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):
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Reply to
David Nebenzahl
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Silicon Chip Magazine here in Oz has had a two month long article.
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So you're not a big, tough, fearless guy any more? How sad. -- Ray
Reply to
Ray Haddad
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.:
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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"?
Reply to
David Nebenzahl
A Google search for "cooler crawler" turned up:
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Reply to
Robert Heller
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.
Scared of Google? How wimpy can you be? -- Ray
Reply to
Ray Haddad
On 11/9/2008 12:55 PM Robert Heller spake thus:
Thank you; 'zactly what I was looking for. You da man.
Reply to
David Nebenzahl
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 descripti 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?
Reply to
David Nebenzahl
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:
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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
Reply to
Klaus D. Mikkelsen
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).
Reply to
Robert Heller
No it's not but you keep on believing it is. -- Ray
Reply to
Ray Haddad
Robert Heller skriver:
Doesn't all decoders have PWM ?
Klaus
Reply to
Klaus D. Mikkelsen
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:
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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.
Reply to
autobus_prime
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
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Reply to
Jason Davies
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.
Reply to
David Nebenzahl
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
Reply to
Klaus D. Mikkelsen
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
rmay at nethere.com http: slash /nav.to slash bobmay http: slash /bobmay dot astronomy.net
Reply to
Bob May
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...:)
Reply to
Whazzit
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.
Reply to
autobus_prime
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?
Reply to
David Nebenzahl
Hi,
Bob May wrote:
[...]
In my university lecture, I remember being told that you still get some heat. The argument went like this - the change itself is not instantaneous but takes a (very short) time. During that time, the switch (e.g. MOSFET) does heat up some. If you switch it on and off fast enough, it'll get cooking hot - compare computer CPUs. Applied to model railroad this isn't significant, but I'd still leave a ventilation opening, especially in very small boxes. A few small holes will be sufficient.
Let me ask a different question.
My layout is analog DC, with 15 power blocks. All blocks can be switched (throttle 1 - off - throttle 2) on the "live" wire and have a common return. Thus I can drive two trains from two throttles. This works fine with the throttles I have running off two different transformators (because of the common return).
What I'm looking for is a circuit for a throttle that does not use DPDT switches to reverse polarity so I can drive two trains off one transformator. I understand that I need one with 2x 12V secundary and attach the throttle itself to +- 12V with the common rail running on 0V. But the circuit itself is still a mystery to me, as I want a few "extra" features:
- a push-button to reverse train direction (only if speed ~ 0) - a linear slider for train speed (stop at one end of the slider) - a "stop" button - Momentum (i.e. train starts slowly and stops slowly) - momentum and top speed configurable via plug mechanism (different settings for each train according to the plug used, also to set a "low speed plug" for children ;-) - some kind of PWM would be fine but is not necessary.
On my own, I can do most of the parts, but I still have not worked out a schematic to do "all at once".
For example, I figured I can use a stereo potentiometer (linear sliding type) with the "lower" ends connected to each other and the upper ends connected to +12V and -12V individually. A relais (ugh) would select forward or reverse direction. Using a resistor and capacitor I can simulate momentum and with an additional resistor between the poti and the +-12V line I can limit top speed (all three resistors put on a plug I get the individual train settings). So now I have a low-power control signal, how do I block the direction button if the train is moving? My high-school electronics is a bit rusty :-( And how do I connect the low-power signal to the PWM-circuit? ;-)
I guess, such a throttle would be of interest to some analog-DC railroaders? You get some of the digital features on non-digital layouts (e.g. loco-adaption). And - at least in my case, my layout is too small for a digital system to have any benefit whatsoever (I can not operate more than two trains anyway and some of my locos may prove difficult to convert).
Thanks for your input!
Reply to
Bernhard Agthe

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