Lecture 6: Chapter 2, Part 4
Matthew Shuman
1
1.1
Measuring Voltage and Current — Not in Text
Ammeter
An ammeter is used to measure current. If the gauge is 50mV at 1mA, what is the value of R
necessary to measure 2A?
Figure 1: This is how an analog ammeter is constructed
1.2
Voltmeter
A voltmeter is used to measure voltage. If the gauge is 50mV at 1mA, what is the value of R
necessary to measure 20V?
Figure 2: This is how an analog voltmeter is constructed
1.3
Digital Multimeter
Dissection of a DMM during lecture.
1
2
Measuring Resistance - The Wheatstone Bridge — Not in
Text
A Wheatstone Bridge is useful for measuring variable resistances. There are three types of variable resistors.
1. Thermistor
2. Photoresistor
3. Strain Gauge
Figure 3: A Wheatstone Bridge is just two voltage dividers
ENGR 201
3
Delta-to-Wye (Pi-to-Tee) Equivalent Circuits — Section 2.7 in
Text
Combining the resistors in a Wheatstone Bridge does not work, because there are no resistors in
simple parallel or series combinations. A Wye-to-Delta or Delta-to-Wye conversion is necessary
for this simplification.
Figure 4: This shows the labeling of resistors in a wye-delta transformation.
Use these equations to convert Y to ∆.
Ra =
R1 R2 + R2 R3 + R3 R1
R1
(1)
Rb =
R1 R2 + R2 R3 + R3 R1
R2
(2)
Rc =
R1 R2 + R2 R3 + R3 R1
R3
(3)
R1 =
Rb Rc
Ra + Rb + Rc
(4)
R2 =
Ra Rc
Ra + Rb + Rc
(5)
R3 =
Ra Rb
Ra + Rb + Rc
(6)
Use these equations to convert ∆ to Y.
ENGR 201