Dynamo Headlight - Does it Still Work?

[NickWi]

Apologies if I'm asking a stupid question that's got a very simple answer, but how do you test a dynamo headlight to see if it's still working if you haven't got a dynamo wheel to attach it to?

I have a Busch & Muller Lumotec IQ2 Luxos U that’s been rattling around the shed after I took it off my old touring bike. As we’re moving house soon and will no doubt be told by she who must be obeyed that ‘all that junk isn’t coming with us’, I dare say I’ll soon be having a shed clearance sale and want to know if the thing still works or not. Dynamos produces 6v AC, a 6v battery gives out DC, but as it's an LED light (which I understand run on DC anyway) is it so simple as to just wire the two together?

I know I could just try it, but I don't want to wreck a hundred quids worth of light if I'm wrong. Do I need a ‘thingy’ to go in between them or am I missing something very obvious?

Polite answers in simple English please as I’m the type of bloke that needs to watch a YouTube instructional video to wire a three pin plug!

[Jdsk]

I don't know. I'd guess it would work on DC at least one way round.

But if you can get it here I'll test it on a SP PV-8 dynamo.

Jonathan

[Brucey]

B&M specify which of their lights will work safely on DC. IIRC most (they say) won't. So if you are unsure, best to find a dynamo (even a bottle dynamo will do) and test it with that.

cheers

[2_i]

I have powered different B&M LED dynamo lamps from ~6V DC, on and off a bike and not ran into any significant difficulties thus far. The only problem I can recall is of something being off with powering of the rear lamp off the front for one of the polarizations. Also I powered B&M LEDs off USB chargers and computer outputs.

[ConRAD]

... regardlessly of polarization I do something like that

[youtube]8qYzDkcCRcI[/youtube]

[Brucey]

why take a risk when B&M tell you if any given model of light is suitable for DC operation or not?

Some of you may be thinking 'risk, what risk?'.

Well the usual thing (especially with headlights) is that the AC is rectified to DC and then used in the remaining circuitry. Apologies if this is b. obvious, but when you feed the lamp with DC, all the current passes through half of the rectifier circuit, instead of being shared throughout it. So you are doubling the load on some of those components.

You may be thinking 'oh but those parts are so cheap, why would you fit the low current capacity version?'. Why indeed... Well, having worked with electronic engineers, I would say that they love to work to a specification, which they will tend to meet exactly, rather than build in safeguards etc; if it (e.g. a safeguard) isn't in the specification, it won't go in.

Note also that a battery is normally not limited in current either, whereas a bicycle mounted generator is limited in current output. So there is another built-in safeguard with a generator.

I'd reiterate that the OP's concerns are not without foundation and it is an unnecessary risk to power a £100 lamp using DC unless B&M explicitly mention this is OK in their documentation.

cheers

[ConRAD]

I think that "powering" and "testing" are indeed something somehow much different, the first being permanent, the second "just for a while". Indeed B&M states that "...the headlight can only be powered by a dynamo (AC). Connection to a DC power source (battery) is not possible". And that is very correct, I believe, because in case of DC powering you'd get a permanent overload on two diodes only instead of four on/off diodes.
To be considered also that prior to be fed to the rectifier bridge the AC dynamo input voltage is Z-downchopped to 6.7V (the same being transfered to the rear light) ... but if you don't like a 9V battery for testing you still may use an USB 5Vdc. IT WORKS!!

[2_i]

why take a risk when B&M tell you if any given model of light is suitable for DC operation or not?

Some of you may be thinking 'risk, what risk?'.

Well the usual thing (especially with headlights) is that the AC is rectified to DC and then used in the remaining circuitry. Apologies if this is b. obvious, but when you feed the lamp with DC, all the current passes through half of the rectifier circuit, instead of being shared throughout it. So you are doubling the load on some of those components.

Well, with all this lecturing, DC is just AC at frequency going to zero. Putting out a product to the market, the manufacturer must assume that the customers will try to use the product in a bit of different manner than originally intended, such as with bottle rather than hub dynamo, battery etc. A safety bracket must be built around the product as otherwise customers will be coming with a flood of complaints about failures and the manufacturer will spend life sorting out which are legitimate and which not and possibly going belly up in the process. Yes, an overengineered product, such as Luxos, can be testing frequency and do this or that depending on outcome. However, a step function in switching on, contains all Fourier frequency and at least the lamp will blink or something, and maybe shine normally, when you connect it to a DC, indicating whether the electric connection is there or is broken.

[Jdsk]

In that case will you indemnify NickWi if he tests with DC? That would solve the problem.

Jonathan

[Brucey]

Well, with all this lecturing, DC is just AC at frequency going to zero. Putting out a product to the market, the manufacturer must assume that the customers will try to use the product in a bit of different manner than originally intended, such as with bottle rather than hub dynamo, battery etc....

Who is 'lecturing' exactly? At risk of repeating myself, that is not how B&M see it. In their instructions they specifically prohibit DC power being used on most of their lights. Only if the instructions for that model lamp specifically allow DC use should battery power be attempted.

To test a Luxos properly requires much more than momentary flash of light from the front; it has several functions which cannot be tested instantaneously.

cheers

[2_i]

In that case will you indemnify NickWi if he tests with DC? That would solve the problem.

Jonathan

I take responsibility for my own actions. Within those, my main bike is equipped with B&M Lumotec IQ Cyo T headlmap, about which B&M says "The headlamp is intended for powering with dynamos (AC voltage) only." On my bike that lamp is powered with current that varies between full-rectified AC and DC from rechargeable LiPhosphate batteries. It is never powered there with the original AC that it was meant for. Other B&M LED lamps were used at different times in that configuration.

[Jdsk]

That would be a "No", then?

: - )

Jonathan

[2_i]

That would be a "No", then?

: - )

Jonathan

Thanks, for spelling this out. There are multiple ways to mess things up, and I would only be taking responsibility if I had the lamp in my own hands .

[Jdsk]

Edited: I think that I read that 180° wrong...

Jonathan

PS: My offer to test it with a dynamo stands.

[ConRAD]

… however for testing its lights, with no distinction between dynamo or battery, B&M suggests its 6 Vdc “Lightchecker Light, type 1516LC1”