Measuring old Soviet Mica Capacitors & Kelvin Clip Issues - Page 1
Forum section: electronics / projects

author: mawyatt
author_rank: Super Contributor
timestamp: September 06, 2025, 05:04:20 pm
content: There's some old Soviet hermetic mica capacitors floating around, and we've been measuring a few for some members here. These are mica layered capacitors encased with a metal tub and with two insulating posts as shown and were asked to make a few measurements with our Hioki IM3536 LCR Meter. The construction is certainly sturdy and the outside tub case is fully painted and galvanically isolated fro the internal capacitor, so the internal mica cap "floats" within the tub.Our initial measurement solution was using quality Kelvin Clips (TH26011CS) over a guard shield plate connected to the meter guard terminal. DUT measurements were conducted on a small cardboard box ~2" above the guard shield plate with the Kelvin Clips suspected above the plate. After the proper Zero Ohm Short Cal, the Open Cal was performed and this posed some interesting problems.Most of the Kelvin Clips (quality or cheap) we have the blades are not shielded and the shielded coaxial cable is unterminated at the Clip Handle end, so the entire Kelvin Clip and Handle is effectively unshielded. This presents a problem with a good Open Cal and subsequent quality measurements. How to position the Clips for the Open Cal and retain thatexactorientation for measurementsCable position and Clip position effect the Open Cal, not to mention with DUT measurements. Sure low Z measurements aren't impacted as much, but high Z DUTs like small capacitors are significantly effected. This is one of the reasons we prefer the direct (no cable) LCR Meter Fixtures like SMD Fixtures and Leaded Component Fixtures like the TH26048A and TL89F1 where the DUT leads plug into the fixture. These are more mechanically and electrically stable than Kelvin Clips and yield better repeatability.Now the next question arrises with the Soviet Caps Tub, this will provide a field termination for the internal mica cap and add additional capacitance between the two cap terminals. If this Tub is left floating it will assume a voltage which will then create displacement currents with the surroundings and Kelvin Clips and cause additional Kelvin Clip and DUT position sensitivity. Seems this begs for the Tub to be grounded or connected to the Guard Shield, however the caps have a fully painted tub with no apparent means for a tub electrical or physical connection? We connected to one cap that had some paint removed and the change in meter readings was significant, not surprising considering the physical size of the Soviet Cap!!Now to add some short wires to one of the DUTs so we can measure with the mentioned Leaded Fixture.Other things to report but will address later.Best

author: mawyatt
author_rank: Super Contributor
timestamp: September 06, 2025, 06:17:14 pm
content: Quick report back. We soldered spade lugs rather than short wires to the DUT to keep the low Z of the Soviets Micas (Low D) and used a Gator clip to the Guard Shield for the DUT Tub connection while mounted to the mentioned leaded LCR Meter Fixture.Result is a significant improvement the DUT sensitivity to local environment (hands) and improved repeatability usually within +- 2 last digit, with Kelvin Clips we saw 2~3X as much.Another consideration for improving LCR measurements is the SNR of the measurement parameters, which in these meters is DUT Vector Voltage and Current (actually another voltage from a transimpedance amp on the meter Low terminal side). With a low Z DUT one wants higher DUT currents to create a higher DUT measurement voltage and with higher Z DUT one wants higher DUT currents which can be very small with high Z DUT at low frequencies. Both of which imply a higher test voltage from the LCR meter to force higher DUT currents in either high or low Z cases. Most bench LCR meters allow a variable or select test voltage (some a select source impedance) and a few allow this up to 5VAC. These meter features allow one to select the test voltage (and source impedance) for a given DUT and test suit. Of course high DUT currents imply higher DUT internal generated temperatures, and higher DUT voltages may negatively impact results for DUTs with PN junctions.Anyway, more testing as time permits. Curious to what others experience?