Lecture 251 DC Measurements

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.

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.

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.

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

Lecture 256 Voltage Measurements HiCom 10V 1V 100V

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

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

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  V in A/D Converter  12VDC Hi Com

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

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

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

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.

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

Lecture 2515 Current Measurements AmpCom 10V 1V 100V

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.

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.

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

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?

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.

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

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  ?