Model Trains and Oscilloscopes?!?

[JTrains]

I’ve been away from the forum for a few months – things around home, but also I’ve taken a bit of an interest in learning more about electronics, largely as a result of my various questions and such on this forum.  Based upon this, a couple of train-related questions (or, more accurately, propositions I’d like to confirm) for those more electrically inclined:

1)A Lionel transformer is essentially an isolation transformer.

2)The output of said transformer has no reference to earth ground.

3)Thus, connecting the transformer’s “common” to earth ground (say, through the ground in a correctly-wired outlet) has no potential – hence, it would neither hurt nor help anything.

4)If one wanted to probe parts of one’s layout with an oscilloscope in place of a voltmeter, connecting the ground/shielding of the probe (which invariably connects to the chassis of the oscilloscope, which is in turn invariably connects to earth ground) to our transformer’s common would be safe and result in the readings one wants.

And, yes – I bought a used analog HP oscilloscope of Da Bay yesterday and am getting ready for when it is delivered next week…

[lionelsoni]

1)A Lionel transformer is essentially an isolation transformer.

As the Wikipedia article says, "Power transformers with specified insulation between primary and secondary are not usually described only as 'isolation transformers' unless this is their primary function. Only transformers whose primary purpose is to isolate circuits are routinely described as isolation transformers."  That being said, it is true that a traditional toy-train transformer does indeed provide essentially the same kind of isolation as a purpose-built "isolation" transformer.

2)The output of said transformer has no reference to earth ground.

True.

3)Thus, connecting the transformer’s “common” to earth ground (say, through the ground in a correctly-wired outlet) has no potential – hence, it would neither hurt nor help anything.

There would be no electrical consequence from grounding the common of the transformer as long as everything is working as it should.  However, numerous electrical appliances have their exposed metal parts grounded, through the round equipment-ground pin in the plug.  The reason is to trip the circuit breaker (or blow the fuse) if those exposed parts come in contact with a dangerous voltage.  This is a reason why you might want to ground the common, to insure that the many exposed rails of the layout cannot become a shock hazard and it is why I have mine grounded.

4)If one wanted to probe parts of one’s layout with an oscilloscope in place of a voltmeter, connecting the ground/shielding of the probe (which invariably connects to the chassis of the oscilloscope, which is in turn invariably connects to earth ground) to our transformer’s common would be safe and result in the readings one wants.

The probe return and the oscilloscope circuit common are not invariably grounded.  My (elderly) oscilloscope has a pivoting ground pin on its plug, which can be swung out of the way to allow the whole thing to float and allow some voltage other than ground to be the reference for the oscilloscope's measurements.  Oscilloscope users often improvise adapters when it is necessary to unground the instrument.

[Lionel 027]

Just out of curiosity, what are you looking for with your oscilloscope? Or is it just a "science project?"

[JTrains]

Bob - thanks for your reply.  I should indeed have been more clear about the possibility of folks floating their scope - with all the exposed chassis on one, that would definitely call for some care.

I'm now the proud owner of an HP 1743A which seems to work great.  We'll see what we can learn about the world with it.  Last night I had Noah stumming a guitar and watching the waveforms on the screen - pretty cool.

Lionel 027

Just out of curiosity, what are you looking for with your oscilloscope? Or is it just a "science project?"

Nothing specific, and probably not much from a train perspective.  I did hook up a small 4150 transformer to it to see how clean the output voltage was (pretty smooth sinusoidal, just a very small hitch on the falling edge).  I'm about to start repairing some old tube radios - next up to buy is a real 1:1 isolation transformer...