Electronics > Metrology

4mm binding posts leakage to ground.

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Alex Nikitin:
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.

Cheers

Alex

P.S. - it is worth mentioning the environmental conditions for all measurements mentioned above, the room temperature was 25C, relative humidity 40% .

BILLPOD:
Good Morning Alex, VERY nice work. :-+ :-+

Stray Electron:
  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?

EC8010:
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?

Alex Nikitin:

--- Quote from: Stray Electron on July 26, 2024, 03:15:37 pm ---  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?

--- End quote ---

Thank 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  :palm: .


--- Quote from: EC8010 on July 26, 2024, 03:48:34 pm ---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?

--- End quote ---

Yes, however this didn't change the measurements for the switch  :(

Cheers

Alex

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