From: dav@crosfield.co.uk (david frith)
Newsgroups: rec.models.railroad
Subject: Relco unit details was Re: High voltage pulsing?
Date: 2 Sep 92 10:20:42 GMT
Organization: Crosfield Electronics, Hemel Hempstead, United Kingdom.

In article <1992Aug27.202532.863@cbnewsl.cb.att.com> sw@cbnewsl.cb.att.com (Stuart Warmink) writes:
>In article <1992Aug26.164822.3140@nmt.edu>, al@nmt.edu (Al Stavely) writes:
>> 
>> Does anyone know any details about what [the RELCO unit] does, and what kind
>> of circuit is inside?  I'd especially like to know what kinds of throttles is
...
>I bought one many years ago when I lived in England; I seem to remember
>that it applied a high frequency, high voltage burst whenever dirt was
>detected on the track.  How it detected such dirt I do not know - I saw
>a demo on TV that was very convincing though.  However, I used a transistor
>throttle at the time and seem to remember reading that it could not be used
>with such a unit - or perhaps not without additional external components.
>Please correct me if I am wrong though - perhaps it was related to switching
>the track voltage instead?
>
>Anyways, I am as interested to find info as Al is - the more so since
>I have a RELCO unit but have lost the documentation and hence don't even
>know how to wire it into the power supply...
>-- 

The Relco unit contains a small transformer and a high frequency oscillator.
The transformer is fairly lossy and not able to produce any voltage into
low resistance loads (ie the locomotives motor) but as soon as a dirty
piece of track is encountered, the load resistance becomes very high and
the output (secondary) voltage of the transformer rises to several hundred
volts (lighting its internal neon light and applying the voltage across the
high resistance dirt to vapourise it). The high frequency is used to keep
the size of the transformer small (indeed its a few hundred turns on a
ferrite pot core in my Relco).

It is usually wired up with the input terminals connected to 12v AC or DC and 
the output terminals connected in series with the controller. It is possible
to connect the input terminals across the output of the controller 
if no separate 12v is available but this is less effective (as the loco
usually stalls on dirt at low speeds when the input voltage is also low).

There is no safety problems with the several hundred volts as the current is
so low you wouldn't even feel it (don't try it on your tongue though!). Your
finger has a low enough resistance to stop it from working by placing it
across the track (the neon goes out or at least dims). It is possible that
the high voltage spikes could get back into the controller but at the low
currents here they are unlikely to cause any problems. (However watch out
if you've used any unprotected CMOS IC's in the feedback loop as these
may be sensitive to the high voltage as well as static).

I've used it on an N guage layout (a few years ago now). Although the neon
flashed on regularly (hopefully cleaning the track as the loco passed) it 
wouldn't cope with very dirty track or track that was badly laid (twisted
points or some of my insulated frog points) where stalls had to be cleared
by hand or full volts!

I had a circuit once for a similar device (not the Relco) but I've probably
lost it by now.
-- 
                                              Those that search for God
David Frith           Crosfield Electronics   at the 11th hour may find
dav@crosfield.co.uk   +44 442 230000 X:4084   that they die at 10.30.
Newsgroups: rec.models.railroad,rec.railroad