High Impedance DC Voltmeter using MOS Op-Amp

Figure 1 

The circuit was designed for the purpose of creating a voltmeter with high impedance that would measure Direct Current voltages across any types of circuits.

• CA3130 – a BiMOS operational amplifier that combines the advantage of both bipolar transistors and CMOS, which can be used in photodiode sensor amplifiers, peak detectors, single-supply full wave precision rectifiers, voltage regulators and followers, high input impedance comparators and wideband amplifiers, long duration timers, fast sample-hold amplifiers, and ground referenced single supply amplifiers
• Voltmeter – a device or an instrument used for measuring the electrical potential difference between two points of either alternating current or direct current electric circuit
• Impedance – a measure of the total opposition to the flow of time-varying and alternating current in a circuit, which consists of resistance and reactance
• Metal Oxide Semiconductor Field Effect Transistor (MOSFET) – a device utilized for switching and amplification of signals

The use of CA3130 is suitable for dealing with DC voltmeters with high impedance because of its MOSFET input. The range by which the circuit measures is dictated by the use of selector switch S1. The presence of the resistors in the voltage divider part of the circuit limits the input impedance to 10M ohms. This is still considered as an acceptable value. The mechanical instrument should have a rating of 100 uA, before the initial switch ON of the device. The pointer should be placed below the scale of zero and this requires the mentioned adjustment procedure.

Upon switching ON the unit, the input is kept shorted while obtaining an exact zero indication which is done by adjusting the trimmer TR1. For the full scale deflection in three ranges 1V, 10V, and 100V, the calibration of voltages is performed by the trimmer TR2. The voltage supply may be between 8V to 20V since the consumption of current in a 12V supply is less than 6mA. The quality of 100 uA instrument is very essential since the voltmeter is highly dependent on it along with the precise calibration done on the unit.

Figure 2 

DC voltmeters can be used as equipment for professional grade automotive test where it covers the testing of electrical components and circuits that are normally found in industrial and marine engines, trucks, automobiles, and other low voltage DC applications. During the measurement, they are being connected in parallel across the component being measured with the black probe typically connected to the negative terminal while the red probe to the positive side of the component.

Source:users.otenet.gr/~athsam/dc_voltmeter.htm