Electronic InstrumentationAnalog and Digital

Voltmeters/Ammeters/OhmmetersDesign & Usage

* In this presentation definitions and examplesfrom Wikipedia, HowStaffWorks and some other sources

were used

Lecturer: Dr. Samuel Kosolapov

Items to be defined/refreshed/discussed

• Operation of Analog Galvanometer• Converting Galvanometer to Voltmeter• Multi-Range Analog Voltmeter & Ammeter• AC Analog Ammeter• Analog Ohmmeter• Outdated Analog Multimeter design• Analog Multimeter with Analog Amplifier

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Items to be defined/refreshed/discussed

• Digital Multimeter

• Arduino based Multimeter

• Usage of Digital Voltmeter to measure resistance (Why not to use Ohmmeter ?)

• Wheatstone Bridge Usage

• What about Capacitance measurements ?

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Analog Galvanometerhttp://pediaa.com/difference-between-galvanometer-and-voltmeter/

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Galvanometer is a device which has parts that move in response to an electric current

Analog Voltmeterhttp://pediaa.com/difference-between-galvanometer-and-voltmeter/

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A voltmeter is a device which, when connected across two points on an electric circuit, measures the potential difference between those two points

Galvanometers can be used to make voltmeters. The needle of a galvanometer moves in response to current,

but if we know the resistance of the coil, then we can use Ohm’s law to determine

the corresponding potential difference between the two ends of a voltmeter.

Voltmeter Calibration:We could set up a scale next to the needle

that reads the values of potential difference corresponding to the needle’s position

+ Manual ZERO correction (rotate spring…)

Analog Voltmeter: ProblemsPeriodical Calibration is neededLow accuracyValues must be logged manually

Galvanometer Voltmeterhttp://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-design/

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How to measure bigger voltages ? Add additional resistor.

Using Ohm’s Law (V=IR),we can determine how much voltagewill drive this meter movement directly to full scale:

V = I RV = (1 mA)(500 Ω)V = 0.5 volts

Multi-Range Analog Voltmeterhttp://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-design/

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More practical design

Analog Voltmeter Impact on Measuring Circuithttp://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-design/

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Every meter impacts the circuit it is measuring to some extent.While some impact is inevitable, it can be minimized through good meter design.

Voltmeters are always connected in parallel with the component under test.A perfect voltmeter has infinite resistance, so that it draws no current from the circuit under test. However, perfect voltmeters only exist in the pages of textbooks, not in real life!

Wrong !!!

Loading Effect

Multi-Range Analog Ammeter designhttp://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-design/

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Shunt resistor values are very low!To achieve these low resistances, ammeter shunt resistors often have to becustom-made from relatively large-diameter wire or solid pieces of metal.

To measure current EE must break the circuit.This is why Ammeters are practically never used in real life electronics

AC Analog Ammeter designhttps://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_12.html

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Problem: Diodes are NON LINEAR Scale is not linear Special Scale for AC must be printed

Analog Ohmmeter designhttp://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-design/

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But scale is NOT Linear !!!Example: if current of 0.5 mA then R + Rx = 9V/0.5 mA = 18 k Rx = 18 – 8.5 – 0.5 = 9 k

Current is a function of R + Rx between black and red leads

Q. Can EE measure resistance of the component on the PCB ?

Outdated Analog Multimeter design

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Analog Voltmeter with Analog Amplifierhttp://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-design/

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Amplifier can has nearly INFINITE input resistance. (FET, OA)

In case galvanometer is used, Voltage to Current converter must be used

High Impedance DC Votmeterhttp://www.circuitstoday.com/high-impedance-dc-voltmeter

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Diodes D1 and D2 protect the IC from accidental excessive

input voltages Diodes D3 and D4 protect the meter from overloads.

Analog Ammeter with Analog Amplifierhttp://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-design/

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Voltmeter with Amplifier

Input Resistance can be set as HUGE

Linear AC Analog Voltmeter with Analog Amplifierhttp://www.angelfire.com/planet/funwithtransistors/Book_CHAP-7.html

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Digital Voltmeter (of Stone Age)

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Digital Multimeterhttp://www.vsagar.org/how-digital-multimeter-works/

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Digital Multimeter. Voltage Attenuatorhttp://www.vsagar.org/how-digital-multimeter-works/

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The commercial DMM has a rotary switch used selecting proper range with many steps in it.

Arduino Based Multimeter. /

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Will be proposed as FINAL Projecthttp://www.instructables.com/id/Digital-multimeter-shield-for-Arduino/

The shield can be inserted on "Arduino" UNO und Duemilanove boards. It can work in three modes:

standalone - the measurement data can be seen at the character or graphical LCMconnected - the measurement data can be read on the PC screen using the "Arduino" IDE "Serial monitor"combined - the data can be observed on both devices

The second mode does not require the presence of LCM, what makes the shield very cheap.

The "Arduino" based DMM has the following functions:voltmeter with 3 ranges : 0-10V; 0-30V; 0-100Vamperemeter - it has a range 0-500mAohmmeter with 2 ranhes : 0-1KOhm, 0-250KOhmdiode, LED, connectivity checkerLED functionality testerNPN BJT Beta meter

Digital Multimeter. Current to Voltage Conversionhttp://www.vsagar.org/how-digital-multimeter-works/

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The commercial DMM has a rotary switch used selecting proper range with many steps in it.

Measuring resistance with DC. Circuit A

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Extremely simple circuit.Problem: Two identical voltmeters are needed.

Q041. Derive relevant equation for R2 calculationby known V1, R1, and Vr2 (Voltage on R2)

Q042. Calculate numerical value of R2

Q. Why not to use Digital Ohmmeter ?A. We want to control the voltage and current while measurements. (We do not know which current and I which direction flows

Measuring resistors with DC. Circuit B

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Switch added Only one Voltmeter is neededProblem: Operator must manually log and process the data

The accuracy of an analog ammeter or voltmeteris usually stated as a percent of the full-scale reading.

Example: The Ququ analog meters are accurate to ±2% of the full scale reading.

Thus for a reading of 1.00V on a 3 volt scale, the uncertainty is ±0.06V. A reading of 1.0V on the 30 volt scale will have an uncertainty of 0.6V.

For a digital multimeter (DMM : XMM1), accuracy is usually specified as a percent of the readingSo a meter with a specification of 1% of the reading will read an actual value of 100.0V as something between 99.0V and 101.0V.

Measuring resistors with DC. Circuit B. Measurement Error

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Reminder: Absolute Errorand Relative Error

𝑉𝑅2 =𝑅2

𝑅1 + 𝑅2× 𝑉1

𝑅2 =𝑉1

𝑉1 − 𝑉𝑅2× 𝑅1

Equation for Error evaluation is not trivial even in this trivial case

Fast evaluation Option is: Worst Case Evaluation

R1 value is known with 1% accuracy.Digital voltmeter XMM1 accuracy is 1% Q043. Calculate Worst Case Range of R2: {R2min .. R2max}

Wheatstone Bridge (By TI Precision Analog Applications Seminar)

http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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A load cell is important sensor. Used in weight scales (balances)

Strain Gaugehttp://www.sensorland.com/HowPage002.html

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Strain gauge is used to measure pressure, load, torque(depending on mechanical design)

Inside: resistive foil which is mounted on a backing material.

When the foil is subjected to stress, the resistance of the foil changes in a defined way

BUT: Change of the resistance of the foil is VERY SMALL Special circuitry is required to measure force properly

Wheatstone Bridge (By TI Precision Analog Applications Seminar)

http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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Problems1. Relation between Rg and Vo is not linear2. Vo has a big “DC offset”. Then it is not easy to measure small voltage changes.

Example: DC offset is 5V, “Voltage change” is 1 mV

Wheatstone Bridge (By TI Precision Analog Applications Seminar)

http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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Invented by Hunter Christie (1833). Studied by Charles Wheatstone

If the current (or voltage) in the cross branch is zero, and THREE resistance are known,

the FOURTH resistance can be calculated

1. IMPORTANT: Voltage of VE is not important2. “ZERO” Voltage / Current is easy to detect

Wheatstone Bridge (By TI Precision Analog Applications Seminar)

http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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Removing the offset:R1==R is selected close to “some” value of Rg

Then offset is eliminatedAnd Vo is nearly proportional to “DELTA”(Because R >> “Delta”0

==

R1:=R 𝑅𝑔 = 𝑅1 + ∆

𝑉𝑜 =𝑅

2𝑅 + Δ−1

2𝑉𝑒

Vo =−∆

4𝑅+2∆𝑉𝑒

Wheatstone Bridge (By TI Precision Analog Applications Seminar)

http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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This configuration is called :“Single-point Bridge Sensor”

Actually, Voltage and not current is measured here

Two identical “Load Sensors” can be positioned in TWO points (Two Point Bridge)Upper and Bottom strain gauges connected oppositely: (R+Delta) and (R-Delta)Q044. Prove that THIS improves scales sensitivity

by a factor 2.

Wheatstone Bridge (By TI Precision Analog Applications Seminar)

http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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This configuration is called :“Four-point Bridge Sensor”

Very Linear and Very Sensitive Configuration !!!(May be I’ll ask to prove this on final exam…)

Strain Gauge in Weighting Scaleshttp://www.ti.com/lit/ml/slyp163/slyp163.pdf

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Typical parameters:Resistances are in the range of 1k“Sensitivity” is expressed in mV/VThis means that 10 V excitation with 2mV/V sensor results in change 20 mV only

Additional important parameters:Offset error: Voltage Produced when the measurement parameter is zeroFull-scale Error: Difference between the ideal voltage when the measurement parameter is at maximumTemperature Drift: change of the above as temperature variesAging error: change of the above as time is runningNon-linearity: Deviation of graph {Output Parameter - Measured Parameter”} from a straight line

Offset calibration for a Weight Scalehttp://www.ti.com/lit/ml/slyp163/slyp163.pdf

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In “Analog” scales some “potentiometer” can be manually rotated.Digital Scales uses software to evaluate and store current offset value (Think How before exam)

Gain calibration for a Weight Scalehttp://www.ti.com/lit/ml/slyp163/slyp163.pdf

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In “Analog” scales some “potentiometer” can be manually rotated.Digital Scales uses software to evaluate and store current “Gain” value (Think How before exam)

Exemplary Design of Weight Scalehttp://www.ti.com/lit/ml/slyp163/slyp163.pdf

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Signal from Wheatstone Bridge is amplified by some Diff OA (or Instrumental OA)Then 0-20 mV range 0-5V range.LPF is a must (remind why)ADC of different types can be used. Vexcitation is used as for Wheatstone Bidge as for REF for ADC

(For better ADC Accuracy)

MCU is Arduino pin A0.5 Sec after power ON Arduino measure “offset” and store itThen “weight” measurements starts.Q045. Write short Arduino Sketch that take into account stored “OFFSET”

Send results every 1 sec to Serial Monitor

Measuring Capacitors with DC and Voltmeter ?

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Practically impossible:Voltmeter is too slow

Oscilloscope and Signal Generator must be used

Control Questions

• What have I learned ?

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Literature to read

1. TBD

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