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> >
> > > If you are referring to Tom's L-Gauge page, the resistor is
> > > used for the headlight to lower the ~12V DCC voltage to 9V
> > > for use with a standard Lego lamp brick. I didn't need to
> > > use this resistor since the RCX is already putting out about
> > > 9V.
>
> A diode, iirc, allows DC current to 'flow thru' in only one direction. What
> would a diode do across the output on the top of the train motor? And why would
> one be good for 'inductive' loads such as a LEGO motor?
>
> I like learning--use small words so i unnerstand please :)
>
> Dave K
Sorry, my earlier post should have been more clear. I was just commenting on the
fact that in general a resistor is _not_ a good way to reduce voltage to a train
motor. And you recalled correctly: diode's are "one-way". Thats why you need to
put them anti-parallel in pairs if you want to reduce voltage so it's
"two-way"again.
In that case the diodes aren't placed on top across the outputs, but in series
with the motor as the point was to reduce voltage. (Putting diodes across the
outputs on top is just lowering the voltage for the shorted circuit..)
one diode:
----[ I]----
current can only "go" to the right
two diode's anti-parallel:
---[ I]---
----| |----
---[I ]---
current can flow in both directions.
As for why diodes are better than resistors: diodes are pretty consistent in
their voltage drop and dont vary (much) with current, resistors however are not
at all consistent in their voltage drop but change with changing current.
Say you've got a resistor in series with the motor so 12V is lowered to 9V (at
cruise speed on a straight).
Suppose your motor is now pulling twice the load in carriages, going uphill or
even through curves(!) and thus drawing more current. The voltage "taken away"
by the resistor will increase, suppose to 6V, leaving only 6V for the motor
while it's doing hard work/struggling. In the more extreme cases this can lead
to no or very little voltage being available for the motor. Resistors will also
cause a different voltage drop for different motors, but since this only
concerns lego motors which afaik are pretty equal that shouldnt be a problem
here.
Aside from the above there's the power/heat dissipation problem. In the above
example the resistor can be quite easily damaged since it needs to dissipate
quite an amount of power and is often undersized in that respect.
I hope these words were small enough. ;)
Esger
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Message has 1 Reply:
 | | Re: DCC for the RCX
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| In lugnet.robotics.rcx, Esger Abbink wrote: <snip> (...) Perfectly understood. Thanks! My problem has always been that I wanted to get into electronics, but all those resistors 'n such used to have all these coloured stipes to denote the different (...) (21 years ago, 30-May-03, to lugnet.robotics.rcx, lugnet.trains)
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Message is in Reply To:
| | Re: DCC for the RCX
|
| (...) I'm no electronics engineer by any stretch-- A diode, iirc, allows DC current to 'flow thru' in only one direction. What would a diode do across the output on the top of the train motor? And why would one be good for 'inductive' loads such as (...) (21 years ago, 29-May-03, to lugnet.robotics.rcx, lugnet.trains)
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