Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: The "typical" high school physics teacher



Herb, et. al.,

It's interesting that my teacher (clear back in 1967) used that exact
example... a toy train.

No. Transformers do not convert AC to DC. Power supplies or power packs
etc. can do that, but not a transformer alone. The power supply at least
has a rectifier, and maybe a capacitor, and maybe a voltage regulator. The
word transformer should be reserved for the actual transformer itself.

Today, it is common for many toys to operate from DC, and the "power pack"
indeed supplies low voltage DC. That's because DC motors are easy to build,
and today rectifiers and voltage regulators are inexpensive.

When I was in high school, rectifiers were more expensive, and solid-state
voltage regulators did not exist. It was less expensive to put an AC motor
in the train than to make a DC power supply. I'm quite sure the typical
model train operated with AC motors in the 1960's (I was an enthusiast).
The "power packs" were just transformers... not "DC power supplies."

In fact, my teacher actually used this idea as his "proof"... if the
transformer didn't produce DC, the train wouldn't run, it would just sit
there and vibrate. I told my teacher that model trains have AC electric
motors in them. He said that was ridiculous because there wasn't such a
thing as an AC motor; motors need DC. That's the point I told him he didn't
know what he was talking about. That's when I was asked to leave class.

* * * *
FYI, AC-motor model trains had at least two interesting problems because of
the use of AC motors:

(1) Reversing direction was a problem. You couldn't just reverse the
polarity of the power on the tracks. Instead the trains had latching relays
(in the engine or coal car) to switch the phasing to the AC motor. Upon
turning-off power and turning it back on the relay released then re-latched
into the other direction. If you stopped the train and restarted it, it
would restart backwards. If you wanted to stop then restart forward, you
had to stop, pulse power on then off, then restore power to go forward. The
on/off pulse moved the relay through the reverse latch and back to forward
again.

(2) AC motors have low starting torque. This made it difficult to start the
train slowly and also difficult to run it slowly. This means realistic
starting was not easy with model trains unless they had fairly big motors
and good gearing (i.e. fairly expensive). Today, with
strong-permanent-magnet DC motors the motors can easily be run at low
speeds, and model railroad operation can be a lot more realistic. I quit
doing model trains when I graduated from HS, but when my kids were younger I
bought my first DC train (I mean my kid's got their first train). I was
simply amazed how nicely it operated with its DC motor and adjustable DC
power supply.


Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817


On the other hand, your teacher might have been correct. Don't some
transformers, such as those that operate toy trains, convert 120 Vac into
a low voltage dc that is connected to the tracks and accessories ?

If so, you should have told the teacher that most transformers will not
convert AC to DC but those that operate toy trains are an exception.

Herb Gottlieb from New York City
(Where people used to give low-voltage DC toy electric train sets
to children for Christmas)