My turn to ask for advice

Controllers ...
I'm currently using a couple of old H&M controllers with an even older Triang one on "standby duty" for a yard area.
I would like to replace them with a modern PWM controller - with (as I have a few tight bends) feedback. I am considering building my own - I do have prior experiance of electronics and have some good curcuit diagrams but having PCBs made up is making me think twice ...
I've been looking at the various adverts for Guagemaster, KPC etc and they are all quite loud on the subject of whether they provide feedback, inertia and all that but none of the adverts state if the controllers are PWM or not.
Any guidence folks?
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All the best,

Chris Wilson
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"Chris Wilson" wrote

Never heard the expression, what precisely is PWM.
John.
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PWM = Pulse Width Modulation
i.e the amplitude stays the same but the duration of the 'on' period is varied. This is how DCC decoders control motors, isn't it?
ROB
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Robert Flint wrote:

Yes.
Something similar is required for DC feedback controllers since they can only measure the BEMF from the motor (the feedback) when the track power is turned off.
MBQ
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On Tue, 14 Mar 2006 16:04:58 -0000, "John Turner"

It's not strictly the width of the pulse but the time interval between full power spikes.
The idea is that the rotor's coils get a kick as they go past the pole pieces so the motor speed synchronises with the pulse interval.
It's not used very often these days because it can cause modern motors to overheat - in traditional motors the rotor has an iron core that acts as a heat sink.

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wrote:

So (silly question time) what makes modern controllers so good compared to the older ones, IOW how do they achieve their performance? I was under the impression that PWM was the dogs 'do-dahs' - subject of course to the problems with coreles motors.
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Chris Wilson
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On 14 Mar 2006 18:53:12 GMT, Chris Wilson

Two kinds:
Voltage regulator, which delivers max current even at the lowest voltage - with an old fashioned rheostat, the resistor draws the current.
Feedback, basically the same but it also turns the voltage off momentarily, and the motor becomes a generator for a tiny instant. It monitors this back EMF and adjusts things accordingly to maintain the same speed. Useful where there are gradients. Early ones weren't suitable for modern coreless motors but most are these days.
Some of the better older controllers were actually pretty good. Variable Transformers could deliver current at all voltages (H&M Safety Minor and the later generation of PowerMaster). Transistor controllers (H&M Electran?) were also good.
Somebody (Buehler?) still makes variable transformer controllers.
At my club (in Kingston, New York - I'm an ex-pat O-gauger from Manchester) we still use a very old setup with variable transformers.
Feedback controllers would be nice because our long, heavy trains need real driving on the gradients. You can't leave them running on their own while you're distracted. They also pull a huge current - fellow members wouldn't believe how little current my 517 and 6-wheel carriages drew.
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On Tue, 14 Mar 2006 14:30:05 -0500, Christopher A. Lee
look Chris I hate to do this - please don't think I am picking on you but this needs correcting:

The motor coils have a resistance measured in ohms. Ohms law states that I (amps, current) = Volts/Resistance. You can't just push current through something. Think of it as water in a pipe. the current is the amount of water flowing, the volts is how hard the pump is pushing and the resistance is the diameter of the pipe. If you turn the pump right down, the current must drop
If the motor has a resistance of 20R and you chuck 12 Volts at it, the current will be12/20 = 0.6A or 600mA
if you wind the voltage down to say 2V, the resistance hasn't changed so the current must... it will now be 2/20 = 0.1
The resistance you measure with your multimeter is equivalent to the off-load resistance - when the thing is under load, the *effective* resistance will change - more on that later.

not exactly. Back EMF is a high voltage spike generated when the magnetic field around a coil (motor) collapses and generates a voltage & current. Motor theory says if you pass a current through a wire suspended in a magnetic field, you get a kinetic motion attempting to push the wire against the magnetic field - this motion is captured in motors and it all goes round. The same rule says that if you move a wire in a magnetic field, you generate a voltage (and if the circuit is not open at one or both ends) a current. When you turn the power to a motor (or any coil) hard-off the magnetic field collapses and as the lines of flux cut through the coil it is the same effect as if you were waggling the wire up and down the field, but it is very rapid and aggresive voltages (damaging ones) get induced in the wiring - this is why you nearly always see a diode across relay coils driven by transistors or other elctronic (as opposed to electrical) signals - to stop these voltages hurting stuff - they can be thousands of volts on big coils. Used exactly this principle in a HV tesla circuit I made as a kid... generated big voltages from a nine volt battery but switching a relay coil on and off and bleeding the HV off through a diode onto a HV capacitor - it did sting I can tell you. On little 12 volt motors they might only be in the order of 70 or 80 volts. However ( and here is the clever part) they are directly proportional to the current flowing at the moment of collapse. As we all know, electric motors work best when under load and the more you stall a motor, the harder it works to turn against you and because the effective resistance (the inductive resistance not the stuff you measured with a multimeter above) drops, the current will rise so if the loco is working hard going up an incline, it will be drawing more current but not going as fast - by measuring the amount of back EMF (the High voltage spike made by the collapsing mag field) you can tell how hard the motor is working and so provide it with a bit more voltage (or effective Volts if using PWM). It now has more power to overcome the forces trying to stall it and the current will drop slightly but the proportions will stay the same and the back EMF will not continue to rise in voltage. so as the current drops significantly you can wind the voltage back to maintain the ratio (it's now coming down the other side)

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snipped-for-privacy@talk21.com wrote:

Sorry uncle, but your attempt isn't much better, and thus are urban myths born.

No, Back EMF is the voltage generated due to the motor acting as a genarator.

i.e. the armature windings are moving in the field of the permanent magnets and the motor acts as a generator, just like Chris said. You obviously have the basic theory so why do you then have to try and complicate things with totally innapropriate theory?

It's nothing to do with collapsing magnetic fields, when you remove power to the motor the armature keeps moving due to inertia and acts like a generator.

That's due to the inductance of the relay coil, which tries to keep the current flowing when it's switched off. The diode acts as a short circuit for that current.
You may indeed get such a spike from a motor coil, but only one, when the power is removed. From then on we are talking about Back EMF which is a totally different mechanism, I'll say it again, the motor is acting as a generator.

Wrong mechanism. Back EMF occurs because the motor is acting as a generator and the voltage is proportional to the rotational speed of the motor. That's how feedback controllers work, by measuring the *actual* rotation speed of the motor versus the desired speed. To work properly, the system needs to be tuned to each individual motor. The best DCC decoders give you full control over the tuning of the Back EMF to suit the motor in each loco.

It depends what you mean by "work best". Maximum torque, maximum power, maximum efficiency? Max torque occurs at zero revs. Maximum power at half the no-load speed of the motor.

Back EMF is being generated by a motor all the time, even when it's being driven. At lighter loads, and thus higher speed, the Back EMF is greater. Back EMF has opposite polarity to the drive voltage and opposes it. It's the lower effective *voltage* that causes lower current at higher speed. The armature resistance is constant. It's nothing to do with inductive resistance. At the extreme, if the motor is totally stalled there is no Back EMF and the total supply voltage is applied across the motor.

You measure the Back EMF generated by the motor (acting as a generator) and it gives you a measure of the rotational speed of the motor.
MBQ
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On Tue, 14 Mar 2006 12:09:11 -0500, Christopher A. Lee

erm, yes. it is.
that's why it's pWm - i.e. the width of the pulse is changed. rising edges (i.e when the power is turned on) happens at a precise frequency and the power stays on for a variable amount of time and then off for the rest. the more kick you give it, the longer the on period and correspondingly the shorter the off period, the frequency of pulses is always the same just the duty cycle (amount of time on versus amount of time off) changes.
Typically in a digital system, run the pulse from an output from a processor having sampled a variable resistor. say that value can vary between 0 & 255 (8 bit), 0=off BTW... the value you read is how long you turn the power on for and then you invert that value (so turn all the 1s to 0s and vice versa) and that gives you how long to turn the power off... so on a very low setting, say, 1, if your frequency is 200Hz (pulses per second per second), you turn the power on for 1x0.005 seconds and then turn the power off for (255-1=) 254x0.005 seconds. then if you think about half way, you turn the power on for 128x0.005 and off for 127x0.005 - but the sum of the on and off periods always adds up to 255. The Motor doesn't see the pulses so much because it takes time to start turning and time to stop so it tends to smooth out the bumps and the perception is of a constant power delivered to the motor. This is called "integration" in electronic circles.
I built my own PWM controller (using a PIC) on just this principle and I could get my farish diesels (and others) to crawl at less than scale walking speed- lovely for shunters and stuff. Oh, and because you are always supplying the full 12volts to the motor (albeit in bits), it overcomes sticky or reluctant beasts - maybe not perfect but much better.
In practice, 200Hz is a little on the slow side and some motors could hum a bit. also, I need to do some work on pulse shaping - the rapid rise in power generates extra heat (because of the inrush current into the inductance of the motor coils. The magnetic field needs to be set up and at the rise times we are looking at with a square wave the energy is quite significant... the only place to dissipate that energy is in the copper of the coils) if I shaped it to rise slower, the current is reduced and so the heat - but then you need to do some work on the timing to get it just right.
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On Tue, 14 Mar 2006 12:09:11 -0500, Christopher A. Lee

sorry... forgot to mention... you are talking about Frequency modulation here because the time is changing but even here, the amount of time the power spends on is significant. even at 1:1 ratio, you are only going to get 50% effective power (after integration) and you need to get the pulse widths right or you get a two speed motor, going and not-going and nothing in between.
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Pulse Width Modulation ... in essence the controller constantly sends "blips" of full power. It can be done in one of two ways, one way (best) the blips are all the same size and what's ajusted is teh speed at which they are are sent by the controller ... low speed a few blips per second, high speed, lots of blips per second. The other way is to ajust the lengths of the blips themselves but at a constant rate. Low speed - short blips, high speed long blips.
The motor thus is only ever fed by full power but it can cause the motor to make a "put put" sound - and potentially overheat. There's also an issue with coreless motors ... but I don't have any of those.
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Chris Wilson
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Chris Wilson wrote:

Why do you say this method is best?

Noise and coreless motors are only a problem if the frequency is too low, hence we have DCC decoders that drive the motor with higher frequency (usually something > 15KHz).
Your "best" method is actually a form of frequency modulation and more likely to result in too low a frequency.
MBQ
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Only from what I've been told previously, I do have more than what can perhaps be described as a basic knowlage but I'm far from being an expert.
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All the best,
Chris Wilson
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On 14/03/2006 15:14, Chris Wilson said,

Making your own PCBs isn't that hard! For a one-off, you can just get some etch resist tape and pads from somewhere like Maplin, some ferric chloride from somewhere like Maplin, and off you go! You don't have to go the whole hog with photoresist and light boxes and all that palaver - just slap the tape on the board, dunk in acid, and drill when etched. OK, there's a little more to it than that, but not much.
Go on, give it a go!
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Paul Boyd
http://www.paul-boyd.co.uk /
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...
Yup, agreed it's no great stress. It's simply that bit more expense; and if it ends up working out as much if not more than a comercial product that I can get off the shelf and not spend an entire weekend putting togther then it's not worth my while.
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Chris Wilson
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On Tue, 14 Mar 2006 15:14:51 +0000, Chris Wilson wrote:

Buy a copy of Roger Penfold's book Practical Electronic Model Railway Projects, published by Bernard Babani in paperback. He answers your questions and uses veroboard layouts. Printed circuit boards are a waste of time other than for bulk production or complex circuit designs, neither of which apply to your project. And they are less easy to modify.
Ken.
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I've got Rodger Amos's book it may be a little old but everything appears to make sense.

Well not necessarily, as Nick points out it's easy enough to knock up "one offs" assuming you have the kit, no different realy to using veroboard except you tend to need far less bridges and you have far less chances of getting shorts.
No my real question was what's on the market now, IOW if I go in to a shop and ask for a PWM (or modern equiv) with feedback just what am I going to be offered and what does it all mean.
many thanks though
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Chris Wilson
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On Tue, 14 Mar 2006 21:36:55 +0000, Ken Parkes

if you are doing the board manually I tend to agree with you, but if you get some PCVB design software (there are some that are free to use below a certain component count) they can make a much better job of it.
Problem with strip board is remebering to make sure you put your cuts in the right places otherwise you can get allsorts of nasties happen. A computer designed PCB eliminates this.
horse for courses - I use both depending on how lazy I feel and how big the project is.
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wrote:

Should not be big for a transistor controller just a Darlington pair and a large power transistor output
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