Torch bulb question.

[Lancs Lad]

I bought a look-alike light bulb a few years ago.

The sort that runs off batteries, with a pull-cord switch, and is probably meant for emergency lighting in the event of a power cut.

It has an old-fashioned incandescent torch bulb, and runs off 4 AA batteries, which suggests to me that it needs 6 volts.

However, it has a socket marked '5 volts', and I happen to have an old Nokia phone charger with a jack plug that fits!

See all the pics.

My question is this: Will I damage either the charger plug, or the bulb, if I run this light from the mains transformer? Obviously, the Nokia plug gives out 5 volts (or thereabouts!)

Five volts seems a big difference from six volts to me, but I'm not an expert, so what do i know?

I just like the old incandescent torch bulb. You don't see many of them nowadays, and I want it to last as long as possible.



I'm not going to use it regularly - just when I want to see a proper torch bulb lit up!

[Graham G3ZVT]

With one proviso you won't damage anything.

Depending on how the lamp is designed, there is a possibility that the charger will attempt to charge the non-rechargeable batteries if you leave them in.

This will cause leakage and corrosion, or worse.

[Lancs Lad]

I'm ahead of you there, Graham!

That was my first thought when I realised the charger plug would fit.

I took the (non rechargeable) batteries out before I plugged it in!

It could have become very messy, unless there was some sort of device in place to completely separate the battery circuit from the transformer socket.

Would that be likely, do you think, given that there was a socket provided?

[radiograham]

Can you still buy torch bulbs?a few years ago i got lost in the infamous maplins online catalogue,the one that even maplins staff could'nt use and found them under "specialist bulbs".

[Jolly 7]

Is the bulb definitely incandescent ? Is there a filament visible ? If not, it could be an led bulb in disguise . That may be the reason for the specified 5V power supply and it could be that there is a 5V to 6V IC based booster circuit in the fixture. Maybe this light was manufactured just before the use of built in lithium rechargeable batteries became widespread, such as in solar lights ?

[JonSnell]

Everyone is over thinking this.
The bulb looks like incandescent and LED were probably not available when it was purchased.
The Nokia charger has a single DC connector and does not control the charge. There is no control sense within the two pin connector.
I think one will find, all Nokia and most other phones, have the charge control on board.
Some that control the charge rate use USB or a bespoke smart socket.

[Herald1360]

The nominal voltage rating for old torch bulbs varies depending on their current draw. This allows for the internal resistance of dry cell batteries to some extent.

A 4-cell (nominal 6V) 0.5A bulb is typically rated at 4.75V for example.

So rating the dc power input for this lamp at 5V may reflect a realisation that the source impedance of a power supply might well be lower than that of the "6V" battery and running the bulb at 6V would reduce its already short, typically only 25 hour, life even further.

Torch bulbs were considerably over run, the cost of batteries/cells soon outweighs the cost of the bulb per unit of light output, well it did when you could get them.. 25 hours is quite long for a torch bulb. If you want it to last for ages use a 6.3V dial lamp.

[Lancs Lad]

Yes, I'm fairly sure the bulb is incandescent.

I bought this thing about fifteen years ago, just as a novelty item, really, mooching aimlessly around Sainsbury's late one night, and have hardly ever used it. I just liked the look of the shape of the traditional light bulb, at a time when those horrible CFL lamps were all that you could buy!

Nostalgia, I suppose!

The actual little bulb is frosted, which is unusual for a torch bulb, so, obviously, I can't see the filament inside. But it does get hot, so I'm pretty certain it isn't LED.

Also, if I leave it on overnight, running off the batteries, it has always exhausted them by next morning.

[stuarth]

Cheap lights with LEDs use 3 cells (and sometimes rely on the internal resistance of the cells provided with it to limit the current). With 4 cells, you either waste the power of the extra cell in a dropper, or you put in a switch mode circuit such as the “Joule-thief” type, in which case a single cell is all you need unless you have a very powerful light.

With filament bulbs, the light output as it dims with failing batteries moves distinctly towards orange, where an LED stays stubbornly the same colour.

I would expect (but not guarantee) the power socket would disconnect the batteries when the plug from an external supply was inserted. Does the light go out if you plug the Nokia supply into the light when the Nokia supply is not connected to the mains?

Stuart

[broadgage]

Agree not to over think this.
The voltage of dry cells drops on load and is generally reckoned as being about 1.4 volts on light load (note 1.4 volt heaters in battery valves)
Down to about 1.2 volts on heavy load.

I would expect an incandescent torch bulb designed for four cells to be about 4.75 volts to about 5 volts.
If such a bulb is supplied with a true 6 volts as from a transformer it will have a very short life.
An external transformer rated at 5 volts should be fine for a four cell torch bulb.
The bulb life may still be rather short since torch bulbs are designed for maximum efficiency rather than for long life.

A torch bulb for FIVE cells would be rated at 6 volts in principle, in practice they were often marked 5.95 volts. An insignificant difference but served to avoid confusion between 5 cell torch bulbs and 6 volt pilot lamps or vehicle bulbs.

[The Philpott]

-On first sight that looks like it is one of the Nokia chargers that runs cool to the touch..?
If i was using one of the older warm ones on a 100% duty cycle i would keep a north eye on it until i was sure it was happy.

It's pleasing that there's a charger that hasn't added to the WEEE pile- helped by the fact that the jack is the correct size- the conductors are usually tiny so changing the jack can be very awkward.

Dave

[Lancs Lad]

Yes, Dave, it's one of the lightweight electronic ones. It's not one of those heavy chunky ones with a copper coil inside. I have a couple of those in a drawer somewhere, but the plugs don't fit.

It's amazing how often Nokia used to change the jack plug size every time they brought out a new phone back in the early 2000s, before micro USB became almost ubiquitous.

I have six old Nokia phones (dating from 2000 to 2014) and five Nokia charger plugs (I lost one when I sold our caravan, but that's another story entirely!)

The jack plugs are all just a tiny bit different sized, so that they won't fit into older phones.

[Brigham]

Those frosted bulbs, with over-size round globes, are usually bicycle dynamo bulbs, intended for a range of voltages.
A cycle shop may be able to advise.

[Glowing Bits!]

Originally Posted by merlinmaxwell

Torch bulbs were considerably over run

Slide viewers are the same, the dimmer switch has the bulb run at normal voltage whilst the boost setting grossly overruns the bulb!

[bobhowe]

Seen these for sale in the Chinese shops in Spain kind regards Bob

[agardiner]

Yes, still readily available. They seem to be sold in various guises, some with a rechargeable lithium cell, others without. I suspect they all have the same control/charge module. I have a few LED battery desk lamps in a similar construction. Some have lithium cells, others don't and have 3 AA instead. All have the input socket.