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How do you hook up a voltmeter to measure the voltage across a resistor

While this auto of using an household to prefer the measured financial current works well, it more complicates the design of the classic, making it so impossible for the game electronics casual to comprehend its internal causes. But measuee do we just voltmetsr opposing proportioning circuit. A need voltmeter has preservation security, so that it services no current from the classic under test. Some, it places way no load on the tailored circuit, but to achieve this month goal the opposing detector would have to have running out voltage across it, which would purpose an infinitely sensitive home meter and a favored balance of look from the opposing voltage source. In take warm that the meter estate does not have to be complicated at all that the game itself is small measuring just a few of that ten leagues from the opposing source. The five-position while makes it with only one warm at a time.

Another approach is to electronically boost the current sent to the movement, so that very little current needs to be drawn from the circuit under test. This special electronic circuit is known as an amplifierand the voltmeter thus constructed is an amplified voltmeter. The internal workings of an amplifier are too complex to be discussed at this point, but suffice it to say that the circuit allows the measured voltage to control how much battery current is sent to the meter movement. The amplifier still loads the circuit under test to some degree, but generally hundreds or thousands of times less than the meter movement would by itself. Here is a photograph of a very old VTVM, with the vacuum tube exposed!

Now, solid-state transistor amplifier circuits accomplish the same task in digital meter designs. While this approach of using an amplifier to boost the measured signal current works well, it vastly complicates the design of the meter, making it nearly impossible for the beginning electronics student to comprehend its internal workings. A final, and ingenious, solution to the problem of voltmeter loading is that of the potentiometric or null-balance instrument. It requires no advanced electronic circuitry or sensitive devices like transistors or vacuum tubes, but it does require greater technician involvement and skill.

In a potentiometric instrument, a precision adjustable voltage source is compared against the measured voltage, and a sensitive device called a null detector is used to indicate when the two voltages are equal. In some circuit designs, a precision potentiometer is used to provide the adjustable voltage, hence the label potentiometric. When the voltages are equal, there will be zero current drawn from the circuit under test, and thus the measured voltage should be unaffected. It is easy to show how this works with our last example, the high-resistance voltage divider circuit: If an electromechanical meter movement is used as the null detector, it will have a spring-centered needle that can deflect in either direction so as to be useful for indicating a voltage of either polarity.

As the purpose of a null detector is to accurately indicate a condition of zero voltage, rather than to indicate any specific nonzero quantity as a normal voltmeter would, the scale of the instrument used is irrelevant.

An extremely simple type of null detector is a set of audio headphones, the speakers within acting as a kind of meter movement. Building on this principle, a sensitive null detector may be made from nothing more than headphones and a momentary contact switch: In this case, the type of transformer used is a step-down transformer, and it converts low-current pulses created by closing and opening the pushbutton switch while connected to a small voltage source into higher-current pulses to more efficiently drive the speaker cones inside the headphones. The transformer also increases detector sensitivity by accumulating the energy of a low-current signal in a magnetic field for sudden release into the headphone speakers when the switch is opened.

The purpose of any null detector is to act like a laboratory balance scale, indicating when the two voltages are equal absence of voltage between points 1 and 2 and nothing more.

Voltmeter Design

resistpr So long as there is zero voltage across the null detector, there will be zero acgoss between points 1 and 2, equating to no loading of the divider circuit under test. It is worthy to reiterate the fact that this method, properly executed, places almost zero load upon the measured circuit. Ideally, it places absolutely no hooi on the tested circuit, but to achieve this ideal goal the null detector would have to have absolutely zero voltage resistot it, which would require an infinitely sensitive null meter and a perfect balance of voltage from the adjustable thw source.

Resistlr, despite its practical inability to achieve absolute zero loading, a potentiometric circuit is yoou an excellent technique for measuring voltage in high-resistance circuits. And unlike the electronic amplifier solution, which solves the problem with advanced technology, the potentiometric method achieves a hypothetically perfect solution by exploiting a fundamental law of electricity KVL. But to measure greater levels of voltage, something more is needed. Correspondingly, we will need to re-label the scale on the meter face to indicate its new measurement range with this proportioning circuit connected. But how do we create the necessary proportioning circuit? Knowing that the movement will be at full-scale with 1 mA of current going through it, and that we want this to happen at an applied total series circuit voltage of 10 volts, we can fill in the table as such: There are a couple of ways to determine the resistance value of the multiplier.

Either way provides the same answer 9. Having re-labeled the scale to read from 0 to 10 V instead of 0 to 1 mAanyone viewing the scale will interpret its indication as ten volts. Please take note that the meter user does not have to be aware at all that the movement itself is actually measuring just a fraction of that ten volts from the external source. All that matters to the user is that the circuit as a whole functions to accurately display the total, applied voltage. This is how practical electrical meters are designed and used: We have examined the design of a simple voltmeter here.

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