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Home , Multimeter, Voltmeter

Electronic Instrumentation Analog and Digital // Design & Usage * In this presentation definitions and examples from Wikipedia, HowStaffWorks and some other sources were used

Lecturer: Dr. Samuel Kosolapov Items to be defined/refreshed/discussed

• Operation of Analog • Converting Galvanometer to • Multi-Range Analog Voltmeter & • AC Analog Ammeter • Analog • Outdated Analog design • Analog Multimeter with Analog

2 Items to be defined/refreshed/discussed

• Digital Multimeter • Arduino based Multimeter • Usage of Digital Voltmeter to measure resistance (Why not to use Ohmmeter ?) • Usage • What about measurements ?

3 Analog Galvanometer http://pediaa.com/difference-between-galvanometer-and-voltmeter/

Galvanometer is a device which has parts that move in response to an 4 Analog Voltmeter http://pediaa.com/difference-between-galvanometer-and-voltmeter/

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 ’s law to determine the corresponding potential difference between the two ends of a voltmeter.

Voltmeter Calibration: Analog Voltmeter: Problems We could set up a scale next to the needle Periodical Calibration is needed that reads the values of potential difference Low accuracy corresponding to the needle’s position Values must be logged manually + Manual ZERO correction (rotate spring…) 5 Galvanometer  Voltmeter http://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-design/

Using Ohm’s Law (V=IR), we can determine how much will drive this meter movement directly to full scale: V = I R V = (1 mA)(500 Ω) V = 0.5

How to measure bigger ? Add additional .

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

More practical design

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

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!

Loading Effect

Wrong !!!

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

To measure current EE must break the circuit. This is why Ammeters are practically never used in resistor values are very low! real life electronics To achieve these low resistances, ammeter shunt often have to be custom-made from relatively large-diameter wire or solid pieces of metal. 9 AC Analog Ammeter design https://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_12.html

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

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

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

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

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

11 Outdated Analog Multimeter design

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

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

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

13 DC Votmeter http://www.circuitstoday.com/high-impedance-dc-voltmeter

Diodes D1 and D2 protect the IC from accidental excessive input voltages Diodes D3 and D4 protect the meter from overloads.

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

Voltmeter with Amplifier  Input Resistance can be set as HUGE

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

16 Digital Voltmeter (of Stone Age)

17 Digital Multimeter http://www.vsagar.org/how-digital-multimeter-works/

18 Digital Multimeter. Voltage Attenuator http://www.vsagar.org/how-digital-multimeter-works/

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

19 Arduino Based Multimeter. / Will be proposed as FINAL Project http://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 LCM connected - 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; 0-100V amperemeter - it has a range 0-500mA ohmmeter with 2 ranhes : 0-1KOhm, 0-250KOhm , LED, connectivity checker LED functionality tester NPN BJT Beta meter

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

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

21 Measuring resistance with DC. Circuit A

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

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

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

Q042. Calculate numerical value of R2

22 Measuring resistors with DC. Circuit B Switch added  Only one Voltmeter is needed Problem: Operator must manually log and process the data

The accuracy of an analog ammeter or voltmeter is 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 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 reading So 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.

23 Measuring resistors with DC. Circuit B. Measurement Error 푅2 푉푅2 = × 푉1 푅1 + 푅2 Reminder: Absolute Error 푉1 and Relative Error 푅2 = × 푅1 푉1 − 푉푅2

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}

24 Wheatstone Bridge (By TI Precision Analog Applications Seminar) http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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

25 Strain Gauge http://www.sensorland.com/HowPage002.html

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

26 Wheatstone Bridge (By TI Precision Analog Applications Seminar) http://www.ti.com/lit/ml/slyp163/slyp163.pdf

Problems 1. Relation between Rg and Vo is not linear 2. 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

27 Wheatstone Bridge (By TI Precision Analog Applications Seminar) http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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 important 2. “ZERO” Voltage / Current is easy to detect

28 Wheatstone Bridge (By TI Precision Analog Applications Seminar) http://www.ti.com/lit/ml/slyp163/slyp163.pdf

==

R1:=R 푅푔 = 푅1 + ∆ 푅 1 Removing the offset: 푉표 = − 푉푒 R1==R is selected close to “some” value of Rg 2푅 + Δ 2 Then offset is eliminated −∆ And Vo is nearly proportional to “DELTA” Vo = 푉푒 (Because R >> “Delta”0 4푅+2∆ 29 Wheatstone Bridge (By TI Precision Analog Applications Seminar) http://www.ti.com/lit/ml/slyp163/slyp163.pdf

Two identical “Load ” can be positioned in This configuration is called : TWO points (Two Point Bridge) “Single-point Bridge Sensor” Upper and Bottom strain gauges connected Actually, Voltage and not current is measured here oppositely: (R+Delta) and (R-Delta) Q044. Prove that THIS improves scales sensitivity by a factor 2.

30 Wheatstone Bridge (By TI Precision Analog Applications Seminar) http://www.ti.com/lit/ml/slyp163/slyp163.pdf

Very Linear and Very Sensitive Configuration !!! This configuration is called : (May be I’ll ask to prove this on final exam…) “Four-point Bridge Sensor”

31 Strain Gauge in Weighting Scales http://www.ti.com/lit/ml/slyp163/slyp163.pdf

Typical parameters: Resistances are in the range of 1k “Sensitivity” is expressed in mV/V This 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 zero Full-scale Error: Difference between the ideal voltage when the measurement parameter is at maximum Drift: change of the above as temperature varies Aging error: change of the above as time is running Non-linearity: Deviation of graph {Output Parameter - Measured Parameter”} from a straight line

32 Offset calibration for a Weight Scale http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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

33 calibration for a Weight Scale http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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

34 Exemplary Design of Weight Scale http://www.ti.com/lit/ml/slyp163/slyp163.pdf

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 it Then “weight” measurements starts. Q045. Write short Arduino Sketch that take into account stored “OFFSET” Send results every 1 sec to Serial Monitor 35 Measuring Capacitors with DC and Voltmeter ?

Practically impossible: Voltmeter is too slow

Oscilloscope and must be used

36 Control Questions

• What have I learned ?

37 Literature to read

1. TBD

38