Subject:
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Electrical Things (was: Railroad Dilemma)
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Newsgroups:
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lugnet.trains
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Date:
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Mon, 8 Mar 1999 13:35:40 GMT
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Viewed:
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3534 times
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I finally got round to having a play with my controllers and my
multimeter.
PUTTING TWO CONTROLLERS ON THE SAME TRACK
-----------------------------------------
Firstly, apologies to Ben, who suggested if you connect two
controllers to the same track, then they'd short only if the
controllers had opposite polarities. Ben was actually correct.
What I actually found was the voltage on the track is the BIGGER
of the two voltages that the two controllers puts out, if they
have the same polarity. If they have opposite polarities, then they
always short out and there's a small residual voltage (about 0.3V)
on the track. I'm a bit puzzled as to what can be going on in
the controllers to cause this behaviour.
Incidently there doesn't seem to be any effect on the speed the train
goes at if you put an extra controller on the track. (I wondered if
there might have been because the controllers could supply more current)
CHARACTERISTICS OF ONE CONTROLLER
---------------------------------
As I suspected the controller supplies a variable voltage. As you move the
speed regulater from zero to the first of the 6 marks the voltage suddenly
jumps up to about 3V. It then increases linearly up to about 9V for the
maximum speed. I say roughly because my controllers, both newish, differ
by 0.2V, so I guess Lego's quality control for this stuff isn't too exact (it
doesn't really need to be).
The motor power unit has a resistance of 8.7 ohms, so at max. speed, it's
eating up about an amp. (or about 9 watts of power).
In more detail, for one of my controllers, the readings were
open circuit voltage with which means
voltage train running internal resistance is
Full power 9.24V 9.04V 0.09ohms
Minimum speed 3.04V 2.99V 0.02ohms
I'm surprised that the internal resistance varies with voltage. I guess
I'll have to go and read up on my basic electronics again to figure out
what's going on.
Simon
http://www.SimonRobinson.com
Simon Robinson writes:
> > > 4) Lastly, what would happen if I had both controllers running at the
> > > same time with the switch connecting them is active (both would be
> > > connected.)
> >
> > ben>> You have essentially connect both loops in parallel with two parallel
> > power supplies. If the two controllers have opposite polarity you have a
> > problem, which will cause a fault and stop the motor and possibly damage the
> > motor if the situation lingers.
>
> I suspect the short would happen even if the two controllers have the
> same polarity, but are set to different speeds. I'm guessing it's the voltage
> they put out which increases as you up the speed. If that's the case and
> you have the controllers set to different speeds then you're still directly
> connecting terminals which will be at different voltages -
> effectively creating a short.
>
> > If both controllers have the same polarity,
> > both trains will run with an amount of voltage from summing the addative • powers
> > of the two power supplies. Note: since they are in parallel it is not a • simple
> > 1+2=3 scenario. The formula escapes me but is a little more complicated.
>
> It's quite a bit more complicated. You're right in that you can't just add
> the voltages. My guess is that if an effective short occurs because of what
> I just said above, then the voltage the trains will see will be the smaller
> of the two voltages being applied by the controller (the bit that's left
> after the difference has shorted). But then how much current the trains
> can draw from that will be different from what they could draw from just
> one controller at that voltage, because you're having to combine the
> internal resistances of the controllers - which are in parallel.
>
> Simon
> http://www.SimonRobinson.com
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Message has 2 Replies:
 | | Re: Electrical Things (was: Railroad Dilemma)
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| (...) I'm not sure I agree that the voltage is variable. People have reported (via oscilloscope analysis) that it's pulse width modulated 9V instead. This will read on an analog meter like a low voltage, because it's taking the average across time. (...) (25 years ago, 8-Mar-99, to lugnet.trains)
| | | Re: Electrical Things (was: Railroad Dilemma)
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| (...) That is because there is a diode to protect from reverse voltage and the voltage drop is about 0.3V (...) The maximum current is about 0.7A MAXIMUM ( maximum voltage and whell not turning) with one controller (...) the internal resistance will (...) (25 years ago, 8-Mar-99, to lugnet.trains)
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Message is in Reply To:
| | Re: Railroad Dilemma
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| (...) I suspect the short would happen even if the two controllers have the same polarity, but are set to different speeds. I'm guessing it's the voltage they put out which increases as you up the speed. If that's the case and you have the (...) (25 years ago, 25-Feb-99, to lugnet.trains)
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