Mercury Arc Rectifier? Where to start

[Steph1997]

Haha I'm glad that everyone's genuinely interested!
regarding the transport, the man had it in a cardboard box laid on its head which is the best way as the mercury don't splash in the anodes, but even still I had both hands supporting it all the way (me being passenger of course!)

I did notice that there was a few drops of mercury sloshing around in the ignition anode half way through the journey which I wasn't to fond about it being there so had to make sure it was at and angle where the risk of the mercury falling was slim! safe to say I had terrible arm ache by home but it was definitely worth it!

The solenoid ( I think its called ) was vibrating against the glass when going around corners which was making my heart beat about a mile a minute!!
But she made it in one piece and was cleaned last night once my heart had returned back to a normal rate!

Regards,
Steph

[Steph1997]

All cleaned up the best I could for now!

Shes a beaut thats for sure!

Regards,
Steph

[julie_m]

She looks absolutely stunning!

Now you just need to give her a suitable home and working environment, to protect her, to protect you from her transformer and to show her best features; a black wooden cabinet with a glass or Perspex front, and maybe have the rectifier compartment lined with black silk or velvet. Perhaps even studded with pieces of mirror tile, so as to catch the glow when she is running! If you use a sewing machine on a wide zig-zag stitch (or buttonhole stitch end section, if the machine just has pre-set stitches as opposed to separate length and width controls), drop the feed dogs and mount your material in an embroidery hoop, you can embroidre shapes, then cut them out with a very sharp knife and the stitching will prevent fraying. Stick pieces of mirror behind the apertures with hot-melt adhesive; this will also seal the edges of the mirror and prevent the reflective metal coating from flaking off.

..... Sorry, I seem to have got a bit carried away with myself She's your baby, after all; you can house her however you like!

[short wave]

Hi Steph glad to here you got your M.A.R.. I have found a link to a PDF book that has plenty of info on the electrical side of M.A.R.s / The book also has some photos of installations that may also be of interest you.
Link

http://www.tubebooks.org/Books/pmaratc.pdf

Regards S-W

[kalee20]

Wow some meaty theory there! With some relevance to my 'day job,' unfortunately not involving MARs.

[Karen O]

I was relieved to find the Birmingham science museum MAR at the 'Think Tank' at Millennium Point, although I suspect it is now festooned in blue LEDs to give the impression of operating.

Could a mock-up be made using such LEDs, silver paint or something, and an agitator?

It would be SO much safer than the real thing

[mark_in_manc]

Alright, I'll ask the daft technical question. I guess the little electrode is to 'ignite' it, and the shorter one poking out at the front of the last picture is the cathode. So why two anodes? Two-phase ac? If so, what would that be like - two phases at 180 deg? And where would it (that is, such a 2 phase supply) have come from?

(OK, should have scanned the text book pdf above, first. It seems the thing 'goes out' during the zero-crossing points of single phase ac, so one needs more that one phase. How much more than one - can you phase shift a few degrees using some kind of reactive network for the second anode, just to keep it alight, but not draw much current through that network?)

[turretslug]

I'd assumed it's a centre-tap winding set-up, like the familiar (and comparatively puny!) HT full-wave arrangement?

I'm somewhat awed by this project, speaking as someone who finds any evacuated glass thing bigger than about a KT88 slightly intimidating. Good luck with it, we're all ears.

[Lucien Nunes]

Yes it's the conventional single-phase fullwave arrangement as per normal valve rectifiers, bulb cathode is DC+ and centre-tap DC-. The utilisation factor for the transformer secondary is not as good as a bridge, but like a valve you can have many anodes in a bulb (but only the one cathode), and the forward drop is significant on low voltage circuits, so this is the most practical arrangement for single-phase supply.

For three-phase supply there are alternatives according to application. Six phase was common for heavy loads, derived from special transformers with two three-phase groups and an interphase transformer to commutate between the groups. For less demanding applications, or where grid-control was used, 3-phase bulbs were popular. A few bulbs were 12-phase, such as that supplying the Type 80 radar.

[kalee99]

Hi all,
Many years ago I worked / ran a cinema I installed a pair of Nevelector MARS. These were single phase and rated at full 100 Amp DC. They were interesting in that they did not use an exciter circuit. The dipper arm just fired the Mecury up enough for the anode arms to take over. Had to be set up dead level or the dipper arm would not hit the mercury pool so no strike up when you brough your carbons together.
The rectifiers came with two new bulbs in transport cases. I got the set up from a cinema closing down. They were delighted for me to take them away as safe disposal was going to cost a lot.
I was very dissapointed with the quality of the picture when we stopped using them and went over to zxenon lamp & long play unit. I still showed the adverts on Machine B which was still on carbons & Nevilector & the difference was astonishing.
As far as I know the rectifiers are still in situe in the ex cinema I ran (its now a furniture warehouse).
You cannot beat a good MARS & carbon arc lamphouse for quality projection.
Paul

[kalee20]

Basically - unless Lucien corrects me otherwise! - it's going to need a transformer with a centre-tapped secondary.

Not having auxiliary anodes (except the starter electrode), there is less complication. The down side is it's going to need a minimum load, 5A perhaps? rather than staying running completely off-load.

Mark-in-manc is right that the thing could 'go out' during the zero crossings. To prevent this, it's going to need to operate with a choke input filter, which (provided inductance is sufficient) will guarantee continuous current flow with the minimum load. The choke goes either in series with the cathode, or in series with the centre-tap, equivalently.

So far then, Steph is going to need a transformer with a centre-tapped secondary, maybe 40-0-40V at 20A, and a DC choke rated also at 20A. Then there's the startup stuff to consider, a solenoid to attract the dipper electrode, and another transformer to drive the dipper electrode, with a chunky contractor or manual push-button. But it all seems do-able!

Completely over the top, here goes...
To make it light up well it seems about 30V at a few amps is needed, why not put a 5V shunt regulator in the cathode? It would be the ultimate USB charger and would be a lot safer than the fake Apple chargers in the news.

[Lucien Nunes]

The ignition can be driven from the main anode circuit without an extra transformer. In the Nevelector units such as Paul mentions, and probably the unit that Steph's bulb came from, a ballast resistance feeds it from the main secondary. When the lamp arc is struck, an AC circuit is completed through the ballast, ignition cutout contactor, dipper solenoid and load. The dipper shorts out the solenoid which releases, the dipper draws the arc, then the main anodes immediately steal it because the voltage drop in the ballast is much greater than the difference in arc drop between dipper and main anode. As soon as the load exceeds a couple of amps, the cutout contactor operates, disconnecting the solenoid and dipper. This interaction of load and ignition is only possible (and necessary) in exciter-less unit rectifiers where the bulb is only conducting when there is load, and a minimum load is guaranteed. In a fixed-output installation the ignition is driven and controlled from the excitation circuit instead.

Some Nevelector models used a bowl-fire element as an ignition ballast, others had a conventional wirewound resistor. During an outing of some projectors from the Projected Picture Trust, the ballast in the Nevelector powering one of the lamps went open-circuit and, for whatever reason, was not easy to repair on the spot. I scrounged a couple of 100W GLS light bulbs to shunt it, which worked fine. They were strung above the rectifier on leads and might have been the world's first bulb-ignition-bulbs.

If I get time this weekend I will make a video of the sequence of operation of our Nevelector, showing the components needed to make Steph's bulb work.

[britishrocket]

I was just wondering...what did the person who drove you to collect your rectifier think of you sitting there cradling the thing?!

Carl.

If it was me, I would do my best for a comfortable ride for both the bulb and Steph. A bit like restoring a radio, think (look listen) and proceed with both pace and caution.

[kalee99]

Originally Posted by Lucien Nunes

Some Nevelector models used a bowl-fire element as an ignition ballast, others had a conventional wirewound resistor. During an outing of some projectors from the Projected Picture Trust, the ballast in the Nevelector powering one of the lamps went open-circuit and, for whatever reason, was not easy to repair on the spot. I scrounged a couple of 100W GLS light bulbs to shunt it, which worked fine. They were strung above the rectifier on leads and might have been the world's first bulb-ignition-bulbs.

Mine had the big tubular resistor. Also the pair could be paralled up to give 200 Amps if needed. Never used that option 45 Amps was enough with 6 & 7 mm carbons. Up a bit for cinemascope or dense prints.
Paul.

[Steph1997]

Hello everyone! I've not been ignoring anyone I've just had some talktalk Internet issues so have been struggling to get onto the forum.

Lucien has answered the questions I see
Everybody seems to think she's an antique glass *****!
Strange people! The amount of stuff I seem to collect people have stopped asking questions these days.

I have a Selmer valve stereo to collect at some point with carlsbro speakers out of a cinema, apparently it's extremely loud!

My projects for 2017 are crazy haha, busy busy busy!
Regards
Steph

[Hunts smoothing bomb]

Originally Posted by Steph1997

Everybody seems to think she's an antique glass *****!
Strange people!

Haha, priceless! were they embarrassed after you had explained to them what they were looking at? That makes me chuckle.

It would be brilliant if you could get this working, I'm keen to see Luciens video of how to fire it up, I've always been interested in these beasties but I have never read up on them and how they work... Great stuff!

Cheers

[britishrocket]

They think it is an antique glass what?

It seems to me that there are three things needed to get this going, an ignition source/solenoid, enough volts to strike and enough current at a sufficient voltage to maintain an arc. I have a thought for the latter two a 50-0-50 transformer with it's primary in series with some ballast (say 20A at 20V for the rectifier, that is 400W so a small heating element would suffice). I will bow down to Lucien's advice on this one (these three!).