Basic DC Controllers

[Graeme Dunbar]

..............It would be interesting though probably uneconomic to design a DC controller that could adjust its parameters on the fly through sensing the motor response.

I think the 'Fidelity' controllers from New Zealand that are no longer produced, did just that?
But I could be wrong.

Brian Meldon

That sound interesting. I was not aware of them. Does anyone know if there is information about them online please?

[Hobby]

Try asking on NGRM, Graeme, I think a couple of people have one and they may know.

[Graeme Dunbar]

Thanks Blair. Will do.

[B.Meldon]

It should also be noted that some of the Lima diesel chassis also have another more fundamental issue in that the brass gears on the back face of the wheels are a very slightly larger diameter than the tread face of the wheels. This will cause them to short out in the electrofrog points every time the teeth of the brass gearwheel touch the point blades. This is not a problem in dead frog points.

In general these Lima chassis with this type of motor are all quite poor chassis, personally I avoid using them myself and I would recommend others do the same. Also, and again in my opinion, it is are certainly not worth spending the time building a loco around one of these chassis as it will never run very well.

Brian Meldon

[Dave Cooper]

Hello Brian - I have run all 3 of my Lima diesels across electrofrog points (Peco) without any shorting problems so far...just lucky I guess.

Two of them were 'free' and the other a low-cost DMU. So, if they end up in the spares bin no great shakes. I won't spend a huge amount of time /money sorting them though - they're just for fun.

Incidentally, there are two very good Youtube videos on how to upgrade the Hornby R965 controller to handle more current (suitable for those low-efficiency motors -so they say !)

Note : there are two different mod's - one for the earlier English unit (essentially a transistor upgrade). The second (Chinese made) a bridge rectifier diode upgrade. Both add higher amperage power leads and proper heat sinks to comfortably handle up to 4 amps output.   

[Graeme Dunbar]

With a bit of digging I found some information on the 'Fidelity' controller.

The designer's name is John Brouwer from New Zealand (as mentioned and which helped trace him) and he wrote:

"I write this reply as a small hobby manufacturer of a controller which has
been available for over 5 years and which has been favourably reviewed
internationally.   That is not a commercial but just to present the context
in which I write.

The class of controller which the CC represents is the "Voltage Feedback"
type, or perhaps more practically, a "constant voltage" type.    The
controller attempts to keep the output voltage at a constant level
regardless of load (high impedance, low impedance, whatever).  This it does
via the diode from the output back to the base of the output Darlington
pair.    This type was described in various forms by Roger Amos in his 2
volumes about model railway electronics (UK).   The Britsh EM gauge society
also publishes a circuit."  

https://groups.google.com/forum/m/#!topic/rec.models.railroad/6wieV8zSxW8

The book he refers to is this one:
https://www.amazon.co.uk/Complete-Book-Model-Railway-Electronics/dp/185260591X/ref=sr_1_1?keywords=roger+amos&qid=1568578400&sr=8-1

It shows its age a bit, but is an excellent book.

[B.Meldon]

Interseting notes about the Fidelity controller.

I was present at a test sesion of various non DCC controllers on various 009/H0e/N gauge chassis a few years ago now.

Some worked very well with the older motors, some worked very well with the newer type motors.
A couple worked ok with both.

However the one that worked best with all types of motors was the Fidelity controller.

Brian Meldon

[Dave Cooper]

In the synopsis of the book that Graeme refers to it mentions op. amps but not mosfets...this may just be an 'age-related' thing, or, maybe because no-one uses brushless motors on their layouts (not sure about DCC) ?

BTW, does anyone know if the Fidelity controller is still available - even if as a DIY project it would be fun to have a go at building one if the merits are as claimed.
Dave

[Graeme Dunbar]

I think you are right Dave and it is a date of publication thing in that the Roger Amos' designs date from a time when MOSFETs were not very common and no doubt expensive. There is also the matter that most model railway motors draw little current at low voltage where "conventional" transistors (BJTs) are perfectly adequate.

It is a different matter in DCC where a command station/booster has to supply power to several or many trains and accessories and in the locomotive decoders where the heat generated by a linear BJT circuit could cause problems.  It makes a lot more sense then to use MOSFETs in the booster and decoders, both using PWM or similar switched outputs.

For a hand-held controller the heat generated by a linear controller can make a useful handwarmer on cold days.  :D

(MOSFETs are very good at switching on and off quickly and behave like a good approximation to an ideal switch with a very low "on" resistance and so dissipate little heat.)

If anyone is interested Roger's web page is here: http://www.rogeramos.co.uk/mre.htm

[Dave Cooper]

Thanks for the link to Roger's designs Graeme - very useful (especially to DIY'ers like myself)....no 'royalties' to worry about just a designer's credit which is fair enough.

[B.Meldon]

BTW, does anyone know if the Fidelity controller is still available - even if as a DIY project it would be fun to have a go at building one if the merits are as claimed.
Dave

As far as I know sadly no it is no longer available.
It was only ever available by ordering from New Zealand.

Brian Meldon

[Stefan Krebs]

As it all is oh so dry I'll add a picture. With a 2nd hand buy came a coreless that self disintegrated. If you wondered how the magic inside looks, here is the detail. Enjoy!!!
And no, don't try that at home: there is no way of putting it together again. I can't even get the anchor back into the shell!  :laugh:

And yes, that's one mm squares on the paper.


All the best
Waldschrat

[Stefan Krebs]

Next picture  ;D

[Stefan Krebs]

[MickT]

Very interesting but I cannot see what it has to do with the thread title: Basic DC Controllers.  ???