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How can I measure resistance of an electronic device?
Let's say I have a motorized toy. How do I go about measuring the electrical resistance of the toy? (I have a multimeter) The whole plan is to test various batteries, so using Ohm's law I can calculate the power output of batteries knowing the voltage and resistance
OK, so how can I measure the power output of AA batteries in general?
Also, how can I measure the current of the toy with only access to the battery terminals?
The resistance characterizes something, which obeys
Ohm's law, for example a coil of wire, or a resistor.
Ohm's law states that in such simple case voltage is
proportional to current, and coefficient is called
"resistance": U = IR
Electronic devices never obey Ohm's law.
Firstly, their current consumption is not proportional
to voltage. Secondly, you can alter current consumption
_without altering voltage. For example if you increase
the volume of your walkman, the current consumption
will increase, while voltage will actually slightly decrease.
For all the above reasons you cannot measure "resistance"
of your toy, because it does not have "resistance".
However, you can measure power consumption P = UI.
The most straightforvard way is to measure both
resistance and current _simultaneously.
In order to do this you need two multimeters.
One should be set into voltage measuring mode, and
its terminals must be connected to the terminals of
battery in parallel.
The current part is more tricky. I suggest that you take
two strips of aluminum foil, and sandwich insulating
piece of scotch between them. The sandwich will
resemble capacitor. Attach the wires of the second
multimeter (set it to current mode) to the strips of foil,
and shove the 'sandwich' in the slit between the terminal
of the battery and the terminal of the toy, so that current
will flow through the second multimeter.
Now you are all set. Turn your car uside down, and let
its weels idle in the air. Measure the voltage U and
current I, and calculate the power. Then apply some
friction to the weels, to load and slow down them.
Make the measurements gradually increasing friction
until the weels can no longer rotate. Plot voltage U,
current I and power P=UI as a function of friction,
or speed of rotation. If not for fun, it will make a
nice science fair presentation.
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