Generally, we use several electrical and electronics appliances in our day-to-day life. All these appliances are designed with some specific electrical power ratings; they must be operated with given ratings for their protection and efficiency. Hence, it is required to measure electrical properties of power supplied to appliances or devices. Instruments used for measuring quantities (electrical quantities such as voltage, current, energy, etc.) are called as meters such as voltmeter, ammeter, phase sequence meter, energy meter, and so on. Voltmeter also called as voltage meter is used for measuring voltage or potential difference between two points in any power supply line or conductor or in electric circuit or across any device. There are different types of voltmeters such as analog voltmeters, vacuum tube voltmeters, field effect transistor voltmeters and digital voltmeters.
Digital voltmeters are used for measuring unknown input voltage by converting the voltage into a digital value and then the measured voltage will be displayed on electronic display boards like LCD displays or LED displays. Digital voltmeters are again classified into different types based on the displays used for displaying the measured voltage and they are LCD digital voltmeters or LCD voltmeters and LED digital voltmeters or LED voltmeters.
Digital Voltmeter Circuit
The major components of digital voltmeter circuit of IC 7107 and seven segment LED display are shown in the figures below.
A high performance and low power consuming integrated circuit IC 7107 that includes seven segment decoders, reference voltage source, comparator, clock, and display drivers as its internal circuitry. The stability of display is very high with less than 10mW power dissipation.
IC7107 consists of 40pins and it is capable to drive seven segment displays (four seven segment LED displays are connected as shown in the figure). It can be used for accurate indication of voltage values present in DC power supplies or anywhere else based on the requirement of accurate voltage indication.
Technical Specifications of Digital Voltmeter
It requires a supply voltage of +/-5V (symmetrical), power requirements 200mA (maximum), it can be used to measure voltages from +/- 0-1, 999 VDC in four ranges.
It is very small in size, easy to construct with low cost using few external components. It is very easy to read the voltage indications even from a distance.
For better understanding the working of digital voltmeter it is necessary to know the working of analog to digital converter (ADC) IC. ADC IC has great accuracy, unaffected by noise, consists of built-in clock. It doesn’t require any sample and hold circuit or any high accuracy external components. Analog to digital converter is also called as integrating converter or dual slope converter. It is very simple to design, reliable, more accurate hence it is preferably used over other types of converters.
Digital Voltmeter Working
The operation of the circuit can be explained in two stages. During the preset period in the first stage input voltage is integrated and at the end of this preset period voltage at the output of integrator and input voltage are directly proportional to each other. Internal reference voltage is fed to integrator at the end of the preset period and the output of the circuit is steadily decreased till it reaches the zero reference voltage. The duration of first operation is fixed and the duration of second operation is adjustable, so it is possible to compare two. Thus, the input voltage can be compared to the internal reference voltage and then the result can be coded and displayed on the display.
The voltage intended to measure is fed to the circuit through resistors and capacitors as shown in the figure. The resistor R1 and capacitor C1 are used to set the internal oscillator (clock) frequency at 48Hz, clock rate at which there will be three readings per second.
The error caused by the internal reference voltage can be compensated and also display can be kept steady using the capacitor C2 connected between pins 33 and 34 of the IC. The resistor R5 and capacitor C3 are used for the integration of input voltage and to prevent division of input voltage that makes the circuit faster and due to probability of error is significantly reduced circuit becomes more reliable. If there is no voltage at the input of the circuit, then the capacitor forces the device to display zero using R2 and P1. The current through the displays can be controlled to provide sufficient current for brightness without any over current using the resistor R6. As we discussed earlier the IC 7107 is capable of driving four LED displays in which first three are used to display numbers from 0 to 9 and the first left LED display is used only to display number 1 or “-“ sign if the voltage is negative. The entire supply to the circuit is given through IC 7107 pins +5V at pin 1, 0V at pin 21, and –5V at pin 26.
The LED digital voltmeter working with circuit diagram can be easily understood with this simple overview description below.
The voltage intended to measure is primarily converted into digital equivalent using dual slope type analog to digital converter inside the IC7107. Then, this value is decoded and displayed on LED based electronic display boards. The voltage intended to measure in integrated for obtaining ramp at the output of the integrator for a fixed period of time. Then reference voltage with opposite polarity is applied at input of integrator and this reference voltage is permitted to ramp for making the output of integrator zero. Thus, time taken by negative slope for reaching zero can be measured in terms of IC7107’s clock cycle. This time is proportional to the voltage under measurement. Thus, internal reference voltage and input voltage are compared and the result converted into digital format to display using digital voltmeter.
Voltmeters are used to check voltage at the receiving end that is given as input for electrical appliances. Analog voltmeters are not accurate and thus, digital voltmeters are used for measuring the voltage accurately. Digital voltmeters displays measured voltage on electronic display boards such as LCD or LED displays. Do you know any electrical projects or applications in which particularly analog voltmeters or digital voltmeters are preferred? Then, explore your technical knowledge and also help to improve the knowledge of other readers by posting your comments in the comments section below.