no account should a DCC system ever be allowed to connect to a DC system. So no cross-over points connecting between two loops etc where a locos wheels can connect the two. If they are allowed to connect one or both system will become terminally damaged!

Thanks Brian for the reply, and in particular for the warnings about accidental interconnection of the various systems - I shall be careful! I've ordered a Lenz Standard+ to try it.

As an aside, what type of DC control do these decoders use to control the loco motor? As it is generated electronically from the AC waveform of the DCC track voltage, I guess it is most likely switched/pulsed PWM rather than a pure DC. But is there any motor feedback? And how do people find the ultra-low speed control compared to a pure DC controlled conventional loco?

Reason for asking is that when I tried a couple of my (DC) locos on a friend's DCC layout using the 'loco 0' feature, I found the low speed control, and in particular the smooth starting from stationary, to be unbelievably smooth compared to the pure DC which I've used to test them on a little oval at home. I guess it is probably because the peak voltage of the DCC track feed is considerably higher than 12V, and this is constantly overcoming the resistance within the motors (ie. that between brushes and commutator). I'd assume the decoder DC outputs won't spike the motor quite as high as the track supply itself, but I suspect PWM and/or feedback control will be better than pure DC for low speed. Am I right here?

Hi
Decoders use PWM at 20KHz and often higher. They also use Bemf to 'read' what the motor is doing and try to maintain the motors speed to whatever is set on the console (to within limits of course!) Much like Cruise Control does on a car.
PWM is probably better in some ways for fine motor control, as it applies full volts to the motor in a stream of pulses. Have a look here for the basic idea... link to item Note; a very small number of motors do not like PWM and it can cause them to overheat, but most motors are fine with PWM. Some DC controllers use it too.
Conventual DC controllers applies 0v to a nominal 12 volts DC to the motor for speed regulation, so to start a motor turning on DC you will need around 4 volts ish applied before the motor can turn, then it has to overcome 'Stiction' - gearing and friction etc, where as PWM applies the full nominal 12 volts in rapid On/Off pulses and this allows the motor to turn easier! More time spaces of Off then On gives the slow running but its always receiving the full volts available - 12 volts or just above this.

Just to update to say I have tried this (borrowing a controller to test) and it did indeed work as intended. Thanks again Brian for the pointers.