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.
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.