Adhesive/cement required for a ceramic insulator

[GreensandsParticipant @greensands]

I have recently acquired a vintage 8mm cine projector which incorporates a ceramic insulator as part of the mains voltage dropper circuit supplying the 110 bulb. Although the projector itself is generally in very good condition and adequately earthed, the ceramic insulator is in two pieces some two thirds along its length as seen in the photo.  I think it would be quite easy to repair given the right choice of adhesive as it is a clean break. Can anyone suggest a suitable choice of adhesive/cement which would be able to withstand the expected temperatures?  All suggestion gratefully received.

 

[MacolmParticipant @macolm]

How many watts are dissipated in this ballast? If it is a resistive dropper for a 110v bulb, then about the same as the bulb itself. This suggests it will get fairly hot. Few adhesives are likely to last. Perhaps high temperature epoxy or silicon rubber might survive.

Alternatively, assuming there are no connections inside that could contact the element, consider making up a metal sleeve that holds the parts together. Just sheet metal, with tabs that fold over the ceramic at the ends to retain the halves together. It needs to be fixed and cut away anywhere it could short.

 

[Brian WoodParticipant @brianwood45127]

If you can source glass tubing of the right internal diameter to splint the existing insulator, that might do the job. Better still if it is silica tube. Where to go I have no idea but searching on Google would be a start.

I don’t think an adhesive would last very long at all

Brian

[Speedy Builder5Participant @speedybuilder5]

Sounds like a NASSA tile adhesive ??

[noel shelleyParticipant @noelshelley55608]

air setting cement of the type used in pottery kilns – alec tiranti may be. I used this material to build a smelting furnace. Good Luck. Noel.

[Speedy Builder5Participant @speedybuilder5]

Space shuttle tile adhesive,  but the tile is the insulator before the adhesive / shuttle body.

Insulcast adhesive made by Permagile Industries Inc. of Plainview, NY 11701. Most likely a derivative their IC 1-215 epoxy.

Insulcast

May be cheaper to re-think the resistor.

[ChrisLHParticipant @chrislh]

Vitcas seem to do a range of high temperature adhesives – no personal experience of them though.

[Speedy Builder5Participant @speedybuilder5]

How hot does it get – use a Thermal Thermometer ??  Because it looks like there are insulated wires running along side the element.

[duncan webster 1Participant @duncanwebster1]

Unless you’re after the ultimate in authenticity how about a lamp dimmer and getting rid of the resistor. It’s tempting to think if you just half wave rectify the mains that would do, no doubt someone who knows what he’s talking about will explain why that won’t work

[MacolmParticipant @macolm]

Unfortunately, half wave from 220 volt mains would supply twice the voltage and thus also twice the current (so four times the power level), but for half the time, about twice the correct power for a 110 volt lamp. However, a suitable thyristor dimmer would be able to be adjusted to the correct power level.

On closer inspection I notice the ceramic is cruciform section, so no through hole. Perhaps, though, a steel strip can run on each side between the top and bottom mounting screws without touching the element, or better might be small diameter ceramic or fibreglass tubes with though bolts or threaded rods. Small ceramic spacers used to be available, and a series of these on screwed rod might do, or fibreglass sleeving over a steel strip. Maybe even just steel wire, tensioned up. It all depends on the exact configuration of what you have got. Keep thinking!

 

[John HaineParticipant @johnhaine32865]

I can’t believe that in 2025 we still want to use a dropper resistor for this!  I’d repair the ceramic with araldite but only retain it for visual purposes, and either use a solid state dimmer or even better replace the bulb with a high intensity LED.

[ZephyrinParticipant @zephyrin]

I would test clay for refractory furnace…

[GreensandsParticipant @greensands]

The ceramic insulator is hollow in section although with restricted access from the top and if you examine the fracture as shown in the photograph it is possible to make out a couple of wires passing down through the centre which, on the face of it, would appear to be conventional pvc insulated connecting wires. This would imply that the temperature in the centre of the insulator is kept to fairly moderate values. I hope to be able to locate a suitable mercury in glass thermometer which I have somewhere in which case it might be possible to monitor what the running temperatures are within the insulator. Certainly parts of the circularly wound resistance wire can be seen glowing red but this is restricted to the outside of course and is fairly limited in extent.

[SillyOldDufferModerator @sillyoldduffer]

Is Greensand’s dropper doing something complicated?  I assumed it was a simple wire-wound dropper, but looking closely shows features I don’t understand.

Top left, a white and blue wire pass inside the element.  The insulation must be something special, not PVC, because Nichrome wound on a ceramic former suggests HOT!  Maybe red-heat.   What are they for?  Could be a heat sensor, possibly a safety cut-out, or maybe an on/off regulator.

The other peculiarity is the element.  It isn’t a single element, it’s four wired in series.  At the top, two close wound single turn elements, connected to a 9 turn less tightly coiled element, which connects to another tight single turn element at the bottom.  Again, why?  The construction is complicated and expensive compared for a simple resistor.  I’m wondering if this is some sort of stabiliser, perhaps compensating for resistance changes as the thing heats up.  Is there any wiring connected on the other side?

If it has a complex function, substituting an ordinary resistive or capacitive dropper, or a diode or dimmer might not be good enough.   Another reason might be an ordinary dropper fitted with adjusters to work on different international voltages from 220 to 250.  Dunno.

I found a suitable goo but it’s industrial – no-one sells it retail.  Therefore I’d try Macolm’s steel strap inside the cruciform.  Maybe with a dab of cement to seat the join.

Dave

[GreensandsParticipant @greensands]

To try and answer SoD points. The projector, a Specto Type 500  Model XC ca early 1950’s is fitted with a manual rotary switch consisting of an OFF position followed by 3 further settings. Close examination reveals these positions to be physically connected to 3 small radially located independent Push ON/OFF switches which in turn are connected to various tappings on the mains dropper resistor, the assumption being that this is the means of extending the life of the bulb. All conjecture on my part and I am quite willing to stand correction.

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