Emergency light transformers

[Radio Wrangler]

I've just replaced some battery-maintained emergency lights. I noticed that in the old ones there were some rather nice small mains transformers used to charge Nicad stacks. I think the secondaries must be about 6v.

Considering the numbers of these things getting replaced once replacement nicad packs aren't available, this must be a good source of transformers for powering fiddly things.

Four years ago my cooking hob packed in, a tiny transformer for a 5v supply for touch-switches. Could I find a transformer on a small enough core? could I hell. So now, yeas after buying an new hob, do I find where to get some for free!

David

[broadgage]

Yes, but do be aware just how crude the circuits in cheap emergency lights are.
The batteries usually fail by developing a high internal resistance, approaching an open circuit. After this has happened the mains transformer should be fine.

Sometimes however the battery fails by developing an internal short circuit, when this has happened it usually kills the mains transformer.
The charging circuit is often a simple bridge rectifier and current limiting resistance. A short circuit battery therefore substantially increases the charging current and burns out the transformer or the resistance.

Before attempting to recover any components from an old emergency light, look at the battery charging resistor, this will be the largest size resistor in the unit.
If burnt looking, throw the whole thing away* If the resistor is not burnt looking then recovering the transformer and perhaps also the high wattage low ohmic value resistor is worth a try.

*apart from the battery which should be recycled.

[mole42uk]

I'm currently recovering a few of these for use as task lights - by replacing the batteries on those units which still work but fail the 3-hour test. They can be fitted with a short length of mains flex and a 13A plug and left plugged in. When a task light is needed, just unplug it, the tube lights, carry the luminaire to the job and it's illuminated.
Recycling or what?

[Refugee]

I have got two very good task lights based on those SLA emergency lights RS used to sell in the 1980s.
They have two 20W halogen lamps that can have there angle adjusted and with a rope handle added are very useful. The lamp and maintenance are on a double pole switch so they do not light up when being carried unless you need them to.

[AC/HL]

A similar source is old Microwave Ovens, the ones with electronic timers, not the cheap mechanical ones. Also scrap CH boilers if you can find one.

[turretslug]

Laminated mains-frequency ironware is rapidly becoming an endangered species, both the copper content and the weight ring alarm bells with production accountants! Also, the latest small, cheap SMPSUs are very efficient and offer wide input voltage range, putting a smile on legislators' faces. Worth looking out for- look how olde-worlde combined HT/LT transformers were common-place and inexpensive on the surplus market two or three decades ago, now anything with a half-decent power rating attracts an unseemly scrabbling and sometimes loopy price on auction sites

I check scrapped things for laminated transformers- as AC/HL says, a lot of things had little transformers to run their control circuitry. I've found a lot of flat epoxy-potted types that mount to a PCB with self-tappers- these can have their holes extended all the way through to the opposite side for M2.5/M3 fasteners, mounting on its back with tags in the air. (Looking a bit like that big potted lump in a Quad 405!) The necessary use of primary and secondary bobbins on separate limbs of the laminate stack means that efficiency and regulation aren't tip-top but the DC isolation is exceptional if you have any "floating" requirements like 'scope CRTs. I haven't measured the inter-winding capacitance but I expect that this would be usefully low.

Anyone got any bright ideas for clamping/affixing the commonly-found type that has an open laminate stack but is secured solely by PCB pins, or is held in place inside a custom moulded plastic box? I wouldn't want to drill holes through the lamination corners, as I suspect that the flux path is pared to the bone as they stand.

Anyone got any bright ideas for clamping/affixing the commonly-found type that has an open laminate stack but is secured solely by PCB pins

Car body filler, cheaper than epoxy and it stays put while it sets.

[G6Tanuki]

Originally Posted by turretslug

Anyone got any bright ideas for clamping/affixing the commonly-found type that has an open laminate stack but is secured solely by PCB pins, or is held in place inside a custom moulded plastic box? I wouldn't want to drill holes through the lamination corners, as I suspect that the flux path is pared to the bone as they stand.

In most cases you can slip a couple of cable-ties through the gaps between the windings and the cores: a few holes in the PCB and bob's yer uncle.

Cable-ties work well with smaller toroids too. I don't worry about the compression effect of the ties possibly damaging the windings' insulation - if you do you could always put a strip of noncombustible plastic (PVC?) between the cable-tie and the windings to spread the load a bit.

[turretslug]

Good call, chaps- I haven't used "gob" for about 25 years since my earnings covered acquiring a decent car- hopefully, the tenacity and flexibility have progressed since then. I think I'd go for bonding the trafo to a plate, then bolting that to chassis-work to give a bit of flexure isolation. I'd considered cable-ties but I've seen too many that have gone brittle and parted in old installations- though that may have been exacerbated by excess tightness, heat and vibration. Also, the well-known Continental discount outlets sometimes do epoxy putty at a price that makes it worth at least a try with a useful recovered component.

As you say, definitely wouldn't entertain toroid fixture using cable-ties without something with a bit of give intervening.

[Herald1360]

What's wrong with folding up a bit of tin into a clamp and letting the pins stick out sideways?

Or just a simple channel section and two long screws or bits of studding?

[turretslug]

True- and I have used off-cuts of DIN rail top-hat section to clamp salvaged transformers- but I know there are some ingenious souls lurking here who might have come up with something right off the beaten track but ideal, as with,

http://www.vintage-radio.net/forum/s...d.php?t=118544

I was a bit put off the ad-hoc clamp set up by working with the products of a well-known European manufacturer of airliner flight-deck displays and other inexpensive and inconsequential items. C-core transformers were secured by use of lengthy studding and sections of steel bar bearing on the lam stack, inevitably they worked loose and moved around, leading to complaints of loud buzzing and fatigued connections.

[Radio Wrangler]

Caulking gun and a tube of "I can't believe they're not transformer brackets" ?

DAvid

[Nickthedentist]

Ha ha! There's a marketing opportunity for you, David. Maybe the BVWS would stock it?

[mole42uk]

Evo-Stik.

Or cable-ties.

[turretslug]

Folk sniff at Evo-Stik and its stablemates (metaphorically and literally....) but I quite rate it for its tenacity and relative flexibility for all sorts of things. I'd certainly give it a go for, say, <6VA types. It does slowly char and go brittle at elevated temperatures, though. I did have a cartridge of that stuff used for securing false floor footings where the substrate is tanked and can't have fasteners through it, like a very thick and still slightly flexible Evo-Stik in effect but naturally can't lay hands on it. I also saw a very impressive mineral-loaded epoxy used to secure threaded eye-bolts into clearance holes in a concrete hangar roof, not long afterwards some high-wire artists were merrily flinging themselves around on wires suspended from them.... Probably either of these too expensive to fritter on a salvaged transformer but I'll keep my eyes open.

Not dismissing cable-ties either, I've been surprised at what some manufacturers get away with using them for, but need to use plenty for redundancy, really. (Rattle, flash, bang....)

[emeritus]

For this sort of thing I use non-corrosive high strength silicone rubber sealant (Dow Corning 3145 RTV clear MIL-A-46146) that I got from Farnell. Not cheap, but it works and does keep well beyond its "use by" date. Evo-Stik is good too, but I think they must have changed the formula a few years ago, as the current stuff seems to be a bit less flexible than it used to be. I used to use it for re-gluing paperbacks and making scrap pads by gluing along one edge, and some books I repaired more than 40 years ago are still in good condition.

[AC/HL]

I have something similar, Plast 2000. We used it to seal external RF connectors at Work.

[FrankB]

GE RTV Aquarium Sealant has worked well for me in the past. But I do prefer something mechanical JIC the bond fails.
Also maybe a "U" bolt over the transformer might work, if the "U" bolt or a chain clamp doesn't if it doesn't disrupt the transformer into thinking it has a shorted turn. Here we have them with rounded tops and square tops. An auto store also carries them as "Muffler Clamps" and "Spring Clamps". If you have a company that makes clamps for replacing car springs, they should be able to bend one to the dimensions you need.