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 19:02:01 GMT
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Viewed:
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784 times
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Ludo - -
Thanks for the great analysis of the LEGO speed control!
- Nick -
ludo soete wrote:
[snip]
> I just opened a LEGO TRAIN SPEED REGULATOR and drawn a shematic of it.
>
> 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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