Executive Summary 4
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- 3.10.2 Analog Voltmeter
- 3.10.3 Digital Voltmeter
3.10.1 VoltmeterA voltmeter is an instrument used for measuring the electrical potential, difference in voltage, between two points in an electric circuit. To measure the voltage, the voltmeter’s probes are connected to two nodes of the circuit. The voltmeter then adds a parallel branch to the points. Since the branch is in parallel to the two nodes, the branch will have the same voltage as the branch that it is parallel to. The electronic components within the meter will then measure the current that enters the added branch. Technically, all voltmeters can be considered as ammeters because they actually measure current rather than voltage. Knowing the resistance of the added branch, the voltage can be calculated by performing Ohm’s Law: V = IR. The voltmeter has a very high impedance for the added branch so that a minimum amount of current is flowing through the meter. This will be very small compared to the current that is flowing in the branch of the two points being measured. The purpose of drawing a significantly lower current is to prevent manipulating the circuit to act differently. A voltmeter is used to find a relatively high resistance that can possibly be an open circuit or a ground. A low voltage can show that there is a poor connection across the points being tested. The leads allow the current to be split between the branch it originally flowed through and the voltmeter creating a parallel connection to the points. The positive lead is placed to the circuit’s positive side and the negative lead will be placed on the circuit’s ground. When using a voltmeter, the user should keep in mind that the voltage will not be exactly correct if the voltage source is not ideal or the voltmeter is not ideal. An ideal voltage source produces a consistent voltage. If the load changes, an ideal voltage source would continue to produce the same voltage where an actual one will adjust for the change in the circuit. With an ideal voltmeter, the impedance inside is infinite and does not add to the circuit. An ideal voltmeter would then act as a open circuit. This would allow the voltmeter to measure the voltage without any effect to the circuit. Since an actual voltmeter wouldn’t have infinite impedance, it creates a parallel branch and splits some of the current. This causes a loading effect to the circuit. Therefore, because an ideal voltage source and ideal voltmeter do not exist, the loading effect of a voltmeter will affect the voltage produced by the circuit’s voltage source. Voltage sources and voltmeters are designed as close as possible to ideal states to eliminate some of this effect. 3.10.2 Analog VoltmeterAn analog voltmeter uses a dial, most commonly with a needle or a moving pointer, to measure the voltage between two points. Analog meters are usually more accurate and are good for picking up small voltages. Analog voltmeters use a galvanometer with a moving coil and resistance to measure the current. A magnetic field suspends the coil of wire which is rotated by a compressed spring when a current is present. This mechanism is referred to as the D'Arsonval movement meter. The diagram below displays a galvanometer using D’Arsonval Movement. Current flows through the coils of the electromagnet. This creates the magnetic field which is opposite to the field of the magnet. The field produced by the current causes the core to rotate. The rotation is directly proportional to the opposing magnetic field, so the larger the amount of current, the larger the magnetic field, and the larger the rotations of the core. 
(http://www.engineersedge.com/instrumentation/electrical_meters_measurement/darsonval_movement.htm) (Reprinted with permission from engineersedge.com)
(https://www.marinenutz.com/products/Blue-Sea-9353-AC-Analog-Voltmeter-0%252d150-Volts-AC.html) (Reprinted with permission from marinenutz.com)
Digital voltmeters use op-amps to create an integrator circuit. The integrator applies a reference voltage to an op-amp to increase the output voltage for a period of time. The unknown voltage being measured is then applied and will decrease the output voltage to zero. The time it takes to adjust the voltage both up and down is used to calculate the measured voltage. The desired voltage is the product of the reference voltage and the time it took for the voltage to increase divided the time it took for the voltage to decrease to zero. The figure below is a picture of a digital voltmeter. 
(http://www.directindustry.com/prod/lutron/digital-ac-dc-voltmeter-25127-90754.html) (Reprinted with permission from directindustry.com)
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