04-08-2012, 12:41 PM
Level 1 Laboratory – Autumn Semester Experiment EL1 – Resistor circuits and load matching
circuit_loading_07.pdf (Size: 28.6 KB / Downloads: 52)
AIM
To become familiar with resistor networks and to be aware of the maximum power dissipated
in a load resistor. The experiments will also illustrate how circuit loading can give rise to
systematic errors in electrical measurements and will provide experience in soldering basic
electronic circuits and the use of the multimeter.
EXPERIMENT
Set up the circuit below. You have been provided with some veroboard on which to assemble
your circuit and a bag of resistors of differing values. Use the power unit supplied.
Part 1 – Instrument loading
In this experiment R1 and R2 should each initially be fixed to a value of 10kΩ. You will need
to refer to the resistor colour-code guide to select the correct resistors (check with a
multimeter or ask a demonstrator if you are unsure). Once you have assembled the circuit and
soldered the components onto the veroboard (taking care to avoid short-circuits!), measure
the voltage across R1 using the analogue (not digital) voltmeter. You should also verify the
supply voltage, V0, by measuring the voltage across the two resistors.
Load matching and resistor networks
Using the resistors provided, keeping R2 fixed at 10kΩ vary the value of R1 in your circuit
(NB: The number of data points may be increased by using series and parallel combinations
of resistors for R1.). Measure the voltage across R1.
Use the relationship
R
P V
2
= to calculate the power developed in R1 as a function of R1.
Plot a graph of P against R1 using the log-lin graph paper provided. It is best to do this during
the experiment to help identify where more data points (and hence resistor combinations)
would be beneficial.
What features does your graph show? At what value of R1 is P a maximum? What is your
uncertainty in this value? Explain why the power approaches zero as R1 tends to very small
and very large values.
Noting that in the above expression, R=R1+R2, mathematically derive the relationship
between R1 and R2 to maximize the power dissipated by R1. (Hint: this involves calculus!).
Does this agree with your experimental results?
Computer simulation of electronic circuits (if time allows)
Crocodile ClipsTM is a relatively straightforward time-domain based circuit simulation
package installed on the PCs at the back of the undergraduate laboratory. Ask one of the
technicians to provide you with the Crocodile clips manual. Attempt the following:
1. Run through some of the tutorial example circuits to familiarize yourself with the
software.
2. Set-up a simple resistor load circuit as per Part 2 of this experiment. Keeping R2 fixed,
using probes, measure the current flowing through the circuit and the voltage across
R1 as a function of R1. Is the behaviour consistent with your earlier experiments?
3. Add additional series and parallel resistors. By probing the node voltages and branch
currents, explore Kirchoff’s current and voltage laws (conservation of charge and
conservation of energy, respectively) in simple resistor networks.