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Old 22nd Jul 2017, 9:18 am   #1
Wishiknewmore
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Default Lamp limiter...no 110v bulbs!

Morning
I was about to start fiddling with a 115v Philips Philetta circa 1960 and I realise I don't have any 110v bulbs.
I was going to use an 220 to 120v 250 va auto transformer.
Could I use a 230v 40w instead?
Can't quite get my mind round this!
Thanks in ignorance.
Philip
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Old 22nd Jul 2017, 9:30 am   #2
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Default Re: Lamp limiter...no 110v bulbs!

Put the lamp in the primary of the step down transformer.
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Old 22nd Jul 2017, 10:38 am   #3
Wishiknewmore
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Default Re: Lamp limiter...no 110v bulbs!

Thank you Colin
Obviously..I think!
Philip
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Old 22nd Jul 2017, 4:05 pm   #4
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Default Re: Lamp limiter...no 110v bulbs!

Try with no load on the transformer first. If the transformer runs a high magnetising current the bulb indication could be misleadingly bright.
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Old 22nd Jul 2017, 4:25 pm   #5
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Default Re: Lamp limiter...no 110v bulbs!

Would a lower VA transformer be preferred Chris, say 100VA ?
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Old 22nd Jul 2017, 11:24 pm   #6
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Default Re: Lamp limiter...no 110v bulbs!

It would probably help.

Magnetising current is determined by by the TX primary inductance and can vary quite a bit between different transformers of similar ratings- it depends on what particular set of compromises the designer has made to achieve the same result. If it's a significant fraction of the current resulting from the secondary load on the TX, it will compromise the usefulness of the lamp limiter.
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Old 5th Aug 2017, 12:35 am   #7
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Default Re: Lamp limiter...no 110v bulbs!

If you wanted to build a 110V limiter (like I have), I can get new 40W, 60W and 72W (72W is halogen) 110V bulbs with Edison screw Bases. Send me a Pm if interested.

Geoff
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Old 5th Aug 2017, 11:34 am   #8
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Default Re: Lamp limiter...no 110v bulbs!

You just need a resistance in the circuit, a lamp gives some indication of current.
A 240V bulb will do fine on 110V, you will not blow it up, it will just not run very bright. Instead of 60W 110V, a 100W 240V could be substituted.
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Old 5th Aug 2017, 11:54 am   #9
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Default Re: Lamp limiter...no 110v bulbs!

60W and 100W 110V ES bulbs should be available from a builder's merchant - they're used in the "festoon lighting" chains you see draped round scaffolding or to provide illumination in partly constructed buildings.

A lot of places are selling the bulbs off cheap now everyone's gone LED.

You can also get coloured 110V bulbs as they're used in things like town-centre christmas-tree displays - though again these applications are rapidly going LED.
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Old 5th Aug 2017, 12:44 pm   #10
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Default Re: Lamp limiter...no 110v bulbs!

The lamp limiter isn't a simple series resistor in the mains feed, it's a lot better than that.

The filament in the bulb changes resistance quite a lot with temperature.

If your radio or whatever takes a reasonable amount of power, well short of the bulb's wattage, then the bulb doesn't light and just glows a little. In this state its resistance is low and the mains voltage to the set isn't dropped much, so the set should work properly. All is well.

If something goes wrong in the set and its current consumption increases, the bulb heats more and that puts its resistance up, so the same current makes even more heat in the bulb and its resistance climbs further, the current available to the radio drops back. Damage is prevented.

So it acts as a current limiter with 'Foldback' meaning that the current available is dropped if the unit under test tries to take too much. The lamp limiter is a very neat invention. The bulb has a useful amount of thermal mass which delays the onset of current limiting and allows a short surg for charging up reservoirs.

Lamp limiters are great for testing valve equipment, but less good with transistor equipment. They aren't fast enough to beat an overstressed semiconductor in the act of dying, but they do act to limit the risk of a big burn-up.

David
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