Shorted BNC dust covers - is it normal? - Page 1
Forum section: electronics / projects

author: zapta
author_rank: Super Contributor
timestamp: October 15, 2016, 03:54:11 am
content: I ordered two BNC dust covers from eBay and was surprised to find that they short the signal to ground (actual short, not 50ohm termination). I planned to use them also for the signal generator output of my DSO so this is probably not a good idea.http://r.ebay.com/FaJ2cnIs it normal? Are BNC dust covers typically shorted?

author: TheSteve
author_rank: Unknown
timestamp: October 15, 2016, 03:58:09 am
content: I know I don't want any shorted ones - I prefer the vinyl caps like ebay # 252381995742

author: joeqsmith
author_rank: Super Contributor
timestamp: October 15, 2016, 04:02:15 am
content: I use the CAPLUGS EP-8 or SEP-08.

author: ataradov
author_rank: Super Contributor
timestamp: October 15, 2016, 04:12:41 am
content: BNC caps come in shorting and open configurations. Chinese Ebay sellers make no distinction, because, well, they don't care. Normal manufacturers specify the type.I'm not really sure why you would want to use them, but all manufacturers of connectors make them, so they must be used somewhere. I guess they can be used for inputs.

author: MatteoX
author_rank: Regular Contributor
timestamp: October 15, 2016, 04:31:48 am
content: Yes, shorting BNC plugs are quite common and they have purpose. They canbe used to short circuit some inputs to prevent false triggering. For example,some HP/Agilent logic analyzers have them on the back panel.The other purpose is to protect very sensitive (wrt static electricity) inputs.Of course, you don't want to connect them to the outputs. But they areeasy to spot since they have the central pin.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: October 15, 2016, 04:52:06 am
content: That's not a "Dust Cover". Its a shorting plug as reported above to quiet inputs. If you still want to use it to cover outputs, then just snap the pin off and chalk it up to experience. These chinese are crazy!

author: tggzzz
author_rank: Super Contributor
timestamp: October 15, 2016, 08:32:43 am
content: One of my instruments has a shorting BNCpanel mount socketon an output. It is there to protect the output (a tunnel diode) from static electricity on a cable - it discharges the static as the cable is inserted; when fully inserted, the short is disconnected.That instrument is battery powered, works in the rain (despite havingfront panelCRT brilliance/focus controls with 2kV on them!) and can be submerged up to 3ft of water.What is it? A Tektronix 1502 TDR. When not connected, the output pulse is limited to showing the distance light travels between the detector and the BNC connector, 75mmThe military service manual mentions Patriot Missile Defence systems, but I've only every used it on a glider

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 15, 2016, 01:45:35 pm
content: Quote from: blueskull on October 15, 2016, 03:58:46 ama good FG/AWG will have 50 ohm output resistance and the output amp should be able to handle that 50 ohm shorted. whats worrying is if the 50 ohm resistor can take such power level at an extended period. btw shorted bnc cap will be usefull to ensure sensitive input fet will not develop static charges, or as someone said, to avoid false input signal during operation.

author: Howardlong
author_rank: Super Contributor
timestamp: October 15, 2016, 03:31:57 pm
content: Quote from: tggzzz on October 15, 2016, 08:32:43 amThat's exactly the scenario I have, although with SMAs/3.5mm APC on a sampling scope test set and trigger unit, for static protection. The manuals go to great lengths to warn of port sensitivities and how to protect them, including the use of shorting plugs when not in use.

author: zapta
author_rank: Super Contributor
timestamp: October 15, 2016, 03:58:32 pm
content: Quote from: MatteoX on October 15, 2016, 04:31:48 amIs it correct to assume that central pin = connected (short, 50 Ohm, etc)?

author: SeanB
author_rank: Super Contributor
timestamp: October 16, 2016, 03:34:38 pm
content: Not really, they could have just put the pin there and left it unconnected, so that all the terminations they make have the same look when not mated.

author: dom0
author_rank: Super Contributor
timestamp: October 19, 2016, 09:52:31 pm
content: Quote from: SeanB on October 16, 2016, 03:34:38 pmBecause that's the obvious chinesish-helpful thing to do

author: zapta
author_rank: Super Contributor
timestamp: October 20, 2016, 12:13:10 am
content: Quote from: dom0 on October 19, 2016, 09:52:31 pmIt simplifies the quality control, no need to to test for conductivity, everything goes. If you are lucky, it will have 50 ohm resistance.