digital meters - 7 - 2014
TRANSCRIPT
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CONCEPTS TO BE EXAMINED
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- Digital multimeterscontd
- Resistance, semiconductors and capacitortesting
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DIGITAL MULTIMETERS
- To measure currents, DMMs alsoincorporate shunts
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- Also load the circuit under test- Convert ac to dc and obtain thecorresponding average value
- True rms readings consider harmonics inthe supply voltage or current
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- For a composite waveform, the DMMindicates the appropriate quantity based onthe selector switch.
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- Measure resistance by using a currentsource to provide a known currentthrough an unknown resistance. Thevoltage developed and Ohms Law areused to determine the unknownresistance.
contd
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CURRENT TO VOLTAGE
CONVERSION
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RESISTANCE MEASUREMENT:
ALTERNATIVE METHOD
This works on the voltage divider
principle: Vo= (R1/RT).E
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RESISTANCE MEASUREMENT:
ALTERNATIVE METHOD II
This works on the principle that:
Vo= -(R
x/R
1).E
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- In the field and the design environment,users encounter low resistances in devices
and circuits that include contacts found inswitches, relays, and connectors
LOW RESISTANCE
MEASUREMENTS
- Likewise, users find low continuityresistances in PCB traces and cables.Typical low resistance measurements arewithin the range of 1to less than 1 m
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- We note that lead resistances cansignificantly impinge on accuracy when
measuring low value resistances.
contd
- High quality DMMs circumvent thisproblem by using 4-wire resistance
measurements.
- Some have resolutions as low as 100.
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contd
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- A DMM can be used on the DiodeFunction to test a diode. Both in-circuit
and out-of-circuit tests are possible.
TESTING SEMICONDUCTOR
DEVICES
- The forward biased connection shouldgive a reading of 650 mV or 300 mV and
the reverse biased mode should indicatean open.
- DMMs can also test BJTs.
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- These measurements are carried out bymeasuring the charge required to change
the voltage across the unknown capacitorfrom zero to reference, Vref.
CAPACITANCE MEASUREMENTS
- This is known as the ballistic type ofmeasurement.
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contd
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- The unknown capacitor is fully chargedfrom zero up to Vrefduring the integrate
period.
contd
- The charge Q is given by:
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contd
Where
Finally,
Or
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The A/D converter integrates the voltage (V)across the known resistor Rrefand latches
the count. The capacitor is dischargedduring the de-integrate and hold period. Themicrocomputer (not shown) calculates anddisplays the capacitance from the latched
count, which is proportional to the unknowncapacitance.
contd
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DMMs also measure:
- Frequency
- True rms voltages and current- Temperature
- Continuity
OTHER SPECIAL FUNCTIONS
For these measurements, manufacturers usemodificationsto the basic techniques or addspecial components to the basic system
components
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THE 8060A FLUKE DMM
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contd
- True r.m.s. a.c. voltage or frequencymeasurement functions are added to thebasic configurationby the use of a true rmsac converter block between the ac input and
the custom IC (MAC)- For frequency measurements, the ac signalis divided by the voltage divider and
buffered by the rms converter. The signal isthen applied to a comparator in the MAC forcounting, where the counter gate iscontrolled by the microcomputer block
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ASSIGNMENT NOs. 1 and 2
1. Define the various functional andcharacteristic parameters of a typical DMM.
2. Differentiate between data logging anddata acquisition systems.
Due date:1. October 02, 2014
2. October 23, 2014