Two locos, one decoder

[propmeup1]

I feel I know the answer but still have to ask. Is it possible to run two N scale locos like an A&B F units off one decoder or is that too much draw from two motors ?

Thank you,

[Tad_T]

This is the way one fellow did it:

https://jamestrainparts.com/2016/01/25/converting-an-n-scale-bachmann-f7-to-dcc/

[Jimbo]

Not sure if it’s possible, but even if it is, I wouldn’t recommend it.

Assuming the decoder was properly rated for the peak current draw of both motors, it could be a problem with the back EMF detection that all modern decoders have.

But even if this is possible, doing so would cause you to lose many of the benefits of DCC.  For example, the motors in each unit will likely have enough manufacturing variation to run at slightly different speeds.  Using a separate decoder for each motor would allow for speed matching.  Another benefit is that the two units have the capability to run independently… no wires needed between the two.  Finally, since most modern locomotive models are designed for plug-n-play decoder installation, using a single decode for two units would be a more challenging installation.

Most good non-sound decoders are inexpensive anyway.  To me, it’s not worth the downside to use one decoder for two units, even if it’s possible.


Just my opinion.  I welcome any other thoughts.


(And I guess I didn’t answer your question.  I don’t know the answer.)

Jim

[Jimbo]

This is the way one fellow did it:

https://jamestrainparts.com/2016/01/25/converting-an-n-scale-bachmann-f7-to-dcc/

I guess we have an answer!

[Rivet Miscounter]

A question I've had as well.    My use-case is for Z-scale GP7 locomotives.   The idea was two-fold....add weight and also allow for electrical pickup to be spread out across 8-axles.  The little AZL GP7's are great little runners and I think have decent pulling abilities.   I'm planning to chop the noses though which will come at the expense of some weight being carved out.   So that weight offset is really what my concern was/is.

The comments about issues running two motors from one decoder are not something I'd thought about.   That causes pause about ditching the second decoder to try to get additional weight.

Then my thought process goes to...I could still pair the pickups and feed both decoders.   So that's still a consideration.

Another development is some super-tiny decoders that also have an option for keepalive.   So either space for more weight, or space for the keepalive to help with the pickup issue so I wouldn't have to pair the locos.

And obviously, any of the above would really apply as discussion points and options for N as well.

[peteski]

I was not really on board with this concept (on a theoretical basis), but there are many Kato N scale Big Boys and H0 scale P42 diesel locos running out there in the wild with a single decoder.

Both locos have dual motors (in both cases Kato coreless motors) with no apparent issues.  I mentioned coreless since those only need fraction of a current required by a standard DC permanent-magnet motor. But if the current requirement of the decoder is not exceeded, there don't seem to be any  problems.

[nkalanaga]

If you want to run them as one loco, but don't want the extra wires between them, you can always program both decoders to the same address.  They could be speed-matched the same as any pair of units, but would respond like a single unit.

[peteski]

If you want to run them as one loco, but don't want the extra wires between them, you can always program both decoders to the same address.  They could be speed-matched the same as any pair of units, but would respond like a single unit.

That is the most basic form of DCC consisting. 

[Rivet Miscounter]

That is the most basic form of DCC consisting. 

Agreed, that is s.o.p. and not one of the problems needing solved here.

[peteski]

Agreed, that is s.o.p. and not one of the problems needing solved here.

I don't believe there is a problem here. Like Nike used to say: Just do it!
With the relatively low current requirement of N scale motors is not going to blow up anything, and if the lashup runs crappy - just undo it. 

[Rivet Miscounter]

I don't believe there is a problem here. Like Nike used to say: Just do it!
With the relatively low current requirement of N scale motors is not going to blow up anything, and if the lashup runs crappy - just undo it. 

Hey, overcomplicating things is a national pastime for model railroaders!!!   

Problem was probably not the right word.   Maybe...consideration is a better one.

Nice to talk through things sometimes though.   I'll probably start with the path of least resistance and see where it goes.   A couple of basic Digitrax decoders should give me a good idea what the capabilities are for the two locos  as independent entities.   If anything, I think as far as I would take it is tying the pickups together.

[nkalanaga]

That would be where I start.  Simply wiring the track pickups together will give much better contact. 

I did that 50 years ago with Minitrix F-units, and my current primary operating locos are a pair of original Kato GP38-2s, wire together.  I also jumpered a Kato EH-10 electric, which, oddly, is a two unit model, with one motor, and a driveshaft to the second unit.  Both units pick up power, but they are NOT connected electrically.  All the driven unit does is power its own headlight.  Now the thing has 16-wheel pickup, and runs much better.

[$2 Bill]

Just my opinion.  I welcome any other thoughts...

Let us ignore motor manufacturing issues and say motors spin @ same RPM... Decoders might still "hate" 2 motors.

Why?

Most or all DCC Decoders uses H Bridges to run the motor(s). It's this and maybe some Caps to "ground" & across the motor terms that likely to have issues here.

If the motor(s) don't draw power much, the H Bridge may still heat the Transistors/MOSFET or All in one "Chip" and because very tight on space and little to no heatsink those parts can Fail. Now add second motor, Heats faster and hotter and fails sooner. Worse, you often see Examples like above link but never tell you if/when it fails later or didn't tell you it's rarely use the result and might didn't fail yet just for that reason.

Back EMF can be a problem too using 2 motors on same HB. Dual Motors might work but may need snubbers etc added to 1 or both motors. Worse Spikes come when the power to the motor(s) shuts off fast by command, derail or just runs into bad section of track. Staying with same link, maybe he's using 1/2 power or less to the motors so limit the Back EMF and prevent problems for now. Back EMF can be Many Times the Volts to run the motor(s), relays, and other Coils.

I won't trust Decoder Makers for Amp Draw and other specs because you don't know how those specs was generated.
If just claim HB parts say 2 Amps Max in the Data Sheet but Data Sheet assumes you have proper Heatsink with Air moving. Even if they have some type of Heatsink, Decoder install in a Engine Body often has little or no air moving on the sink. (Heatsinks might be an extra part or "built into" the PC by how ground plane etc are designed.)
Related Example: Aluminum and some other Power Resistor Watt Rating Requires a Heatsink but many users don't know/care and install without it then fries the resistor to cause a Fire because without the heatsink is now rated 1/4 to 1/3 of the Watts printed on them and easy get Red Hot.

For Cap issues and more on Back EMF on Decoders... A few highlights, see https://www.therailwire.net/forum/index.php?topic=60043.msg827491#msg827491

[peteski]

With all those warnings, first about the DC power packs and now about damaging DCC decoders, we should not even be in the model RR hobby.  Scary stuff! 

N scale motors, especially coreless motors, require very little operating current.  Since the H-Bridge transistors are used as switches in PWM mode (fully on or fully off), their internal "on" resistance is in milli-Ohms, so they do not dissipate a lot of power (read: generate heat). 

Seriously, even model manufacturers don't see a problem powering dual motors using a single decoder.  But thanks for the warnings - I guess I'll take my chances.

[Sumner]

.....Seriously, even model manufacturers don't see a problem powering dual motors using a single decoder.

Agree, the Bachmann 2nd and 3rd series DDA40X's run fine with 2 motors and I've converted the 2nd from DC to DCC and the 3rd series is DCC.  I used a 3rd series to kitbash a DDA35.  Not done yet but running.....

.... fine so far with an ESU 58923 LokSound decoder.

Sumner

P.S.  The 2nd series runs fine................if the gears aren't broken.  I designed some 3D printed gears and...



....you can find more info and print file links here....

https://1fatgmc.com/RailRoad/Locos-1/page-9.html