Digital Voltmeter, Digital Multi-meter, Digital frequency meter
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The document discusses various types of digital voltmeters, including ramp type and dual slope integrator designs, explaining their mechanisms for voltage measurement. It also covers digital multi-meters (DMMs) which measure multiple electrical quantities with high accuracy and detail on resolution and count. Additionally, it describes the structure of digital frequency meters which convert input waveforms into measurable frequencies for display.
Digital Voltmeter, Digital Multi-meter, Digital frequency meter
- 1. Digital voltmeter, multi-meter, frequency meter Unit II: Transistor voltmeter circuits, AC electronic voltmeter, current measurement with electronic instruments, probes Digital voltmeter systems, digital multi-meters, digital frequency meter system 4/4/2017 1 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
- 2. Digital voltmeter systems 4/4/2017 2 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad Digital voltmeters are basically analog to digital converters with digital display to indicate voltage E Vo Attenuator or range selector Amplifier High input/ low output impendence Rectifier Analog to digital converter Digital Display BCD to seven segment display
- 3. Ramp Type DVM • Ramp type uses comparison of input voltage with Ramp waveform. • Ramp output triggers counter, output of which is displayed when counting stops • Counting circuit, BCD to seven segment decoder and digital readout constitutes a 2000 scale counter display. 4/4/2017 3 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad + - Ramp generator Clock generator Decade Counter Latch BCD to seven segment drivers 1 9 9 9 V Vi Display Reset
- 4. Ramp Type DVM • Ramp starts at t=0, when ramp reaches to Vi (t=t1), comparator output switches. • Counting continues in period t1. • No counting in rest time period of ramp t2 • Latch separates display drivers from counters during counting (t1) • At end of t1, latch triggers transfer of counts to display through drivers. 4/4/2017 4 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad RampVi Comparator output t1 t2 Reset Latch Latch trigger to correct display Counting No Counting Clock pulse during t1 Accurate measurement in Ramp type DVM requires precise Ramp voltage and time period.
- 5. Dual Slope Integrator DVM • Dual slope Integrator eliminates requirement of precise voltage and time period, by using a special ramp generator (integrator) 4/4/2017 5 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad Clock generator Counters BCD to seven segment drivers 1 9 9 9 Vi Display Reset + - Integrator Zero crossing detector Frequency divider Constant current source • Integrator capacitor is charged from Vi and then discharged at constant rate. Discharge time is proportional to Vi • During discharge duration, counter counts. • Two slopes; charge & discharge gives its name dual slope. • Accuracy of Dual slope integrator DVM depends on current source
- 6. Dual Slope Integrator DVM • Integrator charges negatively from Vi. In fixed time duration t1 . The charge voltage Vo Vi • Capacitor discharge at constant rate till zero crossing detector output goes negative • Discharge time t2 Vo Vi. • During discharge, counter counts • Counted result is displayed. • If clock frequency drifts, both t1 & t2 change, thus not affecting result. 4/4/2017 6 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad Clock pulse Start counting Stop counting t1 t2 Integrator control waveform Dual slope ramp from integrator Zero crossing detector output
- 7. Digital Multi-meters (DMM) • A DMM measures two or more electrical quantities, principally – AC/DC voltage (volts) – AC/DC current (amps) and – resistance (ohms) • DMM have high accuracy, reliability than analog multi-meters • DMM also come with special features measurement – Frequency – Capacitance – Temperature – Continuity testing/ Diode testing • DMM works on Ohm’s law to measure, voltage/current/ resistance 4/4/2017 7 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
- 8. Digital Multi-meters (DMM) 4/4/2017 8 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad Analog to Digital Converter BCD to seven segment drivers 1 9 9 9 ACV To resistance Constant current source Buffer Amplifier Calibrated attenuator Current to Voltage converter Current to Voltage converter Calibrated attenuator ACA DCA DCV Rectifier
- 9. Digital Multi-meters (DMM) Resolution, digits and counts • Resolution refers to smallest change in value that can be measured. • Digits and Counts are used to describe a multi-meter’s resolution. –A 3 1⁄2-digit meter can display 3 full digits (ranging from 0 to 9), and one “half” digit which display only 2 symbols (0 & 1), with maximum value 1. Thus a 3 1 ⁄2-digit meter can display up to 1,999 (2000 counts ). –A 4 1 ⁄2-digit meter can display up to 19,999 (20000 counts). –A 3 3 ⁄4-digit meter can display up to 3999 (4000 counts). –A 2000/20000-count meter may have volt ranges of 200 mV, 2 V, 20 V, 200 V, 750 V –As the range increase, resolution decrease –DMM are built with 3200, 4000, or 6000 counts. 4/4/2017 9 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
- 10. Digital Frequency Meter System 4/4/2017 10 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad Input waveform Amplifier/ Attenuator Waveform shaping / Schmitt trigger Time base Gated Flip Flop Counters BCD to seven segment drivers 1 9 9 9Display Latch Reset Latch trigger Latch trigger to display input counting 1 s Time base waveform Register reset Shaped waveform 1 s Q output Q’ output No counting Q Q'
- 11. Digital Frequency Meter System 4/4/2017 11 NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad • Digital frequency meter consists of time base circuit, waveform shaping circuit, circuit for gating the waveform to counter, and display circuitry. • Wave shaping circuit converts input wave into square or pulse waveform of same frequency. • Shaped waveform is applied to counter through gated control for the fixed time period (1 s). • Counter output is frequency of input waveform which is transferred to display through decoder/ drivers.