HOW DO YOU MEASURE THAT?
In this lab, we will try to demystify a couple of black
boxes that you may have already been working with. This will be an exercise
not only in basic electronics, but also in experiment design: I will give
you very sketchy directions, and ask you to design most of these experiments
yourself. Your task is to better explain how an autoranging ohmmeter, an
ammeter, and an autoranging capacitance meter work.
How do you make an analog voltmeter out of a galvanometer
and resistor? An analog ammeter? Ohmmeter?
What is the resistance of a digital voltmeter on various
of its scales? If it is an autoranging voltmeter, what determines when
it switches scales? What is a typical current flow through a digital voltmeter?
Compare this to the theoretical value for an ideal voltmeter.
Any ohmmeter works by measuring the ratio of voltage drop
across a circuit element to the current through it, but does a digital
ohmmeter send a constant current signal or a constant voltage signal for
a given resistance scale? Is it a DC signal or a square wave or a sine
wave? If it is autoranging, what is the criterion for the meter to decide
to change scales?
An analog volt- or ammeter consists of a galvanometer
(which measures current) connected to either a shunt resistor (ammeter)
or a series resistor (voltmeter). For electronic measurements, however,
it is easier to measure a voltage directly than a current. How do you make
an ammeter using just a voltmeter and resistor? Find the effective resistance
of an ammeter on various current ranges. Calculate the maximum voltage
across the meter for each range.
How does a capacitance meter do its thing? A capacitor
has an impedance, Zc=1/2pfC, so does
one measure its voltage divided by current at some AC frequency? The voltage
across the capacitor is proportional to the charge, V=Q/C, so does
one send a known amount of charge and then measure the voltage? A capacitor
in series with a resistor has a time constant, t=RC,
does one measure how slowly the voltage on the capacitor decays? Make some
measurements to determine how the capacitance is actually being measured.
You will probably want to do this measurement in the "time domain". What
kind of equipment will you need to do this?
Return to 222 Lab syllabus