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

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!
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Bernhard Agthe skriver:

Your wishes would demand _some_ electronics - actually quite a lot, if it is build with transistors and op amps.
I think you should look into microprocessors instaed. Here you can get full PWM by software, your start and stop momentum cam be programmed to your desires and the "output circuit" could be as simple as 2 resistors and 2 transistors......
What you want the inputs from buttons and sliders to do you program. If you want to change direction only at stop, then you just program that....and so on.
But if I were you I would do the "boring" thing and digitalize the layut. Use your skills to put decoders in the trains, instead of developing something....
Klaus
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Hi,
Klaus D. Mikkelsen wrote:

That's what I'm afraid of ;-)

Good idea, see, if you ask you'll get good answers! Thanks. I'll look into it.

Well, going digital is not "boring" - as I said I have a few locos that might prove a challenge ;-) I'm not even sure how to open them, yet.
On the other hand, for a digital system I would likely spend a few hundred bucks and then I'll still be able to drive two concurrent trains only. And then, my control panel is really great in usability, you wouldn't get that with a DCC system: all switches are controlled by a touch-screen!
;-) You see, I seem to love challenges ;-)
Thanks!
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Bernhard Agthe skriver:

Try....
But thee is no need to change that....
Klaus
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BA:
Hmm...if you mean, can you build several throttles, powered by one transformer winding, to control multiple blocks in a common-return setup, you can't...they must be separate windings, or you'll get a short. If you find a transformer with two separate secondaries, this will work.
If you mean you are using a system that runs half the locos on positive voltage and half on negative voltage, with the common ground at 0, this has been done. I think it's called the "Detroit Method" or something, and requires rectifiers on the motors to block the current that isn't used for each. The motors ran on half-wave DC pulses. I could definitely see this being combined with a pair of PWM throttles, but you'd still get half a wave, if split evenly between both motors.
To reverse, you could build something similar to a Lionel E-unit, activated by interrupting the current, which would switch the motor connections internally to the loco, because you can't reverse by switching polarity...if you tried, you'd just switch control to your other loco.
This is neat. I like reading about clever projects like this. Nowadays so much of our electrical work is described as "plug in the box and follow the instructions". That sort of thing just isn't much fun for me.
Cordially yours: Gerard Pawlowski President, a plywood world with dime store trees.
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Hi,
autobus snipped-for-privacy@yahoo.com wrote:

Well, I partly agree - you can power several throttles from one transformer winding (you need a tap in the "middle", though). The middle tap is the common return and all trains are operated from a voltage between the "ends" of the transformer.
If you try to reverse polarity by switching the wires (with a switch or relais), you'll get a short, right ;-)

[...]
Actually my layout is small enough and the electrical blocks are set in a way, that two locos will always be in separate blocks, anyway. So I don't need a system to run two locos off the same electrical signal ;-)

Yes, that is mostly it. I will continue to think about this while I finish the wiring of the layout (I still miss some turnout motors, anyway). Once I got something feasible, I will come back here and report my findings ;-)
Thanks to all!
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BA:
It looks like the Cooler Crawler could do most of that, actually:
http://users.rcn.com/weyand/tractronics/articles/ccartcl/ccartcl.htm
Move capacitor C1 to the hand unit, then put it and the two brake resistors in a plug-in unit for customization, along with a fixed resistor that would replace the top speed pot. Replace the rotary pot in the hand unit with your slider. Alongside the two brake buttons, put a button that simply shorts SPD to GND...that's your scram button. Paint it red. :D
You could probably do the same with any simple transistor momentum throttle, too.
Cordially yours: Gerard Pawlowski President, a plywood world with dime store trees.
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Hi,
autobus snipped-for-privacy@yahoo.com wrote:

[...]
Most of that, yes. It still uses the reverse polarity feature, so I'd still have to use two transformers...
There's a rather interesting page with lots of different throttle ideas at <http://home.cogeco.ca/~rpaisley4/Throttles.html which has several variants doing most of what I want, but none doing it all ;-)

But then I still need the "fire extinguisher" button ;-)
Thanks!
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Sorry that I haven't been around to keep up on this thread but my eye has been full of blood lately and it takes a while to get it out of the field of view.
Yes, there is heat generated by any electronics. BUT, there is a big difference in heat generated by a switching MOSFET and a transistor of any kind being used in a linear fashion. Technically, the 30mOhms or os of the on resistance of the MOSFET will generate some heat and the switching time will also generate some heat but that is down in the low milliwatts of power while driving several amps of current. A linear power control will have watts of power dissapation with more dissapation at lower voltages - 3amps times 10V is 30Watts of power, a long, long way from maybe 30mWatts of the switching power supply. For the common wiring layout, the best thing is to provide a power supply for each direction and then use which one you need for the job. This gets a little bit more difficult with electronics (power reostats work best in this case) but what you can do is to provide a bidirectional throttle where there are 3 voltage levels at which the output can be. This is the sort of thing done with audio amps and there is a Class D amp design which is a digital PWM power design. The power is either off or on to one side or the other, depending upon the speed of the loco desired. A SPDT switch can be used to drive either one or the other driver as needed.
-- Bob May
rmay at nethere.com http: slash /nav.to slash bobmay http: slash /bobmay dot astronomy.net

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Hi,
Bob May wrote:

Get well soon!

Yes, that's why switched power supplies work so well ;-) However, I wanted to point out that there is some - though very little - power loss by heat - which needs to cool off ;-) In most cases it will be sufficient to let the heat aggregate during operation and leave it to cool over night or to provide a small ventilation hole (or two).
As mentioned, a modern computer CPU is the extreme example - it dissipates more heat per area than an electric kitchen stove... But all it does is switching low-power currents on and off ;-)

[...]
That's basically what it boils down to, though I would eliminate the switch (to switch between + and -) and rather have the driver work the full range in one sweep with the input control set up for switching... Using all the ideas I gathered here and there I would end up with about a hand full of small parts - and still be rather simple compared to the majority of designs I've seen. At the moment I'm finishing my last track connections and working on scenery a bit, but I'll come back to the topic soon and develop a circuit. Perhaps I can post it here and hope for some useful comments?
Thank you all!
Ciao..
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