Subject:
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Re: Eliminate high-speed derailments forever! (and possibly void your warranty)
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Newsgroups:
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lugnet.trains
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Date:
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Mon, 23 Aug 1999 18:39:06 GMT
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Viewed:
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718 times
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In lugnet.trains, John R. Clark writes:
> Okay, how's this for an idea:
>
> Most electronic stores stock a kind of universal power converter. These have
> several output settings, including 9v and 7.5 volt. If you use the 7.5 volt
> setting, the train never gets enough power to derail on curves. This has
> obvious
> advantages at shows, and also anytime kids are given the controls (my 2 1/2
> year
> old loves my trains, but sometimes he runs them a bit fast!)
>
> This reduces the number of different speeds available to only two (instead of
> 6). Specifically, it seems to simply eliminate the 4 highest speeds. But • that's
> perfect for little ones who seem to have only two speeds anyway: full-forward,
> and full-reverse.
>
> So the obvious question is: Am I damaging/shortening the life of my • transformer
> by feeding 1.5 volts less than it expects into it?
>
> And another question that someone with more electronics knowledge than I have
> can answer: The universal converter has removable male jacks, which are
> designed
> to fit many different devices. They are also reversible, which means that the
> tip can be either positive or negative. So does it matter which way it goes
> into
> the transformer?
>
> I leave the answers to these questions as an exercise for the reader.
>
> Rick Clark
> jrclark@nospam.aol.com
> http://members.xoom.com/jrickclark
Hi,
can i joy the conversation?
I just opened a LEGO TRAIN SPEED REGULATOR and drawn a shematic of it.
I did this once before but lost the paper.
Now to the point:
The INPUT of the regulator may be an AC or a DC signal.
Even reversing the polarity (in case of a DC voltage) won't damage the
regulator.There's a full bridge rectifier on the printed cirquit board (pcb).
Lego put on the input of the regulator : 9 - 12V AC, this has to do
with the following things:
1)The voltage regulator is a LM317T (can deliver 1.5A output current)but is
mounted on a small aluminum heathsink, too small to obtain enoug cooling when
drawing 1.5A, so don't do that.
2)The LM317 won't blow up because he's internally protected against to high
tempertures.If he gets to hot,the output voltage drops.But the pcb's layers
are too thin to pull 1.5A out of the regulator.
3) When the input voltage is 12V AC, then the internal DC voltage is aprox.18V
(AC input voltage x 1.41 = DC output voltage) this means that, when the train
runs at full speed (output=9V DC) that there's a voltage of 9V DC OVER the
LM317. Let say that the motor uses .5A, then is the power dissipation
9V x .5A = 4.5Watt
If you run at half the speed (4.5V) then you will messure 18V - 4.5V = 13.5V
over the LM317.
recalculate the dissipation gives : 13.5V x .5A = 7.25 Watt.
As you can see is it verry important to keep the input voltage as low as
possible.So 9V AC is better than 12V AC
When you want to get a lower output voltage, then you have to open the
speed regulator,or the input voltage must be lower than 9V DC !!
The output voltage is normaly independent from the input voltage as you can
see in the formulla below.
The output voltage can be calculated with the followig formula:
Vout=1.25(1+(R2/R1))+Iadj(R2) -> info: data sheet National Semiconductor
where :
R1=200 ohm
R2=200,400,600,800,1000,1200 ohm (depending on the position of the speed knob)
Iadj(R2)= current throug R2.
For fast calculation omit +Iadj(R2),resulting in:
Vout=1.25(1+(R2/R1))
I've done a modification on it so that the maximum output voltage is aprox.
7.8V
If you want to do it too,open the speed regulator.
Remove the yellow speed knob.Keep the green LED to the bottom side.
you'll see on the left side of the 'circle swith' 8 solderpins going from top
to bottom (resistor array).
Count the pins from top to bottom.You'll end up by 8.
Short cirquit the pins '7' and '8' with a solder pad.
re-assamble the speed regulator.
Thats it !!!
With this action is the speed for step 5 and 6 (full speed)equal.
Good luck !!
regards,
Ludo
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