A digital voltmeter (DVM) Circuit displays measurements of AC or DC voltage as discrete numerals instead of a pointer deflection on a continues scale as in analog instruments. Digital Voltmeter Circuit is versatile and accurate instrument that is employed in many laboratory measurement applications. Because of development and perfection of IC module, the size, power requirement and cost of the digital voltmeter as been drastically reduced and therefore, digital voltmeter circuit can actively compete with conventional analog instrument in price, portability and accuracy.
Here, in this article we publish multi-range digital voltmeter circuit has been designed for providing professional performance. The digital voltmeter posted here is of 3 ½-digit type, comprising 7-segment readout with some significant technical advantages like automatic polarity indication, overflow in direction, auto-zeroing and many more. The full scale reading of this digital voltmeter circuit is 1999 with 0.05% resolution i.e. it can measure voltage up to 1999V.
Description of Digital Voltmeter Circuit
The digital voltmeter circuit is designed around analog to digital converter IC, op-amp and. whereas the heart of the circuit is low power, high performance 3 ½ -digit display analog to digital converter IC ICL-7107 (IC1). The CMOS IC (ICL-7107) is a large scale integrated IC incorporates all necessary active device, including clock, references, 7-segment-decorder and display drive to directly drive the 7-segment display.
We can make ICL-7107 a 200mV or 2V full scale deflection type digital voltmeter by selecting the value of resistors R2, R4 and capacitors C1. Here we use a 200mV full-deflection type. As the input impedance of the digital voltmeter circuit is very high in range of 100 M, so it draw very little current from the input.
We can use switch SW1, SW2 and SW3 to select voltage range and its category (i.e. AC o DC). The voltage to be measured is feed to the terminal 1N HI and 1N LO to attenuator made from resistor R9 though R12 and a pot. VR3 and then connected to HI and LO terminal of the IC1.
If the measuring voltage is AC, then it passes through a high-reliable operational amplifier IC (IC2) which is used in non-inverting configuration. If the measuring voltage is noisy, capacitor C3 with resistor R3 have been used for filtering and limiting the input voltage. And is also help in case of overload.
In order to clamp the transient voltage, voltage dependent resistor VR4 is used in input terminals. Normally, VR4 possesses a very high resistance, but when its threshold voltage is exceeds, it acts as a low resistance to shunt out the transients. Thus, it does not affect the input terminals.
Capacitor C1 with resistor R1 controls the frequency of clock which is of the order of 48 kHz for producing the conversion rate of thee reading per seconds. Time constant of the integrator is maintained by capacitor C5 with resistor R4 where capacitor C4 is helpful in auto-zeroing.
Pin number 2 to 25 of the IC1 are used to control the seven segment display, three full-digits and one half-digit display has been connected with their corresponding terminals. Variable resistors VR1 through VR3 has been used for calibration the DC reference voltage and AC input voltages respectively.
Digital Voltmeter Circuit Calibration
In the circuit digital voltmeter, three voltage ranges have been provided for measuring DC as well as AC voltage. If the measuring voltage is less than or equal to 200mV, DC voltage will come directly in the input of the chip without any attenuation.
A DC source is given to the input of the digital voltmeter circuit and makes the reading appropriate in comparison with the reference to another high precision digital voltmeter. Input voltage comes through an attenuator (R12 – R16) at the time of using 20-volt and 500-volt scale. The reading of these ranges may be adjusted by varying the pot VR3. And finally the reading may be checked in comparison with the reference digital voltmeter.
The same procedure is follow while measuring AC voltage except the adjustment of VR2.
PARTS LIST OF DIGITAL VOLT METER CIRCUIT
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1, R8, R17 = 100 KΩR2 = 24 KΩ|
R3 = 1 MΩ
R4 = 47 KΩ
R5, R6 = 1 KΩ
R7 = 5.6 KΩ
R9 = 8.2 KΩ (metal film)
R10 = 82 KΩ (metal film)
R11 = 820 KΩ (metal film)
R12 = 8 MΩ (metal film)
R13, R14 = 150 Ω
R15, R16 = 15 KΩ
R18 = 4.7 MΩ
R19 = 10 KΩ
R20 = 10 MΩ
VR1 = 1 KΩ trim-pot.
VR2 = 5 KΩ pot
VR3 = 1 KΩ pot, o.25% tolerance
VR4 = voltage dependent resistor, 500V
|C1 = 100 pF (polysterene)C2 = 0.1 µF (mylar)|
C3 = 0.01 µF (ceramic disc)
C4 = 0.47 µF (mylar)
C5 = 0.22 µF (polypropylene)
C6 = 0.1 µF (polyester)
C7 = 47 nF (polyester)
C8 = 0.47 µF (polyester)
C9 = 10 µF (electrolytic)
C10 = 1 µF (polyester)
C11 = 10 kpF (ceramic disc)
|IC1 = ICL-7107IC2 = CA3140|
IC3 = CD4009
D1, D2 = OA85
D3 – D6 = 1N914
ZD1 = 5.1V, 400mW zener diode
DP1 = FND508 display
DP2 – DP4 = FND507 display
B1 = 6V battery
SW1 = One-pole three-way switch
SW2, SW3 = One-pole two way switch
SW4 = OFF-ON switch