Rear dynamo light to use with B&M Lumotec?

[JulesP]

I bought a B&M Seculite Plus Rear Mudguard Dynamo Light, thinking it would connect directly to the 'out' terminals on my Lumotec IQ2, but this appears not to be the case. 

The reasons why exceed my near non-existent electrical knowledge and while I gather the rear light can be dismantled and modified to remedy this, that's not something I want to get into.

Is there a simpler fix than snipping wires in the rear light; else can anyone recommend a rear mudguard dynamo light that *will* connect directly to the +/- terminals of the front?

[Kim]

Why won't it work?  Is there some sort of grounding issue?  Hard to make recommendations without knowing what the actual problem is.

"Lumotec IQ2" isn't a specific model, but I'm guessing you've got a Luxos of some flavour...

[Cudzoziemiec]

Swapping the wires round on the terminals shouldn't make a difference but might. I'm not too sure about the Seculite being dismantleable.

[Kim]

Swapping the wires round on the terminals shouldn't make a difference but might.

Agreed.  It's also unlikely to damage anything.

[grams]

The Luxos U (which some people call the Lumotec IQ2) has a rectified rear light output (i.e. It only sends one half of the AC wave). Some rear lights only have a half-wave rectifier, meaning it only takes current from half of the wave. If the halves don't match then you get no light output. As mentioned, easily solved by reversing the polarity - there's no harm in this as all components are designed for the messy AC output of a dynamo.

It's possible you have a problem with grounding if you have a metal frame. The Luxos U definitely isn't grounded, so you can test this by unbolting the rear light and seeing if it suddenly works. This is unlikely, but I can just about envisage a scenario where it might be prevent the light from working.

Failing that, check your cables for damage and continuity.

[Kim]

The Luxos U (which some people call the Lumotec IQ2) has a rectified rear light output (i.e. It only sends one half of the AC wave).

Ah-ha!  That would explain it.  Wonder why they did it like that?  Actually, for all I know, they all do it like that, I've never actually looked at what comes out of, say, a Cyo Senso.


(The front light won't be grounded, but the dynamo might be, so it's possible to fall foul of that if you don't preserve the polarity.  If the combination of half-wave and grounding problems means neither polarity works, it may be worth changing the polarity at the dynamo.)

[JulesP]

The Luxos U (which some people call the Lumotec IQ2) has a rectified rear light output (i.e. It only sends one half of the AC wave). Some rear lights only have a half-wave rectifier, meaning it only takes current from half of the wave. If the halves don't match then you get no light output. As mentioned, easily solved by reversing the polarity - there's no harm in this as all components are designed for the messy AC output of a dynamo.

Thanks, Graham - that kinda makes sense based on what I’ve read.

The IQ2 (yes, the Luxos U) isn’t fitted yet. After failing to decipher its instructions, I found this page: https://oac-dev.myshopify.com/blogs/ocean-air-cycles/84960132-busch-and-muller-luxos-and-grounded-tail-lights, which is where the unfathomable (to me) complications set in.

The rear light terminals are labelled + and ‘earth’ (its mounting bolt is also labelled ‘earth’), so I’m not sure which wire would go where on the front light (with +/-) - and again, the light’s instructions are of little use here...

So should the rear light work when wired to the front, but not bolted the (metal) mudguard on the metal frame?

@Kim - the front light works how ever it’s wired at the dynamo, so I’m guessing flipping those wires around won’t hurt.

[Pickled Onion]

So should the rear light work when wired to the front, but not bolted the (metal) mudguard on the metal frame?

Yes. Only when the front light is on and being powered by the dynamo. It won't get any power when the front light is in standlight mode.

In the rear light, the bolt is electrically connected to the terminal marked Earth. This is for compatibility with some older bottle dynamos with only one wire. I don't think you need to go to the lengths in that web page. First find out if your dynamo is "grounded" to the frame - which model is it?

[JulesP]

In the rear light, the bolt is electrically connected to the terminal marked Earth. This is for compatibility with some older bottle dynamos with only one wire. I don't think you need to go to the lengths in that web page. First find out if your dynamo is "grounded" to the frame - which model is it?

It’s a SON Delux, so my planned setup is:

• https://www.sjscycles.co.uk/dynamos/24-schmidt-son-delux-12-disc-dynamo-front-hub-black-anodised/
• https://www.sjscycles.co.uk/lighting/busch-muller-lumotec-iq2-luxos-u-senso-headlight-for-hub-dynamos/
• https://www.sjscycles.co.uk/lighting/busch-muller-seculite-plus-rear-mudguard-dynamo-light-with-standlight/

I’ll do some experimentation later today...

[Kim]

The SON Delux doesn't ground anything to the fork.  That should mean that the only grounding point is the rear light, so it shouldn't be a problem.

[SA_SA_SA_SA]

...The Luxos U (which some people call the Lumotec IQ2) has a rectified rear light output (i.e. It only sends one half of the AC wave)...

Are you sure it is only half wave rectified rather than full wave rectified? Both give DC but half wave will probably be more flickery at low speed (lamp dependant).

My Herrman S One (V2) doesn't like DC at all, I wonder if any rear lamps are similar?

It also seems strange the Stzvo does not require DC compatibility as it makes aiming lamps simpler....

[grams]

...The Luxos U (which some people call the Lumotec IQ2) has a rectified rear light output (i.e. It only sends one half of the AC wave)...

Are you sure it is only half wave rectified rather than full wave rectified? Both give DC but half wave will probably be more flickery at low speed (lamp dependant).

I can't actually remember off-hand which it is, and no longer own one to test. I didn't mean to imply one or the other with my answer!

I'd hope all LED rear lights still have some sort of smoothing capacitor, even if they don't have a standlight.