滇池海鸥Kelvin (4-wire) resistance measurement
Suppo we wished to measure the resistance of some component located a significant distance away from our ohmmeter. Such a scenario would be problematic, becau an ohmmeter measures all resistance in the circuit loop, which includes the resistance of the wires (R梦想的故事wire猪蹄的做法大全) connecting the ohmmeter to the component being measured (Rsubject):
Usually, wire resistance is very small (only a few ohms per hundreds of feet, depending primarily on the gauge (size) of the wire), but if the connecting wires are very long, and/or the component to be measured has a very low resistance anyway, the measurement error introduced by wire resistance will be substantial.
An ingenious method of measuring the subject resistance in a situation like this involves the u of both an ammeter and a voltmeter. We know from Ohm's Law that resistance is equal to voltage divided by current (R = E/I). Thus, we should be able to determine the resistance of the subject component if we measure the current going through it and the voltage dropped across it:
Current is the same at all points in the circuit, becau it is a ries loop. Becau we're only measuring voltage dropped across the subject resistance (and not the wires' resistances), though, the calculated resistance is indicative of the subject component's re
sistance (Rsubject) alone.
excel加页码Our goal, though, was to measure this subject resistance from a distance, so our voltmeter must be located somewhere near the ammeter, connected across the subject resistance by another pair of wires containing resistance:
At first it appears that we have lost any advantage of measuring resistance this way, becau the voltmeter now has to measure voltage through a long pair of (resistive) wires,
introducing stray resistance back into the measuring circuit again. However, upon clor inspection it is en that nothing is lost at all, becau the voltmeter's wires carry miniscule current. Thus, tho long lengths of wire connecting the voltmeter across the subject resistance will drop insignificant amounts of voltage, resulting in a voltmeter indication that is very nearly the same as if it were connected directly across the subject resistance:
Any voltage dropped across the main current-carrying wires will not be measured by the voltmeter, and so do not factor into the resistance calculation at all. Measurement accura
cy may be improved even further if the voltmeter's current is kept to a minimum, either by using a high-quality (low full-scale current) movement and/or a potentiometric (null-balance) system.
This method of measurement which avoids errors caud by wire resistance is called the Kelvin大血藤, or 4-wire method. Special connecting clips called Kelvin clips are made to facilitate this kind of connection across a subject resistance: 网络安全法规
In regular, "alligator" style clips, both halves of the jaw are electrically common to each ot
her, usually joined at the hinge point. In Kelvin clips, the jaw halves are insulated from each other at the hinge point, only contacting at the tips where they clasp the wire or terminal of the subject being measured. Thus, current through the "C" ("current") jaw halves does not go through the "P" ("potential," or voltage) jaw halves, and will not create any error-inducing voltage drop along their length:
The same principle of using different contact points for current conduction and voltage measurement is ud in precision shunt resistors for measuring large amounts of current.
天上有几颗星星As discusd previously, shunt resistors function as current measurement devices by dropping a preci amount of voltage for every amp of current through them, the voltage drop being measured by a voltmeter. In this n, a precision shunt resistor "converts" a current value into a proportional voltage value. Thus, current may be accurately measured by measuring voltage dropped across the shunt:
Current measurement using a shunt resistor and voltmeter is particularly well-suited for a
基本想法
pplications involving particularly large magnitudes of current. In such applications, the shunt resistor's resistance will likely be in the order of milliohms or microohms, so that only a modest amount of voltage will be dropped at full current. Resistance this low is comparable to wire connection resistance, which means voltage measured across such a shunt must be done so in such a way as to avoid detecting voltage dropped across the current-carrying wire connections, lest huge measurement errors be induced. In order that the voltmeter measure only the voltage dropped by the shunt resistance itlf, without any stray voltages originating from wire or connection resistance, shunts are usually equipped with four connection terminals:
