Dynamo torch.

[keithinuk]

I know this is a silly question, but I need to ask it anyway, I have one or two dynamo torches, one has had the wrong bulb put in it.
So I need to find what is the right bulb for it.
Is it a job for a multimeter and what do I need to measure.
I know its a basic question, a little like my electronics ability, basic.

[robjkmannering]

If the two torches are similar, take the bulb from the working one, it will have the voltage rating stamped on the side. You can then buy one the same.

Rob

[Radio Wrangler]

If you measure the voltage from the dynamo without the right bulb loading it, the voltage will be far higher than the voltage of bulb needed.

If the two torches are dissimilar, then read the ratings stamped on the base of the dead bulb... unless... you've already..... oh dear!

David

[keithinuk]

One torch is sixties and the one I need a bulb for is forties, probably, so fittings and bulbs are different. I would have tried them if I could.

[60 oldjohn]

I had a seventies dynamo torch and it used a 1.5v bulb may be worth trying one.

John.

[keithinuk]

The sixties one I have has no voltage marked on the bulb, so that's no help, the forties Philips version uses 2.5 volts at 100 mA / 0,1 A bulbs, so that maybe a starting point.

As I would like to use a vintage style bulb and they are not that cheap, I will try some different cheap bulbs first; seems to be the only option so far, as I can not seem to find anything much on the make (Melas) brown mottled Bakelite.

[Ian - G4JQT]

I had one and I remember the bulb current rating was quite low, something not exceeding 100mA, might even have been 50mA.

But these days you could experiment with hi-bri white LEDs. Check the polarity of the volts as you squeeze the dynamo and if it's AC have them back-to-back.

If there are enough volts you could put two series - just do some measurements and see what's going on.

Then solder them into the old bulb base.

Ian

[broadgage]

I have an old dynamo torch and a relatively modern one.

The old one made by Philips uses a screw base bulb of 2.5 volts 0.1 amp, no longer readily available.

The newer unbranded one uses an unmarked PR base bulb of a similar rating. Also no longer readily available.

Note that although these items are called DYNAMO torches, that they in fact contain a small permanent magnet alternator with an AC output.
LEDs can be used in inverse parallel such that each LED or group thereof conducts for one half of the cycle.
No current limiting resistor is needed since the high resistance of the miniature alternator limits the current.

[G4XWDJim]

Originally Posted by keithinuk

I know this is a silly question, but I need to ask it anyway, I have one or two dynamo torches, one has had the wrong bulb put in it.

Can you describe or better still post a photo of the type of bulb you think you need or even a close up of the bulbholder. I have various bulbs that I might be able to help you with once I know what you need.

Jim

[Peter.N.]

Originally Posted by broadgage

Note that although these items are called DYNAMO torches, that they in fact contain a small permanent magnet alternator with an AC output

When these things first came out most people had never heard of an alternator but all cars had dynamo's as did many electricity supply generators so it was probably just the use of an accepted term.

Peter

[Alan Stepney]

I have one of these in a cast aluminium case, which dates from just pre-war.

The bulb is a 1.5 volt, and works but is very bright,. and I suspect may not be the original. Perhaps 2.5V would be better.

[keithinuk]

As you can see it has the wrong volt bulb, I wouldn't like to put in an LED, I don't see the point with a vintage item, to be honest I don't know how old it is, but probably before my time.
If I can work out the right bulb, that's what I will use.

[Herald1360]

Can you simply load the torch up with a series of resistors from about 5ohms to 50ohms and plot a voltage vs current curve for the different loads? Then just pick a point on the curve corresponding near enough to an available MES bulb. A true RMS DMM would be useful but probably not essential.

This would also factor out any change in output from a tired magnet, too.

[Phil G4SPZ]

There are many suppliers offering 1.5 volt and 2.5 volt MES bulbs on the usual internet sites. I think Maplin also sell a 2.5 volt bulb. They aren't that expensive; I'd be tempted to try the 1.5 volt first (these seem to come in 0.2 and 0.3 amp variants) as these would have been popular for single-cell cycle rear lamps in the 1940s and it's likely that the makers of this dynamo torch would have opted for a cheap, readily-available bulb rather than a special low-current type.

It's a nice artifact anyway and I hope you can get it going again.

[broadgage]

The alternator winding has a high resistance and therefore can't supply enough current to properly light a bulb that uses two or three times the current of the original.
I believe that a modern 1.5 volt or 2.5 volt bulb of 0.2, 0.25, or 0.3 amps will give a very poor light.

When these dynamo torches were made, 2.5 volt 0.1 amp bulbs were probably readily available. "Back in the day" dry cells had a much lower capacity and a higher internal resistance than today and low current bulbs were widely used from the smaller cells.

[keithinuk]

Well, I got some bulbs, basically what I could get hold of locally, a 2.5v 0.3A, this gives out no light at all, a 3.5v 0.2A pretty good, but probably could be better, so it looks like I need a 2.5v 0.1A.

[richrussell]

2.5V 0.2A MES bulbs seem to be fairly widespread, whereas the 0.1A ones seem much harder to find. Maybe try a 0.2A one - if the 3.5V 0.2A was pretty good, then I'd have thought the slightly lower voltage one would be about right.

Rapid, Farnell, RS and lots of other places all do the 0.2A ones for around 10p each.

[Herald1360]

If the generator is anything like old motorbike alternators, you may find that it's effectively a current source, in which case a 6V 0.1A bulb might work quite well.

[keithinuk]

That did cross my mind, If the 12v 0.2A bulb had a small glow and a 2.5v 0.3A had nothing, so the 6v 0.1A maybe worth a try.