Self oscillating mercury switch - Page 1
Forum section: electronics / projects

author: johnwa
author_rank: Frequent Contributor
timestamp: August 30, 2016, 10:40:52 am
content: Recently I was thinking about how to make an inverter circuit without any power semiconductors. Mechanical switching is one possibility, however contact wear is always an issue. However, it turns out it is possible to arrange a mercury switch in a self-oscillating configuration by applying an external magnetic field. The image shows the basic idea: current flows at right angles to the magnetic field, which forces the mercury drop away from the contacts, breaking the circuit. The drop rolls back under gravity, and the cycle repeats.Using this effect, I was able to build a rather temperamental inverter circuit for running a fluorescent lamp from 12VDC:http://loopgain.net/hgswosc/I wonder if anyone else can get this to work any better?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 30, 2016, 11:31:37 am
content: https://en.wikipedia.org/wiki/Vibrator_(electronic)

author: Seekonk
author_rank: Super Contributor
timestamp: August 30, 2016, 03:50:57 pm
content: I used to work with ignitrons used to discharge a capacitor bank into a coil. This was used to make plastic magnets. Every couple months the mercury would plate to the glass surface and short it out. Then I would have to turn the tube upside down and shake it to collect the mercury. Be aware that may happen to your device if currents are high enough over time.

author: johnwa
author_rank: Frequent Contributor
timestamp: August 31, 2016, 07:10:46 am
content: Quote from: Alex Eisenhut on August 30, 2016, 11:31:37 amYeah, I had heard of these being used in old portable radios and things. I did try a few experiments with self-oscillating relays at one point, but they always seemed to end with the contacts welded together. Though perhaps a purpose-made device, with a tuned reed, might be more reliable.Quote from: Seekonk on August 30, 2016, 03:50:57 pmI did notice at one point, when the mercury switch got quite warm, the mercury vapour starting to condense on the inside of the glass, I suppose it would be possible to completely short it out eventually. Though I think this circuit is far too unreliable for long term operation anyway...Plastic magnets sound a bit odd! (if by plastic you mean polymer) Were these just fridge-style things containing magnetic particles, or something else?

author: ealex
author_rank: Frequent Contributor
timestamp: August 31, 2016, 08:31:05 am
content: I've seen such a thing at the university some time ago.It was used as a time-base for some relay-based railroad safety "computer", something like this:https://en.wikipedia.org/wiki/Interlocking.If I remember properly it was only used to generate timing for blinking lights, etc, as it had a 0.75Hz frequency.I can't find any picture but if I remember it had a spring loaded moving part that had the mercury switches and some kind of latching relays and electromagnets.The main switch was an U shaped tube that had mercury( quite a lot of it ) and a electrodes that connected to the latching relays and electromagnets.Also, for reliability all interlocking relays had graphite / silver contact pads -> they could never weld together (and no springs to wear down).

author: johnwa
author_rank: Frequent Contributor
timestamp: September 01, 2016, 07:19:36 am
content: Quote from: ealex on August 31, 2016, 08:31:05 amInteresting, looks like this idea has seen some practical application. Your description sounds a bit like a mercury contactor design I have heard of, with a steel core floating in the mercury that was displaced by an electromagnet.The interlocking systems are impressive bits of equipment too, especially the old mechanical systems. I guess they would have been one of the first examples of a large scale fail-safe system.

author: Cyberdragon
author_rank: Super Contributor
timestamp: September 02, 2016, 07:06:27 pm
content: I think you need one of thesehttp://www.fripro.com/other%20FAIRBAIRN%20inventions.htmLook for this

author: johnwa
author_rank: Frequent Contributor
timestamp: September 05, 2016, 09:07:24 am
content: Quote from: Cyberdragon on September 02, 2016, 07:06:27 pmI suppose the oscillations have a certain warmth that semiconductor oscillators cannot achieve?Nice avatar there, Cyberdragon!

author: DTJ
author_rank: Super Contributor
timestamp: September 05, 2016, 09:23:10 am
content: Quote from: johnwa on August 31, 2016, 07:10:46 amI recall pulling a valve (tube) car radio apart as a kid in the 70's. It had a vibrator in it to generate "AC" which was stepped up for the valve HT.

author: Cyberdragon
author_rank: Super Contributor
timestamp: September 05, 2016, 04:47:41 pm
content: I melted the contacts in a vibrator connected to a reversed mains transformer while trying to make an arc lamp from pencil leads.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 05, 2016, 05:06:32 pm
content: Properly designed motor brush designs have thousands and thousands of serviceless runtime, industry proven. You could try to modify an existing brush&commutator assembly from a quality DC motor to build an inverter.

author: Tibby
author_rank: Newbie
timestamp: September 23, 2016, 11:16:34 am
content: This device was used as a timing source in old, relay based, railway signalling systems. The mercury oscillates in the U tube. There is a long contact and few shorter ones. Some are used to set the oscillator in motion, when the mercury breaks the contact it swings back, but others re used to provide various timing signals and flash lamps. Electrodes are at different lengths to provide different timing signals, but of course all in phase as they stem from the same source

author: Lukas
author_rank: Frequent Contributor
timestamp: September 23, 2016, 01:32:39 pm
content: I found some (unfortunately german) documents explaining the operation of the mercury oscillators used in railway signalling sytems:http://www.drehscheibe-online.de/foren/read.php?17,6363754,6364082#msg-6364082https://depatisnet.dpma.de/DepatisNet/depatisnet/DE000000536467A_all_pages.pdf?window=1&space=menu&content=download_doc_verify&action=download_doc&docid=DE000000536467A&so=asc&sf=vn&firstdoc=1&struct=&Cl=2&Bi=1&Ab=&De=1&Dr=3&Pts=&Pa=&We=&Sr=&Eam=&Cor=&Aa=&NrFaxPages=3&pdfpage=1&pdfmatrix=1x1&origin=pdf_window&verify_str=null