How does 4-Terminal transistors work?

4 FOUR TERMINAL SENSING

Four-terminal sensing (4T sensing), 4-wire sensing, or 4-point probes method is an electrical impedance measuring technique that uses separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurements than the simpler and more usual two-terminal (2T) sensing. Four-terminal sensing is used in some ohmmeters and impedance analyzers, and in wiring for strain gauges and resistance thermometers. Four-point probes are also used to measure sheet resistance of thin films.

Separation of current and voltage electrodes eliminates the lead and contact resistance from the measurement. This is an advantage for precise measurement of low resistance values. For example, an LCR bridge instruction manual recommends the four-terminal technique for accurate measurement of resistance below 100 ohms.^[1]

Four-terminal sensing is also known as Kelvin sensing, after William Thomson, Lord Kelvin, who invented the Kelvin bridge in 1861 to measure very low resistances using four-terminal sensing. Each two-wire connection can be called a Kelvin connection. A pair of contacts that is designed to connect a force-and-sense pair to a single terminal or lead simultaneously is called a Kelvin contact. A clip, often acrocodile clip, that connects a force-and-sense pair is called a Kelvin clip.

OPERATING PRINCIPLE

When a Kelvin connection is used, current is supplied via a pair of force connections (current leads). These generate a voltage drop across the impedance to be measured according to Ohm's law V=RI. A pair of sense connections (voltage leads) are made immediately adjacent to the target impedance, so that they do not include the voltage drop in the force leads or contacts. Since almost no current flows to the measuring instrument, the voltage drop in the sense leads is negligible.

It is usual to arrange the sense wires as the inside pair, while the force wires are the outside pair. If the force and sense connections are exchanged, accuracy can be affected, because more of the lead resistance is included in the measurement. The force wires may have to carry a large current when measuring very small resistances, and must be of adequate gauge; the sense wires can be of a small gauge.

The technique is commonly used in low-voltage power supplies, where it is called remote sensing, to measure the voltage delivered to the load independent of the voltage drop in the supply wires.

It is common to provide 4-wire connections to current-sensing shunt resistors of low resistance operating at high current.

 3 WIRE SENSING

A variant uses three wires, with separate load and sense leads at one end, and a common wire on the other. Voltage drop in the common wire is compensated for by assuming that it is the same as in the load wire, of the same gauge and length. This technique is widely used in resistance thermometers, also known as resistance temperature detectors or RTDs.

Another example is in the ATX power supply standard, which includes a remote sense wire connected to the 3.3V supply line at connector pin 13, but no sense connection for the ground wires.

What does 4-wire testing buy you?

1.    It eliminates the resistance of your interface cabling. If fixturing resistance is a significant part of the total resistance then using 4-wire will greatly improve accuracy.

2.    It allows you to measure lower resistance values than 2-wire testing. On Cirris hipot testers we use a higher current (up to 1Amp) when performing 4-wire Kelvin tests. This allows us to more accurately measure lower resistances, all the way down to 1 mΩ (0.001 ohm). Our low voltage testers that are 4-wire capable (CR, 1100R+) can measure down to 5 mΩ , but can still resolve to 1 mΩ . (You lose mΩ resolution once the DUT resistance is > 10 ohms)

3.    If you make 4-wire connections on the DUT, not just the connector that mates to the DUT, you can eliminate all sources of fixturing resistance. This extra effort however may not be feasible.

How 4-terminal resistors work

In a 4-terminal resistor, an ultra-precise resistor (green) is connected to 4 terminals through small, but unknown, resistors (red).
These unknown resistors are the combination of lead resistance, screw terminal resistance, connection wire resistance, and other sources of errors.
Typical values for these unknown resistors range from 0.01 ohms to 0.2 ohms, and the values are often unstable
The values can change when you loosen or tighten a screw, for example, or if you substitute a longer test lead.

To use a 4-terminal resistor, we force a current from Terminal 1 to Terminal 2. It's current, so the unknown resistances attached to Terminal 1 and Terminal 2 don't affect the amount of the current. The same number of electrons per second flow through from T1 to T2, regardless of the resistance.

A voltmeter measures the resulting voltage drop across the ultra-precise resistor, measuring through the unknown resistors attached to Terminal 3 and Terminal 4. The voltmeter's input impedance is very, very high compared to the unknown resistors, so the unknown resistors have essentially zero effect (typically less than 0.1 parts-per-million).

So the current flows through the 0.100 ohm resistor, unaffected by the unknown resistors, and we measure the voltage across the 0.100 ohm resistor, unaffected by the unknown resistors.

And that's how a 4-terminal resistor works!

So what errors do we worry about when we're using this type of resistor? We typically measure these errors in parts-per-million, or PPM (one PPM = 0.0001%).


There are five major sources of errors: calibration uncertainty, inductance, temperature, aging, and metal-to-metal contacts.

·         The United States National Institute of Standards and Technology calibrates the PSL standard resistor to an uncertainty of 0.5 PPM - this sets an absolute floor for measurement uncertainty at PSL

·         The resistor is typically made of a length of wire wrapped around a spool; in most precision resistors, the direction of wrapping is reversed after half the turns to minimize the inductance. Still, optimal accuracy is achieved at 0 Hertz, or DC. Even at typical power frequencies of a few tens of hertz, the inductance can contribute errors of a few PPM.

·         The wire is made of a material that has minimal temperature coefficient. Still, a change of a degree or two can contribute errors of tens to hundreds of PPM. It's not just the ambient temperature of the oil bath, either; we have to worry about self-heating when we pass current through the resistor. So temperature control plays a major role

·         All resistors change values when they age. Most studies show that there is an initial aging during the first few years of a resistor's life (sometimes this is intentionally accelerated with heat), then the resistor settles into a stable value. For this reason, PSL prefers calibration resistors that are at least 25 years old

·         It's important to consider the galvanic voltages generated by the contact between dissimilar metals. Although the value of the unknown resistors has no effect on the function of the calibration resistor, even a microvolt of galvanic voltage on Terminal 3 or Terminal 4 will affect the reading by 10 PPM at 1 amp. So we're very careful to use the correct types of test lead connectors on these terminals (and on the terminals of the voltmeter).

Kelvin (4-wire) resistance measurement

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 accuracy 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 caused by wire resistance is called the Kelvin, or 4-wire method. Special connecting clips called Kelvin clips are made to facilitate this kind of connection across a subject resistance:

In regular, "alligator" style clips, both halves of the jaw are electrically common to each other, 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 used in precision shunt resistors for measuring large amounts of current. As discussed previously, shunt resistors function as current measurement devices by dropping a precise amount of voltage for every amp of current through them, the voltage drop being measured by a voltmeter. In this sense, 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 applications 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 itself, without any stray voltages originating from wire or connection resistance, shunts are usually equipped with four connection terminals:

In metrological (metrology = "the science of measurement") applications, where accuracy is of paramount importance, highly precise "standard" resistors are also equipped with four terminals: two for carrying the measured current, and two for conveying the resistor's voltage drop to the voltmeter. This way, the voltmeter only measures voltage dropped across the precision resistance itself, without any stray voltages dropped across current-carrying wires or wire-to-terminal connection resistances.

The following photograph shows a precision standard resistor of 1 Ω value immersed in a temperature-controlled oil bath with a few other standard resistors. Note the two large, outer terminals for current, and the two small connection terminals for voltage:

The Kelvin measurement can be a practical tool for finding poor connections or unexpected resistance in an electrical circuit. Connect a DC power supply to the circuit and adjust the power supply so that it supplies a constant current to the circuit as shown in the diagram above (within the circuit's capabilities, of course). With a digital multimeter set to measure DC voltage, measure the voltage drop across various points in the circuit. If you know the wire size, you can estimate the voltage drop you should see and compare this to the voltage drop you measure. This can be a quick and effective method of finding poor connections in wiring exposed to the elements, such as in the lighting circuits of a trailer. It can also work well for unpowered AC conductors (make sure the AC power cannot be turned on). For example, you can measure the voltage drop across a light switch and determine if the wiring connections to the switch or the switch's contacts are suspect. To be most effective using this technique, you should also measure the same type of circuits after they are newly made so you have a feel for the "correct" values. If you use this technique on new circuits and put the results in a log book, you have valuable information for troubleshooting in the future.