Lamp Limiter

[dave walsh]

I'm sure there would be a general "work around" even if all incandescents vanished from the face of the earth but there no doubt that a simple device which that takes the strain, when a disastrous current is building up, is a bit of a win win. It's interesting that this idea has come into its own [I think] along with the rise of restoration as a hobby and interest. My reading and limited experience doesn't suggest that engineers used them routinely in the fifties and sixties but then equipment wasn't quite as vintage and vulnerable or [as in the case of later solid state gear] subject to invisible destruction!

Dave W

[turretslug]

Originally Posted by stuarth

Would it not be better to use mains Halogen bulbs, sold as replacements for GLS bulbs? Halogen bulbs run hotter than GLS bulbs, so the resistance change should be greater. This would reduce the resistance under "no-fault" conditions, or allow a lower wattage bulb to reduce the current under "fault" conditions.

That's my reasoning, too- that a tungsten halogen type, with its wider resistance excursion, would have even better characteristics for a lamp limiter with visual indication from lower power. I have a nagging feeling that there's a fundamental flaw in my logic that someone will point out.... Hopefully, tungsten halogens will continue to be available for a bit longer than conventional types- though it's difficult to escape the fact that LED types are very efficient at doing what light-bulbs are intended for, i.e. producing light, which must represent about 99.9%+ of their market! The "rough service" dodge will surely be seen through sooner or later, too- if I were a legislator handed a plastic-casing LED "bulb" and a glass-envelope filament type, I know where my instinct would lie as to relative toughness.

Maybe in the past, workshops tended to use variacs more and the ones that fetch keen interest and high prices now are the minority survivors? There's no doubt that the lamp limiter represents a very happy combination of characteristics and simplicity, though.

[ms660]

Must admit I'd never come across a lamp limiter until I joined the forum, when I was in trade I never saw one, occasionally maybe a variac but they were seldom used at the places I worked at, the only test lamp we had on the bench at one place was a bulb on a fly lead to check for live chassis, most TV's were dropper types back then.

I've never used one when servicing vintage sets either, never found the need to, the usual components being checked/replaced first before applying power.

I can see the merits of folks using one though.

Lawrence.

[russell_w_b]

I used a lamp-limiter frequently when testing a thing called an Ignitron Pulse Unit out of a Marconi B6126 HF transmitter. There were several faults that could occur on any of the three toroidal transformer secondaries and it was possible to pinpoint them rapidly by the brilliancy of the lamp limiter, safe in the knowledge that no further bad things would happen. A variac provided no such instantly recognisable visual indication.

A standard 100W lamp has a cold-hot resistance ratio of something like 14:1. A 40W halogen is similar.

[AC/HL]

I don't remember such things either, but the patients weren't 60+ years old then, and spares were easily available. As for rough service lamps, as long as there is a volume demand for them they will be manufactured somewhere.

[Aerodyne]

Lamp limiters were not, to my knowledge, in use during the 'heyday' years of radio (and television) but have come to be used by restorers - rather than repairers - in the latter years.
Their simplicity and extremely low cost coupled with their value as a safety device and testing method make them an essential device and one I would not be without.
Tony

[mark_in_manc]

Mine just came in very handy fixing an old DRO off the milling machine at work. Lacking a circuit diagram, even I could snip out suspect components until the lamp went out, and work from there! So thanks for the tip to make one, forum people.

[Thyristor]

I first came across the lamp-limiter, on Mr. Stenning's main site, when I discovered it in the mid-late 90s.

I do, however, use, either a current-limited supply, and frequently replace fuses with 100R resistors (temporarily, obviously !)

[Refugee]

The first lamp limiter I spotted was in the mid 1970s in the electrical installations form room at college. I made my own with meter sockets across the lamp and a neon lamp in series with the earth when the lamp override switch is set to test.

[Vintage Engr]

Although as people have already said, Lamp Limiters might not have been in common use before radio restoration became the 'in thing', I do remember occasionally using one back as far as the very early '60's.
I didn't use it on a regular basis, but just when the situation demanded. I seem to recall using one on those dreaded Ferguson/ Thorn monochrome portable TV's that used a capacitive dropper to feed part of the heater chain...

Happy days!

[TonyDuell]

I seem to remember that Richard Feynman mentions using a light bulb as a current limiter in the experimental setup he had at home as a kid. Probably in the chapter 'He fixes radios by thinking' in 'Surely you're joking, Mr Feynman'.

I think that was describing work he did in the early 1930s. And I don't think he claimed to have first thought of the idea.