Lamp Limiter Query.

[Philips210]

Hi

I've been successfully using a 100W lamp limiter when testing my old radios but I am thinking of extending it's usage for television as well.

I think I read on this Forum mention of using up to 150W bulbs for higher power consuming TVs. I suppose a 60W or even a 40W may be very useful for battery/mains valve portable radios as well.
The idea is to have a few lamps that can be easily switched in and out for the radio/TV being tested.

One thing I was thinking about is the type of bulbs to use. Clearly the standard GLS filament lamps are tried and tested but I had wondered about using halogen bulbs as a possible option, though the filament may be more difficult to see.
Also, in order to reduce the overall size of the test unit, I thought about using filament golf ball lamps. Technically, is there any reason why I should not do this?
A small wire mesh cage can then be fabricated to protect the bulbs from damage.

Any thoughts on this will be appreciated.

Regards
Symon

[G8HQP Dave]

This question came up a few years ago. The consensus was that any filament lamp will do.

I raised the issue of short life with a cool-running halogen lamp, but it was pointed out that long life is not really a requirement in this application as a lamp limiter might only be used for a few hours.

Personally I suspect that a frosted lamp will make it easier to see how much the filament is glowing, but these are getting harder to come by. I did see some 'industrial use only' frosted lamps in a local cheap shop just a few days ago. The blurb on the packet actually said 'not suitable for domestic use'!

[G6Tanuki]

Halogen lamps work just fine.

Indeed - part of my service-kit is a "12-volt lamp limiter" made from a car H4 main/dip headlamp bulb one of whose filaments is dead. Perfect for use when setting up mobile VHF two-way stuff like Pye Westminsters and M290-series that don't have any high-SWR-protection of the output transistors.

[turretslug]

When the "dim-dip" system was first introduced for car headlamps, the question of reduced operating temperature preventing the tungsten-halogen cycle from operating as intended and shortening filament life was raised but, after a few years, it seems the effect was regarded as negligible.

By my possibly rather simplistic logic, and given the same rated voltage, if a halogen bulb's filament operates at a higher temperature than its similar wattage "conventional" counterpart, presumably it has a lower "cold" resistance? I suspect this effect would be insignificant for this application, mind.

[Philips210]

Thank you for all your replies, I am going to build the lamp limiter using filament golf ball lamps to reduce the overall size of the unit. Once completed, I'll post some pictures of it.

Regards
Symon.

[petertheorgan]

I have a lamp limiter with 3 normal light bulbs , 60w , 100, w and 200 w and a triple light switch so I can short each bulb out in turn , much more use in my opinion than a variac as I can see what is happening by watching the brightness of the bulbs change as you give the set a bit more "juice" , I have even been known to put in a double bayonet adapter and then use 2 200w lamps in parallel ! , here is a photo of mine with both no limit and max limit , neon on left is across mains and neon on the right across output

[emeritus]

The new edition of the "Screwfix" catalogue came in today's post, and I see that they do 60W "Rough Sevice" GLS lamps in both BC and ES, for 99p. The illustration looks like frosted, but is described as "warm white". A local corner shop does "rough service" frosted bulbs in 40W, 60W and 100W.

[FrankB]

Be aware that if you intend to work on TV sets using the 'Dim Bulb Tester" apparatus, that certain TV sets will NOT work on them, due to the high required current inrush.
RCA ( Certain models) and certain brands of rear PTV's immediately come to mind.

I found this out the hard way while servicing them, looking for non-existent problems in start up. The same problem was also encountered using the Sencore and B&K isolated, variable AC voltage power supplies . (PR 57, etc.)

[McMurdo]

Out of all the TV's I've serviced over the years, (70s/80s/90s CRT) few will refuse to start with a lamp limiter, though most will need the degauss coils unplugging. Large plasma and LCD tv's use alot more power and most will struggle to come out of standby.

I see it as useful for trouble shooting an SMPSU rather than the TV as a whole, where a properly working SMPSU should protect itself in the face of a short LOPT etc.

I've never used one for valve radios, the biggest short-circuit risk being the mains suppressor capacitor..everything else is pretty tame wound up on a variac. It's useful on a guitar amp as a valve that flashes over can be observed in a darkened room rather than having to feed the amp packs of 100mA fuses while you find the fault.

[Tim]

I have had mine for years, cobbled together out of junk. Total cost 75p(for the light bulb). I prefer clear bulbs as you can see the filament clearly.