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.

LIBRARY:
How do you make an analog voltmeter out of a galvanometer and resistor? An analog ammeter? Ohmmeter?

LABORATORY:
VOLTMETER:
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.

OHMMETER:
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?

AMMETER:
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.

CAPACITANCE METER:
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, so 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?



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