Lecture 251 DC Measurements. Lecture 252 DC Measurements DC Measurements include current, voltage,...
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Transcript of Lecture 251 DC Measurements. Lecture 252 DC Measurements DC Measurements include current, voltage,...
Lecture 25 2
DC Measurements
• DC Measurements include current, voltage, resistance, and power.
• Section 2.8 should be titled “DC Measurements: a Historical Perspective.”
• A digital multimeter is one type of sensor system.
Lecture 25 3
Digital Meters and Oscilloscopes
• Most multimeters and oscilloscopes are now digital.
• A digital multimeter or a digital oscilloscope has an analog-to-digital (A/D) converter.
• Most digital meters and all digital oscilloscopes have one or more processors.
Lecture 25 4
Data Acquisition Systems
• In many applications, digital meters and scopes are being replaced by data acquisition cards that fit into a computer.
• The data acquisition cards have A/D converters.
• The computer provide processing and storage for the data.
Lecture 25 5
A Generic Digital Meter
Input Switchingand Ranging
Amplifier
A/D Converter
ProcessorDisplay
Lecture 25 7
Voltage Measurements
• For voltage measurements, a more detailed model of the first three meter components looks like this:
+
-
Vin 10G +
-Vin
A/DConverter12VDC
Hi
Com
Lecture 25 8
Voltage Measurements
10V setting:
Input resistance of amplifier is > 10GGain of amplifier is 1
+
-
Vin 10G +
-Vin
A/DConverter12VDC
Hi
Com
Lecture 25 9
Voltage Measurements
1V setting:
Gain of amplifier is 10 (we learn how to make an amplifier with gain of 10 in Section 3.3)
+
-
Vin 10G +
-10Vin
A/DConverter12VDC
Hi
Com
Lecture 25 10
Voltage Measurements
100V setting:
Input resistance of meter is 10MGain of amplifier is 10
+
-
Vin
10G+
-10Vin
A/DConverter12VDC
Hi
Com
9.9M
100k
Lecture 25 11
Model for Meter
The ideal meter measures the voltage across its inputs. No current flows into it; it has infinite input resistance.
10M Ideal MeterHi
Com
Lecture 25 12
10M Ideal MeterHi
ComR
Meter Loading
The 10M meter resistance in parallel with R may change the voltage that you measure.
Lecture 25 13
Loading
• When measuring the voltage across R, we need to make sure that R is much less than 10G or 10M, depending on the meter setting
• If R is close to 10G10M, significant current flows through the meter, changing the voltage across R.
Lecture 25 14
Loading Example
• Without Meter: voltage is 100V
• With Meter: measured voltage is 83.3V
10M Ideal MeterHi
Com2M50A
Lecture 25 16
Measuring Current
• Large Currents (> 100mA):
– The current to be measured is passed through a small resistor (called a shunt resistor) and the resulting voltage across the shunt resistor is measured.
– From the voltage, the current can be computed.
Lecture 25 17
Measuring Current
• Small Currents (< 100mA)
– An amplifier is configured to convert the current into a measurable voltage.
– This is called a virtual ground configuration.
Lecture 25 18
Shunt Resistor
The shunt resistor of known resistance Rs is placed into the current to be measured:
+
-
Vin10G +
-100Vin
A/DConverter12VDC
Amp
ComRs
Lecture 25 19
Shunt Resistance
• What value of Rs should you use to get a full range reading with 1A?
• What value of Rs should you use to get a full range reading with 100mA?
• What value of Rs should you use to get a full range reading with 10mA?
Lecture 25 20
Rs Ideal MeterAmp
ComR
Meter Loading
The Rs shunt resistance in series with R may change the current that you measure.
Lecture 25 21
Measuring Resistance
The known current Is creates a voltage across R which is measured.
Is Ideal MeterHi
Com
Unknown R