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# How does a voltmeter work?

Posted on 2003-11-15
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How does a voltmeter work? I know an ampmeter works because as you increase current flow threw a coil, a strong magnetic pull is generated which can be used to show the flow etc. But dow does a voltmeter work? Increaseing voltate wont change how the meter moves will it?

Arrummzen
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Question by:Arrummzen
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grg99 earned 720 total points
ID: 9758746
Many older voltmeters (the analog ones with an actual physical needle), just use ohms law.  They put a resistor in series with the meter, which is like you surmised, a current meter.  For exaple, a 1,000,000 ohm resistor is going to let 1 microamp thru the meter for every volt.  So the resistor is being used as a voltage-to-current converter.

On newer digital meters, a variety of techniques are used.  The most common one is called dual-slope integration.  There the incoming voltage is again fed into a resistor to convert the voltage to a current.  Then this current is used to discharge a capacitor until the voltage gets down to zero.  Then a known current is applied, and the time it takes to recharge the capacitor is a measure of the input curent.

A faster way is to apply the input voltage to a A/D converter which uses a successive-approximation technique, a version of the old "20-questions" game.  First it compares the input voltage to 1/2 the maximum and decides whether the input is above or below that, then it splits the difference, choosing either 1/4 or 3/4, comparing that, then halving the interval again.  After 16 such comparisons, you have the answer to within 4+ decimal digits (1 part in 65,536), after 24 comparisions you have 7+ digits(1 part in 16,000,000).  That's usually more than plenty of accuracy.

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Assisted Solution

ahoydave earned 280 total points
ID: 9788656
One other tyoe of volt meter not mentioned is an electrostatic volt meter. I believe Ben Franklin came up with this but I may be mistaken. In this type, some mechanism measures the force repelling or attracting plates connected to the voltage. Like voltages repel, unlike voltages attract.

As for digital volt meters, it should be added that all the forms of A/D (analog to digital) conversions, of which dual slope is just one, compare an input voltage to a reference voltage. A process is used where either the input voltage or the reference voltage is adjusted until it is close to the other. The process of adjusting is used to calculate a value for the voltage. The fastest is what is called a flash A/D. This type uses a referece that is divided into  lot of voltages and each of these divisions has a comparator. The outputs of all the comparators are sorted out with high speed logic to arrive at a value. A comparator circuit is simply an amplifier that converts a tiny voltage to a big voltage that is then used to define a binary logic state.
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Thank you both for your time. I have a better understanding now.

Arrummzen
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