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| Kelvin leads for DMM |
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| Electro Fan:
2N3055, mawyatt, others? Anyone up for posting a summary explanation or "theory of operations" regarding the role of the shield and the practical connection considerations? Maybe touch on how a fifth conductor is supposed to work with 4 wire Kelvin leads. Thx "Fairly convenient place to connect shield to analog ground of multimeter is a high current input. I use 10A input on my DM3068." "Yes that is a good spot, however we haven't seen any change in readings when connected or not, but measuring low value resistors (<100k) also." |
| Someone:
--- Quote from: Electro Fan on March 10, 2022, 09:15:47 pm ---Anyone up for posting a summary explanation or "theory of operations" regarding the role of the shield and the practical connection considerations? Maybe touch on how a fifth conductor is supposed to work with 4 wire Kelvin leads. --- End quote --- https://www.tek.com/en/documents/product-article/keithley-low-level-measurements-handbook---7th-edition |
| mawyatt:
One issue is with measuring small capacitance, any coupling between the leads can create errors. Also if the cables move, bend, or change position this can create a change in the setup residual capacitance and corrupt measurements. Having each individual wire shielded reduces the coupling IF the shields are terminated. These are reasons mentioned for using a rigid mechanical fixture for small capacitance readings where things are fixed and don't move. Another issue is potential EMI susceptibility, where shields help reduce the actual external signal coupling onto the inner signal lead, although most DMMs have pretty good EMI filtering since they are designed to work with unshielded cables/probes. With high valued resistance measurement leakage can corrupt measurements, and having shields terminated helps keep the leakage current controlled and confined, in some cases the shield can act as a "guard" where the shield is driven by a voltage similar to the one on the conductor the shield is guarding. This reduced the differential voltage between the inner lead and shield and reduces leakage. This very technique was employed eons ago to effectively reduce the cable capacitance as "seen" by the signal on the cable inner conductor, since displacement capacitive current is proportional to dV/dT, by making dV~0, or shield voltage approximately equal to the conductor signal (unity gain buffer amp to drive the shield), then the displacement current is reduced and capacitance as "seen" by the conductor signal is reduced. We patented a 2 dimensional similar technique to reduce the electron leakage between adjacent pixels in an ultra-sensistive night vision imaging chips (patent 8102452), this is where every electron counted!! Anyway, I'm sure 2N3055 and others can elaborate much more on this issue as we're certainly no expert on this. Edit: That's a great article mentioned by Someone, must read :-+ Best, |
| 2N3055:
Between what "not an expert" :P Mike wrote and Keithley LLM handbook, not much for me to add. These are simply electrostatic shields. As Mike also said, not needed for low impedance circuits, and source of leaks with really high impedance circuits, where you use triax, that combines driven guard and shield. God triax cables also have conductive layer to reduce triboelectric effects ( a cousin of piezoelectric effect in other materials) from moving or vibrating cables. For DC ohm measurements all of that is probably not necessary. For LCR meter OTOH, very useful. I use current socket as a point to connect shield when measuring low voltage AC together with shielded twisted pair for inputs. That is to reject both magnetic (twisted pair) and electric field (shield) coupling into circuit. Microphones use same type of connection: differential + shield. |
| Kean:
--- Quote from: Hydron on March 09, 2022, 09:20:40 pm ---@Kean - have you specifically tried the BK precision/Tonghui clip style from the following link vs the Keysight ones? https://www.tequipment.net/BK/TLDK1/Kelvin-Test-Leads/ It looks like the TLDK1 would have an advantage in ease of use, only needed to plug in 2 connections vs 4 or 5. --- End quote --- The only set I have apart from the KS 11059A is a set for the LCR meter with the 4 BNC box, so I can't do a comparison. I thought my LCR meter set were Tonghui, but they are just an unbranded clone of the TH26011 with the common low-cost plastic handled Kelvin clips. Only my LCR SMD adapter is Tonghui branded, and that is the primary adapter I use. The thing I really like about the 11059A set (and the Pomona/Keithley equivalents) is that the Kelvin clips are a really simple design. The only plastic part is at the pivot, and the rest is just bent metal, heatshrink, and O-rings. Almost as simple as a common clothes peg, so very easy to maintain/repair if ever needed. |
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