Lab Exercise 1: DC circuits - Fysikum

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7 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Elektronikens grunder FK2001


Fysikum

Lab Exercise 1: DC circuits


Introduction

In this lab, the concepts of resistive networks and Thevenins theorem are explored, as
well as measurement techniques for DC circuits. You will also become familiar with
some of the test equipment in the lab.

Prepa
ration

Before arriving at the lab, you should do the following:




Review the lecture notes and handouts on DC circuits and Thevenins theorem.



Read through the lab, and do the calculations in each section.

Lab setup

You will need the following equipment:




A
breadboard for constructing circuits



A DC voltage supply



A digital multi
-
meter for voltage and current measurements



A variable resistor

Exercises

1.

Voltage measurement


The above circuit is a simple voltage divider. Assuming that t
here is an open
connection at Vout, calculate the expected output voltage (show your work):


Vout =

Elektronikens grunder FK2001


Fysikum

Construct the voltage divider with a DC power supply and two 22k resistors. Measure
Vout with a digital multi
-
meter and record the result:



Vout =



Replace the 22k resistors with 5,6 M
Ω
resistors. Again, measure and record Vout:



Vout =



Did you get the same result?


Your digital multi
-
meter has a finite input resistance, typically 10M
Ω
. For ordinary
circuits with resistances in the range of a few
k
Ω
this is large enough that we can treat
it as an open circuit. But measuring circuits with much larger resistance (or to put it
another way: low currents) requires more care.


The equivalent circuit for your measurement of the 5.6 M
Ω
divider is:



Using Kirchoffs rules, assume that the multi
-
meter has a 10M
Ω
resistance, and
calculate the expected value for Vout. Does it agree with your measured value?

Elektronikens grunder FK2001


Fysikum

Thevenin voltage and resistance


Using Kirchoffs r
ules, calculate Vth and Rth of the above circuit, measured at Vout:








In the lab, build the circuit and measure the Thevenin voltage:


Vth =


The naïve method to find Rth would be to short the output terminals and measure the
current. But in practice
this not usually a good idea. Shorting the output of an
unknown device can draw large currents, potentially damaging something inside. A
better method: connect the volt
-
meter and a variable resistor in parallel across the
output terminals as shown:



Adjust the variable resistor until Vout is reduced to one
-
half of Vth. Now measure the
load resistance with an ohm
-
meter (the resistor should be disconnected and/or the
power supply turned off when you do this!) What is the Thevenin
resistance?


Rth =