UNI-T UT612 – Kelvin Clips Test Results, Guard Function & Hacking Feasibility - Page 1
Forum section: electronics / beginners

author: Gerhan
author_rank: Contributor
timestamp: June 25, 2025, 12:08:15 am
content: Hi everyone,I wanted to share some hands-on findings regarding the UNI-T UT612 LCR meter, particularly focusing on its Kelvin clip setup, GUARD terminal, and whether it can truly support 4-wire Kelvin measurements the way it appears to.BackgroundI’ve been using the UT612 with its stock Kelvin clip test leads — the ones where each clip actually carries two wires (current and sense) back to the banana plugs, ending in the 4 front terminals: L, H, –, and +.My goal was to determine whether this instrument implements true 4-wire Kelvin measurement, or just mimics the physical layout without actual internal separation of sense and current paths.Observations from TestingEven when only two of the four cables (one red, one black) are connected, the meter still NOT shows plausible readings in most mid-range R/L/C tests.Switching combinations between H/+ and L/– shows little to no effect on accuracy unless you're dealing with very low resistance (<1 Ω) or very small capacitance (<100 pF).Measuring the continuity between H and +, and L and – using a DMM showed low resistance, confirming that the device internally connects those terminals — suggesting it’s not a true Kelvin implementation.about the GUARD TerminalThe meter includes a GUARD terminal, which I tested for parasitic capacitance reduction during low-value capacitance measurements.When measuring capacitors in the pF to low nF range, grounding the shield of the DUT or test leads via the GUARD terminal did improve stability slightly.For anything above ~1 nF, the GUARD had negligible effect.Conclusion: GUARD is helpful for sensitive measurements, but most hobbyists won't need it for typical passive testing.Hacking Possibility – True Kelvin?I’ve also been wondering if it's feasible to hack the UT612 to support a real 4-wire measurement, by internally separating the current and sense paths between H/+ and L/–.At this point, it’s unclear whether the internal layout would allow it without significant modification or firmware-level support.Has anyone here attempted to modify the UT612 for true Kelvin mode?Or alternatively, has anyone mapped the internal PCB to confirm the terminal routing?Final ThoughtsOverall, while the UT612 looks like a 4-wire instrument, it behaves functionally as a 2-wire meter with semi-Kelvin clips.It's still usable for most general RLC measurements, but for precision low-resistance or ESR measurements, a true 4-wire meter (like DE-5000 or HP/Agilent options) would be required.I’d love to hear your feedback, especially if anyone has tried modifying this unit or probing the GUARD function further.Thanks!GERHAN

author: jwet
author_rank: Frequent Contributor
timestamp: June 25, 2025, 11:27:47 am
content: It sounds like the instrument is doing everything right including true 4 wire kelvin. The expensive part of kelvin sensing is the connectors and hardware, the electronics is near nothing, a couple of op-amps.I would expect the force and sense leads to be connected internally by a low resistance. This is to eliminate open loop error. It doesn't hurt to have a 50 ohm R across them at the instrument, any leads you run will be much lower in R and in parallel with these on the sense side. There is no value in keeping these isolated. The value is getting rid of any voltage drop on the returns (sense). This is common on remote sensing power supplies too- the sense is just to reduce an error in high drive situations.Likewise, I would only expect guard to do anything at very low signal levels like measuring a small capacitance. A larger DUT capacitance can overcome the kind of leakage that a guard is trying to fix.Most LCR's, or pretty much any instrumentation, has a bathtub shaped curve of accuracy vs measured value. At low levels, you're noise limited and at high levels you're drive limited. You get a curve that has a wide flat spot for 90% of the range and then has excursions.Look at a graph from the Genrad 1658 (manual link below) around section 3.3- this is a classic gold standard mainstay instrument.https://www.ietlabs.com/pdf/Manuals/GR/1658%20Instruction%20Manual.pdfWith all measurements, you decide what you're after and then get an instrument that will do it. The problem with all this inexpensive, abundant, GP test equipment is that they will do about 90% of the job really well for 1/10th the cost of the fancy stuff. If you need that last 10%, you're out of luck. Its called the "Casio Effect" in industrial design circles- Casio will give you a keyboard etc for really cheap that almost gets you there...

author: mawyatt
author_rank: Super Contributor
timestamp: June 25, 2025, 02:30:04 pm
content: It's always a good idea to try and understand how instruments make measurements, LCR meters are no exception. Tweezers types can't actually perform 4 wire Kelvin measurements "at the DUT contacts" since they have the Force and Sense leads terminated at the probe opposite ends and not at the DUT contacts. With Kelvin Clips, the tips are F and S and make contact "thru the DUT" when properly used. On our Kelvin Clips we usually note which is HC, HS and LC and LS just for convenience.Kelvin techniques help reduce the effects of DUT contact and test lead effects and work best with low Z DUTs (like low value resistors) and don't provide much benefit for high Z measurements (like small caps).The Guard LCR terminals are usually ground referenced inside the meter, some are directly connected to chassis ground/shield. Using this feature as a "guard shield" helps with sensitive high Z measurements (like small caps or high resistances). Hioki recommends using a guard shield conductive "plate" when making high Z Kelvin measurements, where the measurements are made over the plate surface area (insulated of course). We found this helps with repeatability and "noise" and used the metal magnetic base which came with the PCBrite setup as the Guard Shield connected to our Hioki IM3536 "Guard" external banana terminal.Should mention these "Bridge" type LCR meters don't measure Resistance, Capacitance, Inductance or Impedance directly, this result is inferred from DUT vector voltage measurements and DUT vector current where the current is actually a voltage measurement scaled by the "sense" element as the DUT current is "actively forced" to flow thru this sense element producing a voltage by Ohms Law. The "actively forced" is usually a transimpedance amplifier where the amp directly sources/sinks the DUT current and creates a zero volt reference (ground) voltage result.Anyway, getting quality good repeatable LCR measurements isn't always simple and often requires specialized test fixturing and careful attention to measurement setup layout.Best