Speaking of cooked motors, "pulse power", etc.:

The previous thread on DC voltages, stalled motors and such has made me curious about PWM (pulse-width modulation) for DC model train control.
Anyone have any schematics for a DIY PWM power supply? I did an AltaVista search*, but couldn't find anything on the first try. Must be several floating around out there. (Hello, Robert Heller?)
* I don't use Google; don't like their privacy intrusions, slanted ranking system, etc. There are other search engines out there. If you like Google's search results, you might consider using Scroogle instead (Daniel Brandt's front end to Google that bypasses Google's information-gathering functions): http://scroogle.org .
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On Sun, 09 Nov 2008 11:15:56 -0800, David Nebenzahl

Silicon Chip Magazine here in Oz has had a two month long article. http://www.siliconchip.com.au/cms/A_110926/article.html

So you're not a big, tough, fearless guy any more? How sad. -- Ray
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On 11/9/2008 11:28 AM Ray Haddad spake thus:

Not helpful, "mate":
1. That's the second of two articles, the first having the circuit description, schematics, etc.: http://www.siliconchip.com.au/cms/A_110837/article.html
2. It's a pay site, so we can't see anything without paying for it, which I'm not about to do.

What the *fuck* do you mean by that, "mate"?
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On Sun, 09 Nov 2008 12:28:31 -0800, David Nebenzahl

Yes, indeed. The first is archived there.

Or you could ask me politely for a copy. Bet that never occurred to you. How quickly the tightwads burn their bridges. Go beg somewhere else for free stuff, you idiot.

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wrote:

A Google search for "cooler crawler" turned up:
http://users.rcn.com/weyand/tractronics/articles/ccartcl/ccartcl.htm

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Deepwoods Software -- Download the Model Railroad System
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On 11/9/2008 12:55 PM Robert Heller spake thus:

Thank you; 'zactly what I was looking for. You da man.
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On Sun, 09 Nov 2008 13:10:20 -0800, David Nebenzahl

No it's not but you keep on believing it is. -- Ray
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On 11/9/2008 12:55 PM Robert Heller spake thus:

Further follow-up: while this looks like an extremely useful circuit, it isn't really a PWM source; the description of the circuit says:
Q4 is the output transistor, and is mounted on a sizable heat sink to dissipate the substantial heat generated due to not running Q4 in a PWM switching mode.
So it appears to be running in "pseudo-PWM" mode. It's a good compromise, as the circuit is very simple and the only downside is some heat dissipation in the output stage.
However, the output waveforms look like this (refer to the article for better illustrations):
Full throttle:
__ __ __ __ __ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \/ \/ \/ \/ \ | || || || || | -------------------------------------------
Medium throttle:
__ __ __ __ __ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ | | | | | | | | | | -------------------------------------------
Low throttle:
__ __ __ __ __ / \ / \ / \ / \ / \ | | | | | | | | | | -------------------------------------------
where true PWM waveforms would look more like this (idealized, of course, taking into account that actual waveforms wouldn't be absolutely rectangular):
Full throttle:
______ ______ ______ ______ ______ ______ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | -----------------------------------------------
Medium throtle:
____ ____ ____ ____ ____ ____ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | -----------------------------------------------
Low throttle:
__ __ __ __ __ __ || || || || || || || || || || || || || || || || || || -----------------------------------------------
I realize that a true PWM circuit would probably be more complicated than the Cooler Crawler, requiring different wave-shaping parts, perhaps using triacs instead of transistors.
My questions to you are:
1. Would it be wothwhile to build a true PWM power supply as opposed to the Cooler Crawler implementation? How would true PWM perform in the real world?
2. If the answer to (1) is "yes", do you have any true PWM circuit schematics?
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David Nebenzahl skriver:

Actually I think it is very complicated.

Because what you do is to let the halfwaves from the supply through the transistors. Your switch freq is 50Hz (in Europe). Modern PWM 8as you see it in decoders) rus at much higer freq's.

No, it is much more simple than the crawler.
Us an 555 and a few external components, an example could be: http://www.dprg.org/tutorials/2005-11a/index.html
You can easily replace the FET on the right hand with a normal NPN transistor.

Yes it would. The crawler is not PWM, it is cut down 60Hz pulses. I guess it will make som motors rather noisy....
We have been running true PWM for 12 yeras now on a computer controlled layou wtith 78 PWM trottles and 60 trains running for approx 100 days/year. PWM performs well, the only thing that is betteer is a deconder in each locomotive.

Tonnes of them out there.
Klaus
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wrote:

In theory, a 'true PWM' would perform great. As the article says, running the common PM motors typical of model RR locos with such a power supply causes them to heat up, since they are not really meant to (ab)used that way. The *ideal* way to finely control a DC motor and get full power at all speeds is to apply it full rated voltage to the armature (via its brushes & comutator) and vary the field exitation. Model RR locos are *Permanent Magnet* motors, which is equivalent to a fixed field exitation, so all we can do is vary what we supply to the armature, either a variable DC voltage or PWM or some variation of the two. A lower voltage means lower speed / torque. Full voltage means full torque / speed. Full voltage at less than 100% duty cycle (eg PWM) means full torque, but at reduced speed -- the motor is 'averaging' its speed ('full speed' during the pulses, 0 speed between the pulses). Because the motor is mostly in a 'start at full power' mode, it gets hot, because the current draw during the pulses is high.
I suspect that the Cooler Crawler is a comprise circuit designed to get some of the benifits of PWM without the problems PWM can cause.
The lack of good PWM circuits for model RR locos is probably a 'side effect' of DCC -- many DCC decoders include PWM, so there is little or no need for modelers to bother.

No, I don't think I have good PWM circuit schematic. I have Bruce Chubb's *original* CMR/I book and it *might* have a PWM throttle circuit in it (probably one meant to be computer controlled). I don't have any way of getting it to you (I don't have a functioning scanner).

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Robert Heller skriver:

Doesn't all decoders have PWM ?
Klaus
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Folks:
What a wacky coincidence - the Cooler Crawler popped up in a thread. I just spent some time thinking and Usenet-surfing, so I could figure out how that circuit worked.
What it is is an incredibly ingenious hack. You can read Ken Willmott's cogitation of the issue here:
http://groups.google.com/group/rec.models.railroad/browse_thread/thread/eb089e57ccd2c56f/9b1ccc55703fce9d?lnk=gst&q=ken+willmott#9b1ccc55703fce9d
It's basically a voltage follower *but* with a capacitor across the output transistor, which is isolated from the rest of the circuit by its own diodes. The cap, at crawl settings, charges through the load to provide drive pulses, similar to the old Poorman's Throttle that (I think) was in one of Paul Mallery's books. It discharges through the transistor, hence the separation, to keep this from affecting the base current. At high settings, the cap becomes insignificant, and the throttle operates as a plain Jane voltage follower.
If contact is momentarily poor, it charges less, but charges *more* on the next fullwave pulse, if contact is restored, providing corrective feedback.
Fun with passive devices! Those rascally electrical wizards. To a heinous, ingenious hack like this, I can only take off my hat!
(I've got to build me one for the next phase of my Ampack upgrade...)
Cordially yours: Autobus Prime w/minicon Farebox.
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Coincidence here too. Back around 10 years ago there was an extensive discussion on rec.models.railroad regarding the Cooler Crawler, and I added some wise words in which I basically said I had no idea how the thing worked, even though I'd built one and tried it out. It seems as though, by some mysterious method, it acts as a back-EMF feedback device, where if the train is near stalling it gets a boost in voltage to get it moving. Yet the circuit seems too simple to have any such feature. I'd definitely recommend it to anyone wanting to build their own analog throttle.
The club I'm in uses a PWM throttle system which (ahem) I designed. But it's only PWM for convenience in generating the signal via a microcontroller: from there on it uses analog components to put the voltage on the rails, and the waveform is shaped so that it's not a square wave. We have back-EMF feedback, but it's done digitally by the processor, with the signal sampled via an A-D converter. Our basic control scheme is "one throttle per block", where each block has its own reversing relay, so the difficulty of generating a positive or negative signal doesn't arise. And reversing loops are so simple we're hardly aware that they exist.
One very handy feature of our design is that the actual power-handling element is an LM317 voltage regulator, which is short-circuit proof and overheat-proof. I'd like to say "bomb and bullet proof", but it probably isn't quite that good. Still, we have over 100 of them in use and we've never lost one yet.
Read all about it here: http://tmrc.mit.edu/sys3 /
And another coincidence! We're having an Open House tomorrow! Free! We'd love to see you. http://tmrc.mit.edu/openhouse.html
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Oscillator approx. 200khz --> counter --> magnitude comparator --> out accel/brake buttons-->up/down counter -->
Throw a fet (or 4) on the output to handle the load (and reversing).
or you can do this
http://www.cpemma.co.uk/555pwm.html
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On 11/10/2008 8:02 PM Jason Davies spake thus:

Thanks for that.
Now I'm tempted to build both versions and "race" them against each other.
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David Nebenzahl skriver:

Du you only run 1 loco at a time ?
Then i have the circuit for at pwm regulator with back-emf.
It was posted in elector electronics in 1997, but since it is 5 pages you can only get it by mail (moppe(at) post6.tele.dk)
Klaus
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On 11/10/2008 8:02 PM Jason Davies spake thus:

Just did that latter thing, with a little motor pulled from a VCR for a test. (If you see a VCR tossed out on the street, pick it up: at least 3 small DC motors inside.)
Works nicely, very smooth speed changes, except for one thing: the motor makes an annoying buzzing sound. Oddly, the frequency doesn't change much until you rev it up pretty fast, so it must be the PWM frequency, or a subharmonic. I'll try tweaking it later (the author suggest a capacitor substitution to raise the frequency). Wouldn't want a singing or growling loco now, would we?
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Basically, the idea of PWM is to use a switch to control the power with the width of that switch on time being the percentage of the power desired to the load. Towards this end, the process is really quite simple. You take a source of a sawtooth wave (the 555 timer chip does an excellent job of this in its timing section) and then compare that against a voltage which you control. The ouptut of the comparator goes to the switch. In the olden times, the switch was usually a power transistor but the MOSFETs are a lot better at this as they don't blow up when you abuse them as fast. In addition, there is no high power requirement for a MASFET input to drive it on and off. The nice thing is that the circuits don't need to have any heat sinks that linear supplies have to have and thus a throttle can be put into even a little 1"x1"x2" box without any trouble and still be able to control several locos at a time.
-- Bob May
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wrote:

Gosh, it sure was simpler years ago when all one had to do to get pulse power was to cut and insert a simple on /off toggle switch on the metal full wave rectifier cross rod or whatever it was called...:)
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On Nov 11, 6:26pm, snipped-for-privacy@hectic.org wrote:

W: You can still do that, too. I did so with my Ampack upgrade. Of course, there is a jump if you switch from half to full wave while running, so in practice I just used half-wave for switching...although I didn't use it much even for that, because the full-wave power seemed to give good control already.
Half-wave pulse power does give a useful scaling effect on the speed control range.
The nice thing about rolling your own throttle is that you can make it as simple or as complex as you want.
Cordially yours: Gerard Pawlowski President, a plywood world with dime store trees.
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