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Post by markm on Mar 9, 2017 18:30:35 GMT -5
The PWM on my Arduino runs at the system default about 500Hz. Since this is about the same as the Snail throttle and I'm happy with that throttle, I haven't played with changing the Arduino. Also back in the mid-1970s when transistor throttle designs were coming out there was considerable discussion of the pulse frequency. At that time the feeling was that 100-120 Hz (full wave rectification of the mains) was the maximum effective value. Personally I don't like the higher frequencies because it means faster edge rates and higher current flow. Also square waves are such great harmonics generators. As for the Rokuhan units, their output is essentially DC with a 25kHz microsecond-sized full power pulse for driving lighting circuits: Mark
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Post by zoinks007 on Mar 12, 2017 17:32:17 GMT -5
I've not purchased one of these, but I'm going to: dccspecialties.com/products/powershield_x.htmSolid state circuit breaker...seems like a good idea. They list at $44.95 USD, but saving just one locomotive justified that cost. Also, I'd asked a related question here and was kindly pointed to CV's 6 and 5. The DCC units all (seemingly) can accept 12V, so you limit what goes to the locomotive motor electrically using these CV's. Or certainly at least CV5 (top voltage). My TCS docs that came with the decoder don't specify how that value is indicated, hex or bcd (binary coded decimal), but I'll research that, of course, before I fry something. Maybe I don't care since it's programmed on the track by the controller, and it "knows" what the storage encoding should be.
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Post by zoinks007 on Mar 14, 2017 19:49:24 GMT -5
Ah, gotcha. Yeah, the name is messed up. I think I was starting to understand that, knowing there is no voltage regulator of any kind on the PCB. Would have to limit the duty cycle of the PWM input.
Not hex versus binary...hex versus *bcd*. Different. But you've answered the question. It's hex/octal/binary. Actually it would be unreasonable to assume some random controller would know to write bcd unless all CV's were bcd, which would be nonsense. Learning.
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Post by Deleted on Mar 15, 2017 1:34:27 GMT -5
I come back to my initial idea : what do You think of simple polyswitch-polyfuse components (rated with 150 to 250mA) to avoid short-circuits ? They behave like conventional fuses but reset themselves when the problem has been solved. Perhaps is that what BAZman was meaning with Ohmite, Cirkit ? An electronic limiter (microprocessor controlled ! ) makes me think of a guy who wants to use a bulldozer to crack the shells of peanuts !
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Post by zoinks007 on Mar 15, 2017 6:17:42 GMT -5
Sorry, missed the BCD... funny, you rarely see BCD any more... BCD is backwards, i.e. you are using binary to represent decimal numbers. BCD could not express all the possible values in CV's... True, but it wasn't unusual for the time period in which DCC was developed. In fact it was pretty common then. But I'm glad it's simple binary...
Electrically I can't think of a reason it wouldn't work. The advantage I can see for the more complex units is they serve a dual purpose...that of circuit breaker but also that of a loop polarity reversal controller. They're also "smarter" about the durations of higher current, such as when sound decoders kick off. I wouldn't have thought that important in Z until the recent threads about sounds:
azlforum.com/thread/806/sound-Christmas
So one day, maybe sooner than later, we might need to deal with current inrush for sound decoding. Hoping so.
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Post by Deleted on Mar 16, 2017 11:49:58 GMT -5
To Greg Elmassian : it was an answer to zoinks007 ; I want to insert the polyswitch into the power supply ; as I wrote some time ago, I come back to the idea of only one train running at the same time, so there is only one power supply for the whole layout, and there can't be a short circuit caused by a train running into a track of opposite polarity, for the case that a turnout should not work properly ... And because the layout will be microprocessor controlled, the turnouts would not switch until the train has stopped in the station, this eliminates a second source for problems. The layout will be widely automated, only one pusbutton per train to start it (perhaps a second one to select if it should go to the other station or come back to the initial one). That's the reason why I need many rail isolators (about 30), the microprocessor will look where the train is and automatically increase or decrease the speed, or make the train "go backwards" to the appropriate freight track.
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Post by Deleted on Mar 16, 2017 12:42:01 GMT -5
My first name is a "very french" one : Bertrand (NOT Bart in English, this means Bartholomew ... ) ; Bertrand, like "Du Guesclin, Connetable de France" !
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Post by Deleted on Apr 5, 2017 3:09:56 GMT -5
My new problem : the power supply. I thought that the LM317 provides short-circuit protection ; I read that in fact it does ... by defuncting in some cases ; my problem is still the short circuits caused on a Rokuhan turnout which would not switch properly, I wanna damage neither the turnout, nor the rolling stock. So I am looking for an electronic circuit breaker, which could be used at the output of a CONSTANT voltage source (my PWM will be done by a microcontroller, so I don't need a standard train controller ; and 7.5v could be used both for traction and turnouts) ; there are some standard switching power supplies, in plug or box shape ; unfortunately somebody said that they manage short circuits correctly ... but not too often, say once or twice per day, and that the only power supplies which are not too easily destroyed are those for lab use. A circuit breaker in a former post is only intended for DCC use. So, what can I do ? Does anybody have schematics for such a protection (or a link to a hobbyist page) ? Or is there an IC intended for this use ? Or should I simply use a standard train controller, set to maximum ? Thanks for every help ...
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Post by markm on Apr 5, 2017 8:56:05 GMT -5
Yes, 3-terminal regulators provide short-circuit protection for themselves only. You need a multi-pin device for load current limiting. It's been a long time, but one I can think of is the uA723. It may not meet your needs but it's a place to start. All the switching regulator chips I worked with had current limit/shutdown but I can only remember the manufacturer, Plessey.
Hope this helps a bit,
Mark
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Post by Deleted on Apr 5, 2017 9:59:57 GMT -5
Thanks Mark. I read that the LM317 could be wired as a current limiter, too. But what I want is a real BREAKER. I found a system which triggers a thyristor, here : www.sonelec-musique.com/electronique_realisations_disjoncteur_001.htmlR1 acts as a current-sensing shunt, an adjustable amount of this voltage is directed to the base of Q1, the transistor saturates if this voltage is about 0.7v ; C1 is for a triggering delay. I think the same effect could be achieved with a LM311 comparator. The thyristor-relay combination could be replaced with a RS Latch built from CD4000 series, with two NOR-gates (so the trigger input would be active high, for the transistor version with a pull down resistor) or NAND-gates (active low for the LM311, with a pull-up). The only problem is that reset would still be manual. I heard of some systems which, after a shutdown, send automatically very short pulses to detect if the problem is still present and, if not, reset automatically ... But in worst case, a breaker with a manual reset would be more "boring" but better than a limiter, all the way.
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Post by Deleted on Apr 5, 2017 10:13:18 GMT -5
Thank You Greg for that answer, but what's about the reaction time ? Although I wrote my post into the topic for locomotives (due to the reference to the breaker for DCC), the system should protect from the short-circuits in the turnouts (frog) too, not only from short-circuits inside the locomotive overlapping two sections of opposite polarity. As I wrote in the topic about the turnouts, I don't want to melt the springs (the contacts between the upper shell and the PCB) like our German modeller friend (but he used DCC, where voltage and current are higher). A turnout is rather difficult to replace between other track sections ! I thought of a solution suggested, using power routing to a section of track long enough to stop the loco before it comes to the turnout ; unfortunately I decided meanwhile to 99.8% to run two RDC's coupled together, so the first car would stop on the section but the second would "push" it, beeing still powered. So I don't know exactly what would happen ...
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Post by markm on Apr 5, 2017 23:42:24 GMT -5
Yeah the uA723 is an ancient regulator but more controllable than the LM317 or LM78XX. Giving your discussions a bit more thought, I have a few ideas for you to consider.
The example you linked to suggests you're looking for an analog solution for shutting down the supply. Continuing along that line, if you are still using the L298, I'd use the device's Enable pin to control throttle output. If seems that the circuit you linked to would work with adjustments to component values. You could look into using a silicon controlled switch (SCS) instead of an SCR. The SCS is the same as an SCR but with an additional connection that gives you both an "on" gate and an "off" gate, an analog R-S flip flop. You could connect the extra gate to a timer to allow for a timed automatic retry.
One concern I have with this approach is that with PWM you're going to have switching currents on the order of:
I=CloaddV/dt
which many cause false triggering. For a data point: on my test bench, using the Snail PWM throttle driving an AZL locomotive I see as much as 10mA switching current.
A digital solution may be the easiest way to monitor current and provide overload protection.
Hope this helps,
Mark
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Post by Deleted on Apr 6, 2017 0:36:36 GMT -5
Thanks for Your answers ... 1) The idea with a timer (oscillator with short pulses) to try retriggering seems obvious, it could be used with all systems (including the RS-latch) ... About the current pulses, I think one can leave a safety margin (and use the delay capacitor), without producing immediately "electronically assisted smoke signals" ; I think the permanent short-circuit current would be much higher and could so trigger the breaker safely. 2) I saw that the uA723 (from 1972 ! ) is still available, even at Conrad France ; unfortunately it has no "current overflow" output, signalling when the limiter becomes active, and which could trigger a breaker. 3) The thing that amazes me most is that there is no IC with all the necessary functions, to build a power supply with only one IC ; I saw some interesting circuits by Maxim for a 2-ICs solution, unfortunately I could only find them in SOIC package. 4) If there is a readymade "digital" solution other than the limiter for DCC, or a dedicated real DC power supply (without PWM throttle and pulses for lighting), other than lab power supplies ( ), all tips are welcome ...
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Post by BAZman on Apr 6, 2017 0:37:59 GMT -5
The method of current limiting is called 'foldback'. But, if at any point the load stops to short, the voltage jumps back up. As long as the short sustains, the voltage will remain at '0'
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Post by Deleted on Apr 6, 2017 0:57:59 GMT -5
I saw an example with uA723 here : electronic.de.loisir.pagesperso-orange.fr/ua723.htmlConsidering the low current needed by locos, I think I could discard both transistors and directly connect pins 10 and 2 (uA723 is rated for 150mA). So I will order all parts and test ; in the worst case I will "shoot" an uA723 ... About PWM, progressive acceleration etc : on my former layout in N, the trains ran at only 2 speeds : full and slow, with pure DC (and a supplement ary correction for "downwards", 3% slope ) ; the inertia did the rest, and it was sufficient for me ; so I wonder if I am not again drifting away, to the "gaseous plant" ...
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