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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