Working Principle of Digital Voltmeter

A voltmeter is a significant device used in electronics engineering for measuring potential developed across various devices. Depending on the requirements these voltmeters can be designed into two types that being digital and analog. In this article, we will briefly discuss the two types of voltmeters. Later we will study about digital voltmeter and the working principle behind it. We will also see the block diagram of a digital voltmeter and how it works. The digital voltmeter can further be divided into certain types and we will discuss them too. This description will include talking about their specifications as well. After the discussion, we will talk about the advantages and disadvantages of digital voltmeters which are considered while using digital voltmeters in some real-life appliances. This will conclude the article with some Frequently Asked Questions for user reference.

With the gradual development of technology, there has been a need for more precision while making experimental calculations. This led to the development of digital voltmeters. A digital voltmeter is used for measuring potential differences between two specific points in an electric circuit. Being a digital meter, the values measured either alternating or direct are discrete meaning that output values are more similar to “On” and “Off” values. This results in more precise results ensuring minimum error due to pointer errors.

Types of Voltmeter

Voltmeters are generally categorized into two types namely digital and analog which have been briefly explained

• Digital Voltmeter: A digital voltmeter uses a numeric display to show the exact value of the measurement. This makes digital voltmeters more precise and easier to read. Due to the fragile nature of these voltmeters, they are more expensive than analog voltmeters.
  

  Digital Voltmeter

• Analog Voltmeter: An analog voltmeter uses a needle that moves along a range of scale to measure the value across instruments. Thus makes analog voltmeters more responsive and durable. That being said ,they are more prone to errors making them less accurate mainly because of interference and parallax errors.
  

  Analog Voltmeter

Let us discuss the working of a digital voltmeter. It is important to note that the working of digital voltmeter matches to that of an analog to digital convertor because it basically transforms given input signal into a sequence of pulses . Let us see the working

• The input signal is fed as the input to the pulse generator. The pulse generator generates rectangular pulses of width as that of input signal.
• The output of the pulse generator acts as the input the AND gate whose another input is a sequence of pulses. This creates a positive triggered switch resulting in positive triggered sequence of pulses having width same as the pulses generated by the pulse generator.
• The positive triggered pulses i.e. the output of AND gate is fed to invertor which inverts the output of AND gate.
• The output of invertor is fed as an input to the counter which counts the total pulses and time duration between the pulses.
• The last step involves showing the precise output mainly on LED and the reading is calibrated and shown in Volts.

Now we can see the similarity in working of digital voltmeter to that of an Analog to Digital convertor

Let us see the block diagram of digital voltmeter and see how it works.

The voltmeter begins working when it receives an input i.e the input signal whose voltage is to be measured. The input signal can not be read as such and thereby is converted in rectangular pulse by the pulse generator. The frequency and width of the generated rectangular pulse is selected using digital circuitry present inside the generator. The AND gate is added mainly to ensure that train pulses have the same duration as the rectangular pulse. The train pulse and rectangular pulse are fed as the input and the output is high only when both are high.

The decimal display section performs the task of counting total number of impulses and the time difference between them. This count is generally displayed on an LED screen after calibration in the required scale.

Here are the different types of digital voltmeters:

• Ramp Type Digital Voltmeter
• Integrating Type Digital Voltmeter
• Successive Approximation Digital Voltmeter
• Dual Slope Integrating Digital Voltmeter

Ramp Type Digital Voltmeter

There are circuits that generate output signals in the shape of a ramp. The voltmeter used for measuring such ramp signals in Ramp type digital voltmeter. Timing plays an important role in these type of signals .The name of voltmeter is mainly because it measures ramped up signals.

Working: The voltage is measured by providing an unknown input signal to the ranging and attenuation .Depending on the needs ,the signal is attenuated or made stronger by amplification. The ramp generator is a device which generates a positive or a negative ramp and our unknown signal is compared to it. The comparator compares input signal to the ramp signal. If the input voltage matches with ramp voltage then gate is opened with the pulse and after the ramp signal reaches 0 ,the gate is closed. The time period between two events is called gating time interval. This generates the ramp voltage

Integrating Type Digital Voltmeter

This voltmeter measures exact value of input corresponding to the constant of time. This circuit usually uses a voltage-to-frequency converter device which works on the feedback control system. The main characteristic of this voltmeter is that the output from the integrator is compared with the fixed level voltage of reference source.

Working: In this, as soon as the input voltage is applied, the output voltage begins to increase which fed to level detector. After the output voltage reaches a certain value, detector sends a pulse to pulse generator gate. The integrator output is compared to fixed level voltage of the internal reference source resulting in an output pulse. This output pulse from level detector. This pulse opens gate which passes pulse from oscillator to pulse generator. The pulse generator like a Schmitt trigger , generates pulses with fixed width and amplitude .So for each wave a pulse is generated which helps to determine the input voltage.

Successive Approximation Digital Voltmeter

In this voltmeter the output of digital to analog convertor with a certain unknown reference voltage. This meter can measure up to 100 readings per second. The voltmeter uses an amplifier to choose a required range of input voltage and minimize the noise which can cause distortion.

Working: The input amplifier of this device helps to choose a range of input voltage and eliminates any noise. The input is then sent to comparator through an S/H circuit. The comparator generates a signal when it receives the analog signal. This is further sent to AND gat whose output become one if and only if output of comparator is positive. In the end , a digitized digital signal is generated from control registers giving us the required voltage.

Dual Slope Integrating Digital Voltmeter

It consist of an integrator circuit which takes the unknown input for certain time measured using a clock. The circuit consist of comparators ,capacitors ,clock and counters which generate the reference voltage with time. In this way we can measure the required voltage.

Working: In this, the integrator circuit receives an input signal for a specific period of time which is measured using clock frequency. During this time, the capacitor gets charged and the charge is proportional to the input voltage. At the end of this time, the switch is shifted from input voltage to a reference voltage and the charging of capacitor decreases creating a downward linear ramp voltage. This generates the required voltage.

Digital voltmeter has various advantages and some of them have been stated below

• Since it does not involve reading from a pointer, it eliminates the human error that are caused by reading at an angle. This means gross errors are now removed giving precise results.
• They are more reliable and stable as compared to analog voltmeters which are comparatively unreliable while making readings.
• The output from DVMs can be provided to memory devices for the purpose of storage. This means that we can directly store the result of digital voltmeter in devices like flip flops.
• We don’t require any extra manuals for using digital voltmeters as they are easy to read and give precise and accurate readings.
• Digital voltmeters are more durable than analog voltmeters. We say this because they provide accurate readings without being affected by external factors like atmosphere, temperature and moisture.

There are certain limitations of digital voltmeter like:

• They are comparatively fragile than Analog voltmeters and need to be handled with care. They can get heated up on prolonged use and give wrong readings in that case.
• The speed of digital voltmeters is dependent on digitizing circuit being used .This might reduce the operational speed of the voltmeters making them slow.
• There is a certain threshold value associated with digital voltmeter and exceeding that value can damage the voltmeter without any prior warnings.
• In case of fluctuations in the output, digital voltmeter fails to detect them and can even give wrong readings in that case giving wrong output.
• Due to the designing of digital voltmeter in a certain way, it becomes almost impossible to measure transient voltage using this device. Hence measuring transient spikes through this meter is a difficult process.

DVM is a useful device and has many applications like:

• It is used in laboratory and labs for determining accurate and precise voltage level across various devices mainly resistors, diodes, capacitors in electronics circuits.
• Often times unknown current values are determined with the help of DVM. As DVM helps to calculate voltage and once voltage level is known we can calculate the corresponding current in the circuit.
• DVM are employed in for verification purposes like they are used in cathode ray tubes to verify the accuracy of the findings and readings. They help in electrical maintenance and inspection of circuits.
• In certain circuits tree ammeter method and the three voltmeter method are used for measuring of power. They are used to calculate the power factor of a specific load in circuit.
• DVM is largely used for the purpose of circuit designing and prototype. They help in measuring and verifying voltage levels at every stage. This is used for designing the required circuit.

The article presents us a brief introduction to the types of voltmeter and a detailed picture of digital voltmeter. The working and block diagrams of voltmeter have been discussed to give readers some insights into the DVM. The applications of DVM shows us how important DVM is to the electronics industry mainly because of the precise readings it can give. That being said , developments in technology are peaking and there is a large possibility of further improvements in DVM to make it better in terms of fragility and other factors. Readers can refer to the FAQs for any doubts.

What is Analog voltmeter?

Analog voltmeter is a device which is mainly used for measuring AC voltage. It uses a pointer to display the reading. The pointer is fixed on the calibrated scale and can point to a range of values.

What is meant by calibration?

Calibration is the process of configuring an instrument to provide a result for a sample within an acceptable range. By controlling the calibration we can decrease or minimize the errors.

What is Analog to digital convertor? How is it similar to DVM?

Analog to digital convertor is used to convert an Analog signal to a digital signal using some circuits. Its working is similar to DVM as it transforms the Analog input signal into a sequence of pulses .