Strange Idea?

[Wobblinwheel]

I'm looking at my little N gauge M7 loco, which runs "ok" on my DC layout, but it's speed control leaves a lot to be desired. The motor voltage required is so low that it seems t "surge" a bit at slow speeds, partly because of random slipping, and partly (I think) the throttle is barely past "0" before the loco begins to move. Very difficult to control at very slow speeds. In other words, it needs a decoder. From what I've read so far, this loco may not have room for a decent decoder, and is not "DCC ready". I've been struck by a "brilliant?" idea, and here it is: Since all I need from a decoder in this case is speed/motor control, why cant't I make up a "remote decoder", with a specified address for the M7, which is mounted near the track, with it's track-power leads attached to the DCC controller, but NOT to the track. Only the MOTOR CONTROL leads attached to the track? Couldn't I then have programmable momentum, start voltage, and BEMF controlling my M7? Kinda like "DCC controlled DC". Couldn't the motor control leads from a decoder, attached to the track, run the loco just as if it's mounted in the loco itself?? As long as the DCC system is supplying power only to the decoder, and not the track, with only the motor leads going to the track!Wouldn't I have DCC-like motor-control characteristics on my normally DC layout? Am I NUTS? Don't answer that.....Could it be the track would reduce the current going to the loco too much to work? Could an apperatus be devised that you could simply attach your DCC wires to, that contains it's own motor-control decoder, that then connects to your track, to run your DC locos? Go ahead, tell me I'm stupid....

[96RAF]

@wobbly

Wasnt  there a similar method of applying an adjustable controlled voltage to the track to control one loco called DC.  😛

 

If you have an old decoder just give it a try on a small circle of track and see if it will work.

 

Could be ideal for a xmas tree base layout with a single loco. Worse case scenario you will have one less decoder cluttering your spares box. Best case scenario you may be onto something.

[Fozzy Bear]

I started a thread a while ago about getting DC from a DCC controller and one of the suggestions was the very same idea. It would work fine but whether the slow speed running would be any better , I don't know.

The best solution would be to hook up a 12V battery to the track and have a 1 amp fuse and a 470Kohm 10W pot in series, in the one of the leads. You would get the smoothest DC available and have very fine voltage control.

Of course, it might just be your loco doesn't run well at low volts....

[choralc]

https://www.hornby.com/uk-en/forum/dc-from-an-elink/?p=3/#post-158445

[dadad]

Hi there,

I think that you may be onto something, the output from the chip for the motor is pulse width modulation not variable voltage, so in the dead time (spaces) you can have back emf feedback which will give you a steady speed.  You will, of cource, have the usual problems with dc control, keeping the track clean etc.

 

John 

[Wobblinwheel]

I have an R8249 decoder that I might experiment with. If I blow it up, I ain't lost much. My two DC controllers, as well as the DCC controller I have, attach to track via RCA "phono" plugs. I figure I will come up with an enclosure to house the decoder with a (female) phono jack on one end, with a 2-wire lead with a male phono plug on the other. My DCC controller would plug to the "track power" leads of the decoder, and the output to the track will be the "motor control" wires. The 8249 decoder usually requires quite a few throttle steps to get a loco moving (no start-voltage adjustment), which would be perfect for the "snappy" little Dapol. The constant BEMF should smooth out the speed regulation, and the momentum should help a lot too. The 8249 decoder would not be my ideal choice, if this idea works, as I would like to be able to adjust start, mid, and max voltage, as well as the coasting "brake" function of the Digitrax decoders, but this one will demonstrate if my idea even works or not. My next experiment will be (if this works) will be to "split" the output from the DCC controller to a second decoder (set at a different address) to run the second track on the layout. I have multiple isolated "blocks" on the layout so I can control two locos from the same handheld. This might work since the two tracks (and trains) would be electrically isolated from one another, while being idividually controlled from the same power source.This idea is gonna make me RICH one day.......

[SteveM6]

WW, a little while ago, I tried to control something (can't remember what now) by using a devoted in much the same way - it didn't work. But to be fair that may have just been me.

However, the N gauge M7 can be converted to DCC using the Digitrax Dz126 decoder - I have two which I converted this way. From memory, there is enough room for this decoder in the coal bunker.

[96RAF]

Here's a how-to Wobbly

Dapol M7 Conversion

[Wobblinwheel]

I still gotta try this, as I don't want to convert my little N gauge layout to DCC. Too many DC locos running around. I still have it in my head that the motor outputs from a decoder are still pretty much DC, and will run any loco I have, with much better control. I may be wrong, but I need to find out...

[poliss]

There is no space problem at all in fitting a decoder into the M7, provided you use the right decoder. A Z scale one would be fine. Once the correct decoder is fitted it will work fine on DC too.

It looks like there are quite a lot of TV suppression components in the M7, so using a decoder connected to the track may not work unless they are removed.

[Wobblinwheel]

There is no space problem at all in fitting a decoder into the M7, provided you use the right decoder. A Z scale one would be fine. Once the correct decoder is fitted it will work fine on DC too.

It looks like there are quite a lot of TV suppression components in the M7, so using a decoder connected to the track may not work unless they are removed.

You got point there. A lot of junk in there. Then again, I've seen decoder-equipped locos with that same "junk"!

[poliss]

Yes, but are they working as they should?

[StephenD]

I still gotta try this, as I don't want to convert my little N gauge layout to DCC. Too many DC locos running around. I still have it in my head that the motor outputs from a decoder are still pretty much DC, and will run any loco I have, with much better control. I may be wrong, but I need to find out...

Am I mistaken or was it not confirmed that this would work by the thread which Choralc pointed us to in his reply above?  It was even tried out by Choralc.  I assume it means that the output from the decoder is DC which would mean that you don't even have to modify the loco in any way.

Stephen

 

[poliss]

It's not straight DC Stephen. It's around 12 volts pulsed DC. The decoder output is the full 12v, but pulsed, like flicking a light awitch on and off very fast, to control the loco speed. TV supression components can interfere with the pulses.

[StephenD]

It's not straight DC Stephen. It's around 12 volts pulsed DC. The decoder output is the full 12v, but pulsed, like flicking a light awitch on and off very fast, to control the loco speed. TV supression components can interfere with the pulses.

Sorry for showing my ignorance and thank you Poliss for the clarification.  I don't pretend to understand it but I accept what you've said.  Funny though that it wasn't mentioned on the previous thread or maybe I missed it.

 

Stephen

[Wobblinwheel]

Well, I have built my little "box" using RCA jacks for connections, and even mounted an 8-pin DCC decoder socket inside. I will be adding a second socket to control a second track on the layout. I have to say, even with the lousy 8249 decoder, this thing works like a CHARM! I have not seen my old N gauge run so well! The slow-speed "creep" is amazing, having momentum is great, and even the headlights work! I can see I will probably switch-up to two much better decoders, and probably go with "keep-alive" since size is not an issue! I am extremely happy with the results! I've got some really good-running locos, that I never knew ran so good.....picture to come.

[Wobblinwheel]

Here it is./media/tinymce_upload/2a3bbcd796a6b681e86b01120d3a2e14.jpg/media/tinymce_upload/7592cfa0942e4ad7b6fc711511b5edd3.jpg

[Wobblinwheel]

Oh, and the little Dapol M7 runs almost perfectly. It creeps along flawlessly and smoothly, except for the slight "buzzing" from the not-so-good decoder. Of course, at higher speeds, this is gone. For some reason, it even pulls more stuff...it's now pulling three coaches AND three wagons AND a brake van!! Slips occasionally in points, but still goes through after a slight "delay" sometimes.....I am impressed....

[96RAF]

@wobbly

surely for stay alive to work it needs to be in the loco as that is where the loss of power occurs when crossing points, not at the decoder/rails.

 

interesting that it works so well - good job.

[Wobblinwheel]

@wobbly

surely for stay alive to work it needs to be in the loco as that is where the loss of power occurs when crossing points, not at the decoder/rails.

 

interesting that it works so well - good job.

I sure am glad you brought that to my attention!!! I would've felt like a complete nit-wit had I spent the extra money for stay alive. Not many "breaks in current" are gonna happen with the power supply wired directly to the decoder!!! Man I feel stupid, but grateful for you, my good man! By the way, what kind of "issues" should I be experiencing because of the capacitors and rf chokes on the M7's motor? It seems to be running fine, at least compared to the way it was. Would it improve if I cut that stuff out? I have found that two of my oldest locos don't work that great with the 8249's "pulsing" at very slow speeds. The cheap Digitrax decoders I like to use have something they call "supersonic" speed control (or something like that), that makes the "pulsing" effect almost unnoticeable. You don't see it much in the headlamps, either. I'll be ordering two of those soon, as I think they will correct the issues with the older engines. They also have a "shunting mode" that comes in handy when you have gobs of momentum and need to slow down....quick. Looks like I might be doing away with DC controllers all together......thanks so much for the input!!

[96RAF]

You could still put Stay Alives  in the loco, wired into the motor circuit, but you would need some diode arrangement in there to prevent those big caps feeding back and charging up your whole set of rails, so I guess it's not worth the effort.

 

Those inductors and capacitors are there to knock out any interference the motor may generate by brushes arcing, poor contact with rails, etc, to allow the manuf to gain a certificate of compliance with the 'electrical noise' regs, but as a decoder has this built in and they are generally considered to possibly be problematic for DCC most  people recommend their removal, but if they are not affecting the loco running or seem to be having no effect on the DCC signal then leave well alone.

 

It will be interesting to see how you bigger and better decoders perform driving 'remote' locos.

[Fozzy Bear]

Arggh, just typed a nice explanation of capacitors etc and when it came time to post "you are not logged in". This forum needs some serious work.

Basically, don't remove anything from the loco, the caps are there to filter the DC and if you want to add large caps to help across points, go for it. You won't need diodes and it will work fine.

Good to hear that the engine is running well.