"Joerg" wrote in message
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"Joerg" wrote in message
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Oppie wrote:
"ian field" wrote in message
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"Oppie" wrote in message
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"ian field" wrote in message
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Does anyone know the fancy/technical name for a model railway track
cleaner that's based on flyback pulses from a blocking oscillator to
break down oxide and crud on the rails?
Googling "model railway+track cleaner" produced a few examples of
commercial units but none were referred to by the technical name, I
need some schematic examples to base a similar gadget to clean switch
contacts in a 12V system that occasionally get corroded.
TIA.
Will this help you?
http://freespace.virgin.net/c.gardner/semg/relco.htm
also http://www.merg.org.uk/resources/faq.htm
Track cleaner A device for cleaning track so that dirt or other
deposits do not impede the flow of electric current. A number of
methods exist to accomplish this. MERG Technical Bulletin A4/1
features the Relco Track Cleaner which superimposes a high frequency,
high voltage signal over the traction current. This burns through
track deposits when traction is interrupted. Mechanical abrasive
rubber-type products are also available such as the Peco PL41 Rail
Cleaner and the MERG kit 83, PCB and Track Cleaner. These must be
rubbed over the track to clean it. A variety of solvents applied with
a clean cloth are also suitable as are a number of wagon-mounted
abrasive blocks. A popular MERG method is to polish the rail tops
with the rough surface of common hardboard.
July 2006
That's one of the pages I found googling and it doesn't mention the
technical name for an electronic track cleaner - I'll have to phone the
local model shop and ask them.
From what I can make of it, the device monitors the load and when it
drops out, superimposes HF pulses on top of the DC feed. Sounds similar
to the arc lamp ignitors I work with - a 20 turn winding in series with
the DC feed to the lamp (track) and a 2 turn primary fed by a spark gap
connected to a 600V charged capacitor. Spark gap breaks over and dumps
capacitor into primary. Nice sparks on the secondary!
Can't imagine this is too kind to any electronics installed in the
layout but then again, probably doesn't need the 14kv that my lamps
need.
When I was a kid I built myself something similar for a Fleischmann model
railroad. Those are DC operated. I don't have the circuitry or schematic
anymore but basically I ran the pulses all the time. They got shorted out
as long as the locomotive made contact and the spike voltage only ran up
when dirt got in between.
I built a simple pulse generator with Ge transistors (and blew quite a
few in the process), a few Hertz, then hung a reversed transformer from
an old tube radio in series with the variable transformer output.
Didn't the resistance of the transformer's primary drop most of your
available suply voltage when the train was drawing (tried to draw)
traction current?
Well, sure, but read on.
Since
I had also built the regulator myself it compensated for the DC drop
across the added transformer winding. Well, to some extent because I had
no clue about loop stability and all that back then. Guess I lucked out.
Of course, at that age I was blissfully unaware of EMC and all that
stuff. So don't do this at home ;-)
I was just a highschool kid so most of the time I scavenged schematics
from books in the library and combined things I found. There, they had a
nice book about railroad electronics with an electronically stabilized
"Fahrstromtransformator", that's the usual rheostat-type transformer you
could buy in those days and they had not electronics back then. Now, I
looked at the circuit and gasp placed the pulse transformer winding
between the emitters of the pass transistors and where the feedback loop
taps off. No reverse Vbe diode, nothing. In fact I didn't even know what
reverse Vbe is. I consider it a miracle that the whole thing never went
unstable and smoked out the motor of the rather expensive locomotive I had
on the track. I am also still baffled by the fact that it didn't melt down
the base-emitter junctions.
Then I caused a really stupid blunder: The old Fleischmann transformer had
a fixed voltage output for street lamps and such. Running that big green
thing next to my new sleek electronic version wouldn't look good. I always
created fancy enclosures, blending in with the surroundings. So, looking
at the schematic, tada! I figured I could just tap the secondary and
that'll be it. Drilled a couple holes, mounted cable posts so it'll look
neat and professional. It didn't occur to me that it had a much higher
voltage because of the regulator overhead and ... flash - POOF. At least
$20 worth of bulbs, gone. This was in early-70's Dollars, very painful.
--
Regards, Joerg
http://www.analogconsultants.com
If I wanted it for model railway use, one of the first things I'd try to get
away with is a couple of capacitors on one side of the rectifier to make it
a voltage doubler - not sufficient capacitance to significantly increase
traction current but enough to double the peak voltage when crud insulates
the wheels from the track, say about 0.22uF mylars. The rheostat was usually
a simple variable resistor (not a potential divider) so the doubled peak
voltage would be applied when contact was lost, at any rheostat setting.