2-Wire vs. 4-Wire Resistance Measurement (Kelvin) + Wave Winners!
Is a 4-wire resistance measurement worth the trouble? Find out!
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Today's tip is all about how 4-wire resistance measurements work, as well as how a power supply remote sense capability uses the same principles.
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4 wire Kelvin resistance measurement overview:
When can you use a two wire measurement, and when should you switch to a 4-wire measurement? If you don’t know you’re in luck, because that’s what we’re covering today!
measurement. Measuring resistance is often straightforward – you just grab your leads and go crazy.
There are times when a standard 2-wire measurement just won’t do the job, so you have to switch over to a 4-wire measurement. The two main cases I’ve seen people do this, and we’ll get hands-on with each of these in a moment.
The first is using a DMM to measure small resistances. The second is supplying dynamic currents with a power supply. Let’s start with the resistance measurement of a DMM.
A DMM makes resistance measurements by supplying a known current out to the device, and then measuring the DC voltage drop across the outputs. We can actually see this if we measure the device’s output during a resistance measurement.
So, let’s actually make a measurement. Here’s a quick measurement of a 1 ohm resistor – let’s store that value away for later. You might think this looks good, but there’s a problem. The DMM’s test leads are in series with the device. The resistance of the leads causes a small voltage drop, tricking the DMM into thinking there’s a higher resistance. For larger resistance measurements this isn’t a problem, but when you’re measuring small resistances this effect can be significant. Fixture resistance is also a problem in automated testing environments where you have long cables, multiple chained connections, and a multiplexer or relays between the device and the DMM. So, what we can do instead is move the voltage measurement from inside the DMM to the point of test with a second pair of leads.
Now we have four leads, hence a 4-wire measurement. These two leads source current, and these two leads make the actual measurement. Because the voltage measurement has a large internal resistance, the current through this path is nearly 0, and the impact of the lead resistance is negligible.
When measuring that same 1 ohm resistor with a 4 wire measurement, we see the new measured value. If we pull out our 2-wire measurement, we see that there’s a 6.5% difference! Clearly, a 4-wire measurement was the superior choice in this scenario.
An easy way to decide if you should use a 4-wire vs. a 2-wire measurement is to measure the resistance of your measurement lead system and compare it to the range of measurements you’re making on your device. Do the math, look at your error budget and decide if it fits within your acceptable tolerances.
The second place I see 4-wire measurements used a lot is for power supplies. Most pro-level power supplies have a capability known as “remote sense” to control the output. In a power supply, you typically set an output voltage, and the supply tries to provide that voltage regardless of the load. As the load changes, the power consumption changes. So, the supply has to sense the voltage at it’s output and adjust for dynamic loads. The problem they run into is the same – as the current changes, the loss in the supply leads changes based on Ohm’s law, and the actual power delivered to the device changes. So, you can run sense leads out to your actual device under test and the supply will adjust its output based on the voltage of the device, not the voltage at the output of the box.
#DigitalMultimeter #DMM #DMMMeasurement #DMMResistance #MeasureResistance #HowToMeasureResistance #4WireMeasurement #KelvinMeasurement #ResistanceMeasurement #DMMResistanceMeasurement #electronics #electricalengineering #computerengineering
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