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Hi all,
I seem to remember seeing a story on "Tommorrows World" many moons ago about a device that gave you a Pullsed DC output so that a smoother running could be had.

Does anyone know of such a thing and is it still around.?
tx.
 

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QUOTE (Paul-H @ 17 Jun 2009, 17:42) <{POST_SNAPBACK}>Many moons ago Pulsed width modulation was supposed to be the next big thing in model rail control, don't know if it ever did though.

Ok so you have to go down the DCC route for this answer but....

"Virtually all DCC decoders use a form of PWM (Pulse-Width-Modulation) to control motor speed."

See it here on this very site...
 

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Alan D
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Old H&M Controllers have a half wave thingy which is very good for slow control. Also gaugemaster feedback controllers use pulse modulation to improve slow running these also provide increased voltage when the engine comes under more stress such as sharp bends or inclines and tries to maintain a constant speed throughout.

Cheers

Alan
 

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****There are three things being mentioned here - all different. It can be complex so I will try to keep this simple.

1/ Half wave rectification as per the old H&M - basically it only rectified half of the AC waveform, which gives a "goose"to the engine at the same AC frequency as the mains power.... a very simple form/method of coaxing a motor into keeping turning at lower speeds. Not at all suitable for modern can motors, but OK on the heavier open frame/tender drives of yore. A bit noisy sometimes.

2/ PWM or pulse width modulation - this applies full voltage at all times for a greater or lesser period of time - the longer the pulse, the faster the loco goes. In its primitive form, it is sort of a "chopper"circuit at mains AC frequencies so it can also be rough on motors and noisy too

- anyone who has used the cheapo Lenz decoder will have heard it and its B annoying. In a more sophisticated controller the pulse frequency is higher and so becomes almost totally silent. It can be used with DC or DCC controllers. In a quality DCC chip, the PWM pulse is usually well above 10k & the frequency goes up to 40k times per second in some decoders, so it is usually silky smooth.

3/ Back EMF - the controller is designed to listen for and respond to increased load at the motor, and modifies the power delivery to keep the motor speed close to where it is set, adding more and reducing power as needed.... It reads the momentary back EMF from the DC motor each time the brush passes from one pole to another - thats many many times per second... and obviously more on a five pole motor than a 3....

In the real world this method and its derivatives was used to control motors for constant speed on turntables and cassette decks, and now in a more sophisticated way in the high speed disc drives - In the hobby its applicable to DC or DCC equally as wel and will indeed if set to do so, create a form of "cruise control", but its most useful thing for us is its ability to help slow running qualities.

Its important to know that all feedback/back EMF and PWM or pulse power controllers aren't the same - and you will probably "get what you pay for" in these areas - Half wave is a doddle to do so adds only a switch to costs really.... But a proper back EMF control system or PWM control system (or a combination) uses complex software in a processor, so even for DC applications, it'll never be a 'lower priced" product... especially if it allows ädjustments" which it should (as in DCC) to get the best from it.

Richard
 

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QUOTE (Richard Johnson @ 18 Jun 2009, 04:20) <{POST_SNAPBACK}>****There are three things being mentioned here - all different. It can be complex so I will try to keep this simple.

- anyone who has used the cheapo Lenz decoder will have heard it and its B annoying. In a more sophisticated controller the pulse frequency is higher and so becomes almost totally silent. It can be used with DC or DCC controllers. In a quality DCC chip, the PWM pulse is usually well above 10k & the frequency goes up to 40k times per second in some decoders, so it is usually silky smooth.

Well explained Richard - I've still got a few of the Lenz 1024/5 decoders from my early DCC days & they are noisy, especially at low speeds with the FLM 3-pole motors.
 

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Hi
Just to add and may be of interest, I contacted Hornby several months ago to determine what sort of output all their current range of controllers supplied..
Their reply I have copied and pasted below....
Which was rather surprising to see as even their basic controller has a PWM output!
Hello Brian,
R965.. is a basic PWM type controller. It does not incorporate any load compensation control system. i.e. what is sometimes termed "feedback."
HM2000.. is a PWM controller which does incorporate "feedback."
Hope this clarifies..
Kenn
HH DCC Support
 

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Maplin produced a PWM kit some years ago called Control-A-Train and was suitable for relative beginners to electronics.
The design was by Robert Penfold

I never built it for various reasons - time and house moves etc. and now I am using DCC it is going spare.
It is complete with instructions and printed front panel. It only needs a suitable box and a 15v AC transformer.

If anybody would like it for the cost of the postage send me a PM.

Chris
 

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Alan D
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Excellent Richard, That explains it so well. I knew about feedback controllers a long time ago but never really understood them.

It has been hard to pry the information from people at shows or shops. Either they, like me, knew just that they worked, or like magician's, they were keeping all the secrets to themselves.

I never understand why people who are exhibiting are reluctant to explain their stuff, maybe they are just shy. But that's another topic.

Not meaning to hi-jack this thread but thanks for sharing that info, it has helped me understand a great deal.

Cheers

Alan
 

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Back in the late fifties, early sixties there was a train controller marketed as the "rollacontrola' which used a mechanical drive to feed pulses of 12 volt power to your model railway. I saw it demonstrated at Westminster Hall at the then Easter Model Railway Exhibition. As it was a school trip, which included a visit to the Science Museum and the Natural History Museum, we ended up at the MRE late in the afternoon, but we saw Frank Dyers 'Borchester Town' and other layouts but it was the trade stands which held my fascination. Never before had I seen such a cornucopia of Model kits, bits and handbuilt loco's and coaches on sale, I spent a long time athe EAMES stand and K&C Keyser (K's) and spoke at length with Bob Wills of Wills Finecast (now South Eastern Finecast). But it was the trade stands like H&M, Welkut, and Kings Cross Models where you could the electrical bits and pieces, and a demonstration of the 'Rollacontrolla' (I'm not sure of the actual spelling but I seem to recall it was a part of the Rolls Razor Group - someone correct me if I am wrong)

After seeing the RC demonstrated, our science teacher, the legendary Tom Marshall of The Grove School St. Leonards, set out to design and build a replica of the controller we had seen, I know it had a synchronous motor out of a fan, with the autual moving parts machined from copper rod which was held together with the then new "Araldite'. Several prototypes were produced as projects by the School Radio Club and were tried with some success on a hastily constructed School Model Railway. the unit was powered from the science laboratory's low voltage ring main which was on every bench and could be set by us lab stewards to any combination of voltage and amperage required for experiments - we had long since passed on from dry cells at that time.

The construction of the RC meant that the actual handle was' live' to the operator which was OK for 12 volts but someone had tweaked the power up and when we came to run the railway at lunchtime, the first operator got a very big shock and the Triang Jinty we were using had terminal melt down in the motor, this resulted in all further experiments being banned unless they were teacher supervised. Anyway at that time we had progressed on to the building a prototype radar unit which used magnetrons and was in a whole different league from model railways.

I'm sure if you googled Rollacontroller you might get a hit or several.

Cheers
 

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QUOTE (mikemasey @ 26 Apr 2007, 00:11) <{POST_SNAPBACK}>Hi all,
I seem to remember seeing a story on "Tommorrows World" many moons ago about a device that gave you a Pullsed DC output so that a smoother running could be had.

Does anyone know of such a thing and is it still around.?
tx.

How many moons ago?? In the late '70's I was playing around with home made thyristor controllers.
 

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QUOTE In the late '70's I was playing around with home made thyristor controllers.

Didn't RM have an article on building a thyristor controller? I know I built one and I think it was from there that I got the circuit.

A few years later I built a controller which used a 555 timer to switch a voltage regulator on and off. I never quite got the current handling capacity right before I drifted away from modelling for about 20 years.....

David
 

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The Rollacontroller (not sure of the spelling), was made by Rollason in Letchworth, in Hertfordshire. I think it would have been late fifties or very early sixties. The working principle was rather crude and it was reputed to burn out the motors in locomotives. He made a number of models too - I remember signals and street lights - that were hand made in brass. I don't know what happened to the company, but when he gave up the model railway products he gave a lot of them, including a small demonstration layout (without the controller) to my schoolfriend, whose father was a friend of Rollason's, and I still occasionally find fragments of them in the scrap box.
 

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QUOTE (andrew @ 11 Jul 2009, 17:22) <{POST_SNAPBACK}>whose father was a friend of Rollason's, and I still occasionally find fragments of them in the scrap box.

Would that have been Len Rollason who latterly worked for H & M just before Hornby bought them out ?
 

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Brian - Just received the following from the aforementioned schoolfriend:

Thanks for dredging up a piece of my past that I had completely forgotten. I was about to deny all knowledge, when it slowly came back to me. As I read, I feared you were going to ask me if I had kept the bits and pieces because you wanted something. My policy of keeping seemingly useless documents has paid off, as he is in my parents' old address book, but his name was Bill, initials WA, so not the man you hoped.

I think that's a "no"!

andrew
 

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I have a Gaugemaster feedback controller series DF, i find it makes some of my Hornby loco drive motors noisey, not so much
a problem with Bachmann locos, any one else had this problem.
 

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With PWM controllers (that includes feedback controllers) you need to be able to tune the PWM frequency to match the motor for the best performance/minimum noise. If there is no PWM frequency adjustment available your controller will have been set to a compromise setting that will work better with some motors than others.

If the frequency is too high you will lose torque and possibly overheat the motor, and if too low you will get noise, so the compromise tends to be on the noisy side.
 
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