The Tech Blog

This is a simple resistance measurement technique and fairly widely known, but I rarely see folks whom I usually interact with, using it. The name “Kelvin” comes from Lord Kelvin, who made a bridge circuit to measure smaller resistors.

To measure any resistor, the default method used is to pass a known current through a resistor and measure the voltage across it. Then via Ohms law calculate the resistance value. To understand why 4-wire is needed, you need to understand the issue with a 2-wire measurement. In 2-wire measurements, the resistance of the wires and contacts adds to the resistance of the component under test. This becomes a problem when the component value under measurement is very small. Check images. As the current in both arms are the same, the voltage measured includes the voltage drop across lead resistors also.

The 4-wire method solves the issue by using one pair of wires to supply the current and another pair to measure the voltage drop across the device. This separation ensures that the voltage measurement is not influenced by the voltage drop in the current-carrying leads, resulting in a much more accurate reading. Since the current flowing to the voltage measurement part is negligible (because of high impedance), measured voltage accurately reflects the resistance of the component alone.

Their application mostly involves measuring milliohm-level resistance values. These can be for measurement of shunt resistors, PCB traces, internal battery resistors. They find their way in precision and calibration instruments.

There are also shunt resistors with 4 terminals that allow you to do this measurement on an actual PCB. Please check my older posts or search the website on how to place and route them in PCBs. Also, if you want to use them, you will get 4 wire Kelvin style connectors with clip on leads. Worth purchasing you if you do a lot of low value resistance measurements.

I was recently doing some background reading for a project and came across a cool concept called Gyrators. Sometimes also called synthetic inductors, they are circuit elements that emulate the behaviour of inductors using active components like opamps or transistors along with capacitors and resistors. The concept behind a gyrator is that it converts impedance, effectively transforming a capacitive or resistive load into an inductive one without needing a bulky coil.

A “real” inductor (coil of wire wound on a magnetic core) passes DC easily but its impedance rises with frequency. However, it can pick up stray magnetic fields, it has a winding resistance, and its self-capacitance can cause resonances in the audio range. By contrast, a gyrator-based inductor can exhibit very low resistance, virtually no self-capacitance, and isn’t sensitive to external magnetic fields.  By using gyrators, you can shrink the circuit footprint, reduce weight, and potentially cut costs. In many analog designs, such as active filters, parametric and graphic equalizers, and other low-frequency circuits, gyrators provide a way to precisely fine-tune frequency responses by simply changing resistor or capacitor values.

Design-wise, a common gyrator approach involve an op-amp configured with a few resistors and a capacitor to generate the same impedance that a real inductor would offer. The only drawback is, that these gyrator circuits expect one end of the inductor to be grounded. This means floating inductors can’t be replicated with gyrators. Also, gyrators generally aren’t suitable for high-frequency or power applications as they rely on the supply voltage of the op-amp. Works great in the audio range though.

There is a nice white paper by Rod Elliot on Active Filters Using Gyrators that shows the gyrator circuits in real life and its equivalent spice simulations. It’s a nice read if you want to explore this one further.

Fun fact: if you replace the capacitor in a gyrator with an actual inductor, the circuit behaves like a capacitor.

#BackToBasics #Electronics #Inductors #Capacitors #Filters #Audio