E
ehsjr
Guest
I helped a friend's 10 year old son build a science fair motor:
a coil of wire (20t around a pill bottle former) supported
above a magnet by paper clips with the tops bent into a U.
The coil has #14 copper soldered to each end, looks like this:
**
___* *___
* *
**
The construction is such that the #14 is rigidly attached
to the coil, so it acts as the axle as well as the conductor.
A D cell makes the thing spin nicely. (Usually you have
to start it spinning by hand.)
The lad asked "how long will the battery last?" The problem is
that, as it spins, it arcs & the current is interrupted often
and with no repetitive pattern. In addition, the arcing burns
the paper clip & wire which eventually stops the motor and
(I assume) lowers the current draw over time as the motor
runs.
He could just run the thing until the battery is "dead", where
dead is some chosen terminal voltage, and note the elapsed
time. That is the "science fair" answer we went with for the
"how long" question. But, from a practical viewpoint, how would
he measure the current? My knee jerk reaction is a series
resistance to develop a voltage & charge a cap with a DMM
measuring the voltage. Then, assuming worst case, he could
predict a minimum run time. Is that approach valid? (I realize
that with intermittent contacts and resistance changing, the
battery won't discharge as quickly as worst case predicts.)
Bear in mind that he is a 10 year old who needed help with
this "electrical project", so he's not a computer wiz kid
who can whip up an interface and take a whole bunch of samples
over time, record them with a PC & spit out a prediction.
A "side note" - the paper clips have been replaced by a couple
of small angle irons screwed to a piece of wood. Paper clips
scotch taped to the lid of a plastic container can't take the
"handling" of a 10 year old boy.
Ed
a coil of wire (20t around a pill bottle former) supported
above a magnet by paper clips with the tops bent into a U.
The coil has #14 copper soldered to each end, looks like this:
**
___* *___
* *
**
The construction is such that the #14 is rigidly attached
to the coil, so it acts as the axle as well as the conductor.
A D cell makes the thing spin nicely. (Usually you have
to start it spinning by hand.)
The lad asked "how long will the battery last?" The problem is
that, as it spins, it arcs & the current is interrupted often
and with no repetitive pattern. In addition, the arcing burns
the paper clip & wire which eventually stops the motor and
(I assume) lowers the current draw over time as the motor
runs.
He could just run the thing until the battery is "dead", where
dead is some chosen terminal voltage, and note the elapsed
time. That is the "science fair" answer we went with for the
"how long" question. But, from a practical viewpoint, how would
he measure the current? My knee jerk reaction is a series
resistance to develop a voltage & charge a cap with a DMM
measuring the voltage. Then, assuming worst case, he could
predict a minimum run time. Is that approach valid? (I realize
that with intermittent contacts and resistance changing, the
battery won't discharge as quickly as worst case predicts.)
Bear in mind that he is a 10 year old who needed help with
this "electrical project", so he's not a computer wiz kid
who can whip up an interface and take a whole bunch of samples
over time, record them with a PC & spit out a prediction.
A "side note" - the paper clips have been replaced by a couple
of small angle irons screwed to a piece of wood. Paper clips
scotch taped to the lid of a plastic container can't take the
"handling" of a 10 year old boy.
Ed