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Max, this has been an outstanding thread. You've gone above and beyond to determine exactly what's going on, and furthermore, proposed--and demonstrated--a practical workaround to address the problem.I'd vote this as a "best of" thread for the DCC/Electronics section.
Yes, but the catch there is that they would probably have to use a better transformer. Transformers all experience some voltage drop as the current increases, and the closer you push a transformer near its maximum current rating, the harder it drops (as of course, you know). So to get a 1/2 amp output with a fairly stable output voltage would probably require using a better transformer than the one they have in there, and probably one with more like a 1A current rating. I'd venture that a zener for a few pennies would be more desirable to somebody in the accounting department than a better transformer, cynical though I may be.
Yes, adding the Zener diode limits the top voltage while retaining some reserve voltage coming out of the transformer. Basally you added a voltage regulator to the circuit.Changing the voltage divider arrangement of the speed-control potentiometer would also limit the top voltage out of the transistor. For example changing the pot from 20k to 15k and inserting a 5K resistor between the top lead of the pot and the positive output of the bridge rectifier should result in limiting the top output voltage of the transistor.As far as transformer quality is concerned, I have never heard of different levels of their quality (of standard industrial grade transformers utilized in consumer products). Transformer is an iron core with couple of copper wire windings. They are designed to have input and output voltages, and handle certain wattage (volt-amps). I don't think there is a better and not-so-good quality. All MRC had to do is to get a transformer with a same wattage, but lower output voltage, to retain the lowest component count throttle design.I suppose that the iron core material can have slightly different magnetic qualities, but I think that the effect of that on the "quality" of the transformer's ability to deliver the rated wattage to the output would be minimal. After all, I'm discussing a low cost commercial quality parts (not some esoteric high-end piece of equipment with a transformer made with some fancy ferromarnetic core, and oxygen-free 99.9% pure copper wire).
I think what I really meant by "quality" was whether or not the transformer really delivers its stated output voltage at its stated current. It might say "12.6VAC 3 AMP", but in fact only output something like 11 volts when you get up to 3 A. I don't know how many cheap "wall warts" I've played with that have a certain voltage and current rating on them, but have severe voltage drop when you get anywhere near the advertised current on their label. I've almost come to expect a transformer to not really hold its rated voltage much over 1/2 its rated current.
I see now. I tried it.It does reduce the maximum pulse height, but you have to change the resistor to 2.2k in order to get a full 12V average DC outat full throttle. Otherwise, you only get about 9.5v and I don't think that's good enough. With a 2.2k, the pulses are reduced to 20v from 25v, which is a nice improvement, and the maximum DC out is about 12.3v at full load, which is fine.What I don't like about this is that it changes the fundamental waveform. You lose the intermediate half-height pulses, which I think would improve motor performance at low speed, and I like the way they gradually "catch up" to the full pulses as you raise the speed, thereby giving it more smooth DC at higher speed, which is desireable for motors.
It depends, too, upon what you intend to power with your throttle.Dual-mode locos may need over 7VDC applied just to begin moving.For comparison, according to MR test data, the Kato FP7 reaches 103 s(cale)mph at 6VDC, 178 smph at 9VDC and 248 smph at 12VDC.Also, I wonder if those full voltage pulses are still necessary for decent performance with 5-pole, skewed armature and coreless type motors versus older 3-pole motors.If Jason from Rapido can't perform an analysis on the failed decoders, perhaps he could ship a few off to peteski, if he would be willing to investigate them.
It would be interesting to scope out Kato's power pack to see what voltages it has. As I understand, it is also a very simple transistorized throttle using full-wave unfiltered DC.
There's some info on the Kato unit here: http://www.sumidacrossing.org/Musings/files/131222_Kato%20DC%20Power%20Pack.php
Thanks! So yes, it is a very basic transistorized throttle and its output voltage is more inline with NMRA recommendations.
I agree, because with a 17 VAC RMS supply, that's 24V peak, and there are always a few voltage losses through the diodes and transistor, so yeah, it's not going to be more than 20 on the output peaks.
To be honest, I have never thoroughly tested any of the AC wall-warts I used (and I never load them up to more than roughly 70% of their rated current). But I have seen what you describe in the DC wall-warts. Those utilize a simple bridge rectifier and a usually too-small-value electrolytic filter cap. With no or little load, the voltage will reflect the peak (not RMS) voltage. As the load increases, so does the ripple, and the voltage gets closer to the true RMS value.
Note that some newer "wall warts" are actually switching supplies.
Instead, here's my idea of putting in a 20v zener diode. This limits the maximum pulse height to only 18v, and as you can see, it preserves the alternating pulse height behavior. I do admit I'm wondering if the clipped square wave shape at higher speeds will create more heat or noise from a motor. But I rather doubt it, since it's certainly no more harsh than the square waves from a PWM.A 20v 1W zener costs about 50 cents, and that's a consumer price, buying only 10 of them (which I just did on eBay... ).So we are talking pennies here from a manufacturing standpoint.The second plot, at half throttle, doesn't show the voltage on the voltmeter. Instead, there I was measuring the current through the zener. As you can see, it's miniscule, so a 1W zener can easily handle this and it won't even get warm.
Could you run a simulation that shows how much current flows through the zener diode when the throttle (R2) is at the full setting?
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Topic: Railpower 1300 testing (Read 40988 times)
