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4-Wire Resistance Tutorial | Workbench Wednesdays

Bench DMMs have an extra set of banana jacks called “sense.” Known as a Kelvin or 4-wire resistance measurements, these inputs accurately measure small resistors. Like, milliohms small. This video shows how to make a 4-wire measurement, prove when it is accurate, and alternatives to 4-wire.  See below for an explanation of the alternative method shown.

Behind the scenes

Doing yet another DMM episode was a tough call for me. However, I am working on a project that requires me to characterize both a 1 ohm and a 100 milliohm resistor. The element14 community was kind enough to send me an MP720028 bench DMM. As shown in the picture, it has an extra set of banana jacks called “4-wire sense.” These connections make 4-wire resistance measurements. In this video, I show the same resistor measured with the traditional 2-wire and advanced 4-wire configuration. (Spoiler Alert! The 2-wire measurement was almost twice as big as the 4-wire!)

Alternative 4-Wire Resistance Measurement

In the video, I show one optional method of using a multimeter with 4-wire resistance capability. The trade-off is that it requires at least two multimeters. Since the 4-wire measurement is making two measurements at the same time: voltage and current, the alternative method does the same thing. A bench power supply, ideally with current limiting, applies a voltage to the resistor under test (RUT). One multimeter is used to measure the current through the resistor and, another measure the voltage across it.

In a pinch, you could use the bench supply’s ammeter for the current, however, its precision is probably much less than the average DMM. Once the voltage and current are known, a little bit of ohm’s law determines the RUT’s resistance!

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Digilent Analog Discovery 2 Review | Workbench Wednesdays

The Analog Discovery 2 combines all the equipment found on a typical electronics workbench into one small package. It packs an oscilloscope, logic analyzer, power supplies, spectrum analyzer, and so much more. As impressive as the hardware is, the Analog Discovery 2’s software, called Waveforms, is fantastic as well. You can configure it for any measurement situation, and it has extensive scripting capability. See if you should be considering adding the Analog Discovery 2 to your electronics workbench.

My day job causes me to travel on a regular basis. Sometimes that means taking my circuits with me. I picked up an Analog Discovery 2 so that I could take test equipment with me. It’s a small enough box that fits nicely into my bag. The one significant trade-off is that it requires a PC to be functional. However, for travel, that works out great.

The key functions the Analog Discovery 2 provides are:

  • 2-Channel oscilloscope
  • 2-Channel function generator
  • Voltmeter (not DMM!)
  • 2-Channel Power Supply w/ negative voltage
  • 16 Digital Channels for Logic, Protocol, and General Purpose I/O
  • Network Analyzer (Bode Plots!)
  • Spectrum Analyzer
  • Impedance Analyzer

There are probably other functions, but that covers the major ones. Years ago, I reviewed the Virtual Bench from National Instruments. While the Virtual Bench has better specs than the Analog Discovery 2, the difference in price points is staggering. You can get the AD2 for $200-300.

Check out how some of the features work and my thoughts on this device in this Workbench Wednesdays review.

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Workbench Wednesdays

Function Generator Basics | Workbench Wednesdays

Generating signals with a waveform, function, or arbitrary generator lets you test all kinds of circuits. Learn how to get a function generator to output a signal, the 3 types of waveforms you can create, and which controls matter. James, the Bald Engineer, explains the difference between analog and digital generators.

The video introduction is my first attempt at doing an LGR style “Tech Tales” story. It is short with only a few images. However, it is something I would like to do more of in the future. My professional career started at Agilent right when they split off from Hewlett Packard. Most of my co-workers had been there while Bill and Dave still worked at the company. Even though the computer division took the name, Agilent was HP’s core: test and measurement.

Someday I hope to see an actual 200A. Their place in history has become legendary. Some people say that Disney’s Fantasia could not be produced without it. However, Bill (or Dave) set the record straight. Disney’s engineers could have used another piece of equipment, however, they did select the 200A.

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Workbench Wednesdays

Logic Analyzer Basics | Workbench Wednesdays

Logic analyzers capture digital signals and then display a waveform or list. Serial busses like I2C, SPI, or UART (Serial) can be decoded or triggered on when there are problems in your circuit. In this video, learn the basic controls you need to use (almost) any logic analyzer. You’ll learn how to set up a simple trigger, make measurements, and set things like sample rate or memory depth.

When I first graduated from college, I started as an Application Engineer for Agilent Technologies. Based in Austin, I supported computer companies like Dell, Compaq, HP, IBM, and Tandem. I helped their engineers set up million-dollar logic analyzer configurations to measure various PC busses.

I did embellish a tad during the introduction. Shown in the video is an HP 16500. While I did occasionally help program a trigger on these analyzers, my career really started with its successor, the HP 16700. Both were amazing analyzers for their time. It is mind-boggling to think that a small $150 USB-based device has very similar measurement capabilities to those old beasts.

Previously, I made a written Logic Analyzer introduction tutorial and benchmarked digitalWrite() with a Salee Logic 8. Contrary to what people have said, the LA2016 featured in this video does NOT use Salee’s excellent Logic software. However, the user interface is clearly copied from it.

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Workbench Wednesdays

Spectrum Analyzer Basics | Workbench Wednesdays

A spectrum analyzer displays signals in the frequency domain. To use one, you only need to know about four controls: reference level, center frequency, and resolution bandwidth. In this episode, see how to use a spectrum analyzer and determine the transmitting frequency of a device.

The example device is my microphone transmitter. It operates around 500 MHz with FM modulation. You’ll see how I step through the spectrum analyzer controls to find the exact frequency. At the end of the episode, I show some advanced measurements you can do with a modern spectrum analyzer. My favorite one is the demodulation.

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