What is this component in a Gossen Metrahit 29s? - Page 1
Forum section: electronics / beginners

author: Assafl
author_rank: Frequent Contributor
timestamp: September 03, 2016, 07:32:11 pm
content: So I tried to figure out what this is an am stumped. It is in the input section so my initial guess was some sort of gas discharge for overvoltage protection.I am pretty certain it is for protection. The case - opposite this component has a shiny silvered aluminum sticker affixed right opposite from it. So my guess is that the aluminum is to prevent it from making a hole or burning the case. That spells protection circuit.But it is adjustable so any gas would leak. As an adjustable air gap, there is an air gap component next to it - so why would they need two?

author: Zero999
author_rank: Super Contributor
timestamp: September 03, 2016, 07:36:20 pm
content: It looks like a variable capacitor to me.

author: Assafl
author_rank: Frequent Contributor
timestamp: September 03, 2016, 08:06:03 pm
content: Does look somewhat. But it is a rather big can with a screw in the middle... Sort of Layden Jar...It measures about 13M Ohm across. But I am not sure it isn't wired in parallel to anything else.I am not tracing it (yet) as I am loath to unsolder the board. Cant remove the board without desoldering.I am hopeful somebody can save me the trouble.Please note that the 3rd picture - the foil shows a specular reflection due to the flash - the round gray area is a highly silvered metal (reflective) disk.

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 03, 2016, 08:23:13 pm
content: +1 for trimmer cap, it can't really be anything else, it must have to do with AC compensation. Do you need to know apart from curiosity?

author: Assafl
author_rank: Frequent Contributor
timestamp: September 03, 2016, 08:27:30 pm
content: Curiosity.Why would they protect a cap with a metal foil? It isn't grounded so as shielding it is not so good...Also - as a cap it is very tall. there is a big metal can at the base and a screw through the to. A piano wire spring is wedged against the screw and holds the plastic surround which covers the majority of the component.

author: SeanB
author_rank: Super Contributor
timestamp: September 05, 2016, 06:40:45 pm
content: It is a cylindrical variable capacitor, there to provide AC compensation for the input with increasing frequency. It forms a potential divider with the variable capacitor and the input capacitance of the meter input circuit, and is adjusted so that the ratio of these capacitances is the same as the ratio for DC voltage. Thus the response of the meter is constant for DC and Ac voltage. This is more important at higher frequencies like over 1kHz, where the capacitances will pass enough reactive current to have a noticeable effect on the voltage at the actual ADC input of the meter.Cylindrical so that the outer electrode ( connected to the input) provides shielding to the inner more sensitive electrode so hand movement near the meter will not affect the reading, as this small capacitor is the most easily affected by nearby conductors. If you trace it you will find it is attached across the input sampling resistor network ( the blue resistor array to the right of it in the last picture) and will be on the first and second pins, the ones with the 13M resistor inside, and the ones with the large creepage space. The capacitor has to be rated for operation at more than the meter is rated for, as it cannot break down in service without destroying the meter.

author: Assafl
author_rank: Frequent Contributor
timestamp: September 06, 2016, 09:12:55 pm
content: Indeed it is wired in parallel to the 13Meg resistor.Since it is sensitive - it probably also means that the foil in the case opposite the capacitor is simply shielding for the inner electrode?A very strange capacitor - still can't find it in catalogues... Wonder who makes them...