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📌Tags
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#digitalvoltmeter #diy #electronics
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📌Description
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The circuit given here is of a very useful and accurate digital voltmeter with LED display using the ICL7107 from Intersil. The ICL7107 is a high performance, low power, 3.5 digit analog to digital converter. The IC includes internal circuitry for seven segment decoders, display drivers, reference voltage source and a clock. The power dissipation is less than 10mW and the display stability is very high.
The working of this electronic circuit is very simple. The voltage to be measured is converted into a digital equivalent by the ADC inside the IC and then this digital equivalent is decoded to the seven segment format and then displayed. The ADC used in ICL7107 is dual slope type ADC. The process taking place inside our ADC can be stated as follows. For a fixed period of time the voltage to be measured is integrated to obtain a ramp at the output of the integrator. Then a known reference voltage of opposite polarity is applied to the input of the integrator and allowed to ramp until the output of integrator becomes zero. The time taken for the negative slope to reach zero is measured in terms of the IC’s clock cycle and it will be proportional to the voltage under measurement. In simple words, the input voltage is compared to an internal reference voltage and the result is converted in a digital format.
We have designed basic 100V DC Voltmeter using design steps given in data sheets. It displays 3 digits i.e. 99.9V maximum. Actually ICL7107 have 3 and 1/2 Display we are not using 1/2(half) display. Reference voltage of 100mV is generated using 1.2 V Zener and Variable resistor forms a voltage divider. Input is divided by 100K and 100Ohm resistor to get 200mV in-proportion.
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📌 Components Value Selection
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🏷️ Integrating Resistor:
Both the buffer amplifier and the integrator have a class A output stage with 100μA of quiescent current. They can supply 4μA of drive current with negligible non-linearity. The integrating resistor should be large enough to remain in this very linear region over the input voltage range, but small enough that undue leakage requirements are not placed on the PC board. For 2V full scale, 470kΩ is near optimum and similarly a 47kΩ for a 200mV scale. Select 47KΩ.
🏷️ Integrating Capacitor:
The integrating capacitor should be selected to give the maximum voltage swing that ensures tolerance buildup will not saturate the integrator swing (approximately. 0.3V from either supply). In the ICL7106 or the ICL7107, when the analog COMMON is used as a reference, a nominal +2V fullscale integrator swing is fine. For the ICL7107 with +5V supplies and analog COMMON tied to supply ground, a ±3.5V to +4V swing is nominal. For three readings/second nominal values for ClNT are 0.22μF and 0.10μF, respectively.
🏷️ Auto-Zero Capacitor (Pin 29):
The size of the auto-zero capacitor has some influence on the noise of the system. For 200mV full scale where noise is very important, a 0.47μF capacitor is recommended. On the 2V scale, a 0.047μF capacitor increases the speed of recovery from overload and is adequate for noise on this
scale.
🏷️ Reference Capacitor: (Pin 34, 33):
A 0.1μF capacitor gives good results in most applications. However, where a large common mode voltage exists and a 200mV scale is used, a larger value is required to prevent roll-over error. Generally 1μF will hold the roll-over error to 0.5 count in this instance.
🏷️ Oscillator Components:
For all ranges of frequency a 100kΩ resistor is recommended and the capacitor 100pf
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Copyright disclaimer under section 107 of the copyright act 1976
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Copyright Disclaimer under Section 107 of the copyright act 1976, allowance is made for fair use for purposes such as criticism, comment, news reporting, scholarship, and research. Fair use is a use permitted by copyright statute that might otherwise be infringing. Non-profit, educational or personal use tips the balance in favour of fair use.
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