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Lecture 251 DC Measurements

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Lecture 252 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.

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Lecture 253 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.

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Lecture 254 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.

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Lecture 255 A Generic Digital Meter Input Switching and Ranging Amplifier A/D Converter ProcessorDisplay

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Lecture 256 Voltage Measurements HiCom 10V 1V 100V

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Lecture 257 Voltage Measurements For voltage measurements, a more detailed model of the first three meter components looks like this: + - V in 10G + - V in A/D Converter 12VDC Hi Com

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Lecture 258 Voltage Measurements 10V setting: Input resistance of amplifier is > 10G Gain of amplifier is 1 + - V in 10G + - V in A/D Converter 12VDC Hi Com

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Lecture 259 Voltage Measurements 1V setting: Gain of amplifier is 10 (we learn how to make an amplifier with gain of 10 in Section 3.3) + - V in 10G + - 10V in A/D Converter 12VDC Hi Com

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Lecture 2510 Voltage Measurements 100V setting: Input resistance of meter is 10M Gain of amplifier is 10 + - V in 10G + -10V in A/D Converter 12VDC Hi Com 9.9M 100k

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Lecture 2511 Model for Meter The ideal meter measures the voltage across its inputs. No current flows into it; it has infinite input resistance. 10M Ideal Meter Hi Com

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Lecture 2512 10M Ideal Meter Hi Com R Meter Loading The 10M meter resistance in parallel with R may change the voltage that you measure.

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Lecture 2513 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 10G 10M , significant current flows through the meter, changing the voltage across R.

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Lecture 2514 Loading Example Without Meter: voltage is 100V With Meter: measured voltage is 83.3V 10M Ideal Meter Hi Com 2M 50 A

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Lecture 2515 Current Measurements AmpCom 10V 1V 100V

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Lecture 2516 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.

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Lecture 2517 Measuring Current Small Currents (< 100mA) –An amplifier is configured to convert the current into a measurable voltage. –This is called a virtual ground configuration.

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Lecture 2518 Shunt Resistor The shunt resistor of known resistance R s is placed into the current to be measured: + - V in 10G + - 100V in A/D Converter 12VDC Amp Com RsRs

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Lecture 2519 Shunt Resistance What value of R s should you use to get a full range reading with 1A? What value of R s should you use to get a full range reading with 100mA? What value of R s should you use to get a full range reading with 10mA?

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Lecture 2520 RsRs Ideal Meter Amp Com R Meter Loading The R s shunt resistance in series with R may change the current that you measure.

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Lecture 2521 Measuring Resistance The known current I s creates a voltage across R which is measured. IsIs Ideal Meter Hi Com Unknown R

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Lecture 2522 Measuring Resistance What value of I s should you use to get a full range reading for 100 ? What value of I s should you use to get a full range reading 10k ? What value of I s should you use to get a full range reading with 1M ?

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