4mm binding posts leakage to ground. - Page 1
Forum section: electronics / general

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 26, 2024, 12:56:36 pm
content: 4mm binding posts commonly used on measuring equipment, including metrology level meters, calibrators and references. The isolation leakage to ground is not a problem for a typical use in an average multimeter and even less so in a power supply etc. However if you chase ppms it might easily become a serious concern. While 1Gohm sounds a lot, it may introduce an error of 10 ppm for a 10K resistance and 1000ppm (0.1%) for 1M resistance. And if you are looking for a 1ppm error at 1M level, a minimum leakage resistance acceptable would be above 1 Tohm. That is the main reason I've decided to place this thread in Metrology section.I have means to measure these kind of leakage resistances and my Keithley 617 electrometer routinely set up for measuring leakages to ground for up to +/-100V test voltage. The self leakage of the measuring system, including a 1m coaxial cable and connectors, is below 5fA at 100V, or >20000 Tohm = >20 Pohm.A little while ago I've bought a number of good quality (Tektronics spares, most likely Superior Electric BP21) binding posts from a forum member, with the idea of building some useful switching boxes, mostly to use with my HIOKI DM7275 which has only one input so gets abused with constant cable switching. My usual binding post of choice is Pomona 3770 Cu-Te however they are expensive, at about $15 each, so I had some hopes that a cheaper alternative would work for me in a switching box and other not so critical places.The Pomona 3770 series uses polycarbonate for isolation, but the Superior Electric BP21 uses nylon, so I did expect leakage to ground to be larger, however I didn't realise how much larger it would be! My measurements on a post mounted in a metal box gave figures up to 30nA (!) for 100V or about 3.3 Gohm. The Pomona 3770 leakage measured at about 10pA at 100V or 10 Tohm, not perfect but 1500 times better (and generally good for most purposes you might use a 4mm binding post. I did also measure leakage to case ground for some of metrology items I have in the lab. The SR1010 resistors arrays measured from 100pA at 100V (1Tohm) for 100K set up to 1.8nA at 100V (~55Gohm) for 10K set. The Fluke 742A-10k measured 31pA or respectable 3.1Tohm leakage resistance to ground. So I thought I've wasted my money on these Tek posts!After some consideration I've decided to check if it is feasible to add insulation to Tek posts, using some PTFE washers, however I've found that suitable sizes are not available from stock. To order a custom size would be prohibitevely expensive. Nevertheless, I kept looking till I've found a cheap spacer from Rosenburger (available from RS), made from Polystyrene to isolate panel mounted BNC connectors, with perfect dimentions to take a 4mm binding post from Tek/Superior. Come next day, I've mounted a Tek binding post using a couple of these spacers and got a very respectable leakage - below 30pA or 3 Tohm. On par with the Fluke so I was reasonably happy and started to assemble the switching unit. After I've mounted all 9 posts I've measured the leakage and was somewhat surprised by the big differences between posts, the worst ones measure over 30pA, the best - under 2pA. So I've found a softer place on the wall and banged my head a bit on it, while another 20 spacers were washed in IPA. After thorough drying the spacers and re-mounting the posts, the leakage went down to 10-30fA on all nine, making roughly a 1000 times improvement. My head deserved the punishment as I should've washed the washers before assembling the lot first time!Below are some photos, now I am not entirely happy with the switch I've used (an old ceramic-based Soviet military type) as it is the leakiest part in the whole assembly (around 3-5pA or 20-30 Tohm) however it is still better than the Fluke, so I'll live with it! The switch box allows for two 3-wire lines to be switched to one output (plus the OFF position), and two BNC connectors switched in the same way to one BNC output. The next test is to see how bad the thermal voltages are in this setup, with brass contacts.CheersAlexP.S. - it is worth mentioning the environmental conditions for all measurements mentioned above, the room temperature was 25C, relative humidity 40% .

author: BILLPOD
author_rank: Frequent Contributor
timestamp: July 26, 2024, 01:34:23 pm
content: Good Morning Alex, VERY nice work.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 26, 2024, 03:15:37 pm
content: Yes, that's a nice bit of detective work! I'm glad to see some real numbers and some real measurements for projects like this.That rotary switch appears to be a copy of the American made (Switchcraft? brand) that were made in the 1960s. Have you tested any of those for leakage?

author: EC8010
author_rank: Frequent Contributor
timestamp: July 26, 2024, 03:48:34 pm
content: Interesting stuff. To save further head-banging, you did scrub the switch wafer front and back with flux cleaner then rinse it with copious isopropyl alcohol, didn't you?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 26, 2024, 03:57:43 pm
content: Quote from: Stray Electron on July 26, 2024, 03:15:37 pmThank you, as I've mentioned, the switch I use (4way 2 positions) leaks about 3-5pA at 100V. I have a similar switch 3W3P and it is considerably worse due to a somewhat different design with a single phenolic ring for all moving contacts (see the photo attached), it leaks about 5nA.Quote from: EC8010 on July 26, 2024, 03:48:34 pmYes, however this didn't change the measurements for the switchCheersAlex

author: TimFox
author_rank: Super Contributor
timestamp: July 26, 2024, 04:10:20 pm
content: A known problem with Nylon insulation is that it absorbs a measurable amount of water from ambient moisture, up to maybe 2%.https://www.plastics.toray/de/technical/amilan/tec_003.htmlPolycarbonate and acrylic are much better.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 26, 2024, 06:19:29 pm
content: Quote from: TimFox on July 26, 2024, 04:10:20 pmYes, it is a known problem, fortunately the solid PS (polystyrene) has one of the lowest water absorption coefficients, so the spacers help in that respect as well.CheersAlex

author: EC8010
author_rank: Frequent Contributor
timestamp: July 26, 2024, 06:21:22 pm
content: Quote from: Alex Nikitin on July 26, 2024, 03:57:43 pmWhatwerethey thinking of? There appear to be far more different designs of ceramic wafer switches than I ever imagined. I look out for them at radio rallies and now have five incompatible wafer designs and mechanisms. Mostly, they vary centre distance between mounting pillars, but even when they have 1/4" shafts (not guaranteed), the thickness of the flat varies, making it necessary to pop a mechanism into the mill to make a shaft fit a wafer.I've successfully used polished clear Perspex (methyl methacrylate) for low leakage. Be warned that (like all plastics) when you drill a hole in Perspex, the resulting hole issmallerthan the drill. If you get "eee eee" noises when tapping Perspex, it's because you needed to use a larger drill than recommended tapping size.

author: mendip_discovery
author_rank: Super Contributor
timestamp: July 26, 2024, 08:02:38 pm
content: Handy info on insulative materials,https://web.archive.org/web/20190422012506/http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_3.htmlThanks@Alex NikitinI have not gone too deep into the measurements of large resistances other than to play with some insulation testers. However, I would be interested to know how much the leakage you are getting affects measurements at 1G, 100M, 10M etc.

author: TheDefpom
author_rank: Frequent Contributor
timestamp: July 26, 2024, 08:10:57 pm
content: Perspex, a trade name for Acrylic, proper name PolyMethylMethAcrylate (PMMA) is also hygroscopic, meaning that it does absorb moisture, which will certainly increase leakage.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 26, 2024, 08:40:18 pm
content: FWIW, I had some leakage problems with the banana jacks on an HP null meter. It turned out that the aluminum panel had corroded a bit and somehow managed to creep or dust it's way to the center conductor. Very slight leakage, but enough to notice and measure. I'm now much more careful about cleaning panels, connectors and hardware before assembly and preventing corrosion. I suspect some clear conformal coating might not be a bad idea, maybe Krylon clear spray.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 26, 2024, 08:57:23 pm
content: I've seen recommendations for a different style of binding post for use with high voltages (picture attached). They're constructed so that there's no chance of internal arcing to the chassis. Would this style have lower leakage than the two-piece style that Alex used?Ed

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 27, 2024, 12:01:37 pm
content: Quote from: edpalmer42 on July 26, 2024, 08:57:23 pmThe leakage depends mostly on material, cleanness and temperature/humidity, as these posts on the picture most likely use Nylon for isolator, the leakage might be substantial. 4mm binding posts should not be used above 1000V anyway. From purely mechanical construction point the traditional design is much sturdier in my view.CheersAlexP.S. - I run the switch overnight with copper wire shorts on both inputs and the output connected to the HIOKI DM7275. The offset has drifted from about -50nV to about -150nV overnight with about 1.5C change in temperature, and does not visibly change when I switch between two (shorted) inputs (so the change is roughly below 50nV). On the graphs below the vertical lines indicate the point of switching between inputs, there is a small (0.1-0.3mV) spike due to the contact interruption. The first graph is zoomed in for 1h so the switching is better visible. Vertical scale is 100nV/division, 1uV full span.

author: EC8010
author_rank: Frequent Contributor
timestamp: July 27, 2024, 12:43:00 pm
content: Further to the issue of switch leakage, I used a Keithley 6517B to measure the leakage from wiper to chassis of four different types of ceramic wafer switch with 100V applied inside an earthed conductive enclosure:CRL (USA): 7pA had ceramic spacers tooS402 (USA): 30pA cleanAB "H" Type (UK): 10pA thicker ceramic than the CRLOAK (UK): 16pA but very dirtyIt seems that we can expect a leakage of around 10pA at 100V from a clean ceramic wafer, corresponding to 10 teraohm. The S402 appears to be a different (and inferior) type of ceramic. The CRL switch was salvaged and had been cleaned very carefully whereas the others were as found at radio rallies.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 27, 2024, 12:53:01 pm
content: Yes, that seems to be the limit. If I need to do a fA level switching, I have some really nice reed relays for it with only 2-3fA leakage on the open contact at 100V, and these can be operated with only a permanent magnet moved in and out. For switching of a voltmeter input with a <20pA input current, such as my Hioki DM7275 or HP3456A, these ceramic wafers are barely OK.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 29, 2024, 02:05:23 pm
content: I've run the offset test for 55h and the temperature dependency is clearer, and it might be the voltmeter drift, however with about 100nV/C it is good enough for my applications. Perhaps I should buy more of these 4mm posts.CheersAlex

author: TizianoHV
author_rank: Regular Contributor
timestamp: August 03, 2024, 05:02:39 pm
content: I have a 1.5kV analog electrostatic voltmeter. It came without binding posts. Originally it had a special double insulated binding post to ensure minimal leakage.It was important to choose the right binding post, in fact the meter by itself has practically no leakage (<1pA). After charging it at 1.5kV it can hold charge for days into its parasitic capacitance (only a few pF)._____________________________________________________I tried using normal biding posts, but they were leaking (it was discharging in a few seconds)._____________________________________________________So, I tried with some of these cheap safety banana posts:https://it.aliexpress.com/item/1005004657741735.html?spm=a2g0o.order_list.order_list_main.52.6dab369668CxMN&gatewayAdapt=glo2itaLeakage at 1kV was in the pA's (pracitronic MV40). Thanks to the long isolation distance they were fine up to 4kVDC.I tried it, in one day (50% humidity) the voltage dropped from 1.5kV to 1.3kV. Not perfect but more than good enough for me.______________________________________________________So, if you are interested only in low leakage, and not in uV's they can be a good choice.

author: Hydron
author_rank: Super Contributor
timestamp: August 03, 2024, 07:35:02 pm
content: I used safety banana jacks for a breakout panel for my SMU and they don't give any detectable leakage (i.e. less than a few 10s of fA at up to ~1kV). What I did do however is I used a PCB panel to mount them in, and surrounded the sensitive ones (Sense/Force HI) with guard potential, which is kinda cheating and not an option for something like the meter above.On the back side of the panel this is trivial (pour unmasked copper around the holes and solder to guard) but on the front I had to compromise and only put a thin ring that's hidden under the plastic shoulder - guard potential on a K236 can be up to 1.1kV so can't have it accessible to finger-poking!See this post for some pics:https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg5541853/#msg5541853

author: Gyro
author_rank: Super Contributor
timestamp: August 03, 2024, 09:22:45 pm
content: Quote from: TizianoHV on August 03, 2024, 05:02:39 pmYou may be interested in a characterization I did on a similar 1kV electrostatic voltmeter. It clearly demonstrates (2nd plot) the importance of surface cleanliness...https://www.eevblog.com/forum/metrology/characterising-and-electrostatic-voltmeter/

author: Overspeed
author_rank: Frequent Contributor
timestamp: August 04, 2024, 05:21:53 pm
content: Helloa very nice materialhttps://www.corning.com/worldwide/en/products/advanced-optics/product-materials/specialty-glass-and-glass-ceramics/glass-ceramics/macor.htmlthat can be machined with carbide tool without problemother solution cheaper is to machine flange in PTFE look like a square washer with a central hole as the banana body diameter and 4 holes for screw as a N connector for chassis so the Banana connector is round up by PTFERegardsOS

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2024, 05:35:20 pm
content: MacorTMis a good glass-ceramic material that can be machined with carbide tooling.It is a rigid material, so you need to add squishable washers under the screw fasteners to avoid cracking.

author: The Soulman
author_rank: Super Contributor
timestamp: August 24, 2024, 11:49:01 am
content: To add another binding post to the list I've measured my favorite "affordable" one the:https://www.sks-kontakt.de/en/test-measurement/products/product?nr=9301037Hirschmann PKI10 Au measures 400 Giga Ohm

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 24, 2024, 01:01:56 pm
content: Just a curious thought. Would making some cutouts to the material the terminals are attached to help reduce leakage, making the path longer.

author: The Soulman
author_rank: Super Contributor
timestamp: August 24, 2024, 02:23:14 pm
content: If that material is able to conduct electricity than no.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 24, 2024, 02:28:57 pm
content: Quote from: The Soulman on August 24, 2024, 11:49:01 amNot too bad for PA (polyamide = nylon) isolation.CheersAlex

author: The Soulman
author_rank: Super Contributor
timestamp: September 08, 2024, 01:24:29 pm
content: Quote from: Alex Nikitin on August 24, 2024, 02:28:57 pmSurprised me as well, it is much better than my diy-made-from-lampshade-cord-four-wire-resistance-measurement-cables.